Precision HubFind TI’s latest analog content at TI.com/analogwirehttps://e2e.ti.com/blogs_/archives/b/precisionhub/atomTelligent Community (Build: 11.1.7.15705)2016-08-19T09:00:00ZHow to balance efficiency and settling time with highly integrated DACshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-balance-efficiency-and-settling-time-with-highly-integrated-dacs2017-05-05T15:12:39Z2017-05-05T15:12:39Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/DAC8775" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC8775</a></div><p>In my first <a href="https://e2e.ti.com/blogs_/b/precisionhub/archive/2017/03/07/dacs-enable-performance-in-analog-output-modules" target="_blank">post</a>, I talked about the need for modular, flexible and smart design in analog output modules and explored methods for improving efficiency in a typical high-side voltage-to-current converter used to drive 4-20mA outputs. Figure 1 shows an implementation that involves a buck/boost converter in a simple feedback network to supply just the necessary power to the load. While this implementation makes 4-20mA generation highly efficient and thermally optimized, it comes with a reduced settling time. So in this post, I will explore how to balance efficiency without sacrificing settling time.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/0118.DAC8775_5F00_1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/0118.DAC8775_5F00_1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: High-side voltage-to-current converter with a buck/boost converter</b></p>
<p>As an example, let’s assume a load of 1kΩ. If the digital-to-analog converter (DAC) code changes from a 4mA output to a full-scale 24mA output, the buck/boost converter will boost from approximately 7V to 27V, as the system is fully adaptive. Since the buck/boost converter needs to charge the large storage capacitor, the settling time of the system could be as large as a couple of milliseconds, as shown in Figure 2. In such a fully adaptive system, the settling time is completely dominated by the buck/boost converter. However, some systems need to have a relatively faster settling time while still meeting efficiency requirements.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_2_5F00_small.jpg"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_2_5F00_small.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Settling time for a fully adaptive system</b></p>
<p>You can meet this goal using TI’s <a href="http://www.ti.com/product/dac8775" target="_blank"><span>DAC8775</span></a>, a quad-channel 16-bit 4-20mA DAC with adaptive power management. This DAC integrates an analog-to-digital converter (ADC) to sense the load and set the buck/boost converter to a fixed value.</p>
<p>The block labeled “Auto Learn” in Figure 3 is a high-level representation of this system. Based on the ADC’s calculation, the buck/boost converter clamps to a fixed value, which satisfies compliance for a full-scale output current at a given load. Once the DAC code written exceeds quarter scale, auto-learn mode kicks in, starts calculating the load in the background, and sends the information to the buck/boost converter to adjust its value. This results in the buck/boost converter clamping the supply to the value needed to support the maximum current for the given load. The settling time of the system then becomes dominated by the DAC which is typically more than an order of magnitude faster than the buck/boost settling. </p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5808.DAC8775_5F00_3.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5808.DAC8775_5F00_3.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: The DAC8775’s integrated ADC senses the load</b></p>
<p>Back to our example 1kΩ load: once enabled, auto learn will calculate the load as 1kΩ and set the buck/boost converter to 27V. This achieves the maximum power saving without sacrificing settling time. As you can see in Figure 4, after the buck/boost output settles, all of the consecutive DAC code updates settle within 10µs. This is a significant improvement in settling time.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_4_5F00_small.jpg"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_4_5F00_small.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 4: Setting time with auto-learn mode</b></p>
<p><b> </b></p>
<p>Another advantage of such an implementation is that it makes modules more flexible. System designers don’t need to spend engineering time to optimize modules for different loads. This implementation enables the system to “learn” the load automatically and adjust the configuration as necessary. In addition, because auto-learn mode runs in the background, it saves costs during installation.</p>
<p>We will continue to cover topics around trends in factory automation in subsequent blogs. Until then, learn more by checking out TI’s broad <a href="http://www.ti.com/lsds/ti/data-converters/dacs/precision-dacs-overview.page" target="_blank"><span>precision DAC</span></a> portfolio or the DAC8775.To get posts like this delivered to your inbox, sign in and subscribe to Precision Hub.</p>
<p></p>
<p><span style="text-decoration:underline;"></span><b>Additional resources</b></p>
<ul>
<li>Download the <a href="http://www.ti.com/product/DAC8775/datasheet" target="_blank"><span style="text-decoration:underline;">DAC8775 data sheet</span></a>.</li>
<li>Evaluate the DAC8775 with the <a href="http://www.ti.com/tool/dac8775evm" target="_blank"><span style="text-decoration:underline;">DAC8775 evaluation module</span></a>.</li>
<li>Download these reference designs:
<ul>
<li>“<a href="http://www.ti.com/tool/tipd216" target="_blank">Quad Channel Industrial Voltage and Current Output Driver Reference Design (EMC/EMI Tested)</a>.”</li>
<li>“<a href="http://www.ti.com/tool/tipd215" target="_blank"><span style="text-decoration:underline;">Less than 1-W, Quad-Channel, Analog Output Module with Adaptive Power Management Reference Design</span></a>.”</li>
</ul>
</li>
<li>Watch this <a href="https://training.ti.com/node/1128104" target="_blank"><span style="text-decoration:underline;">video about DAC8775 features and uses</span></a>.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669568&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Tsedeniya Abrahamhttps://e2e.ti.com:443/members/346497Zero out your system error with zero drift, zero crossover and zero hasslehttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/zero-out-your-system-error-with-zero-drift-zero-crossover-and-zero-hassle2017-04-21T16:30:00Z2017-04-21T16:30:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/DAC8830" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC8830</a>, <a href="https://www.ti.com/product/OPA340" class="internal-link folder product" title="Link to Product Folder" target="_blank">OPA340</a>, <a href="https://www.ti.com/product/OPA388" class="internal-link folder product" title="Link to Product Folder" target="_blank">OPA388</a></div><p align="center" style="text-align:left;"><i>This post is co-authored by </i><a href="/members/1828216" target="_blank"><i>Richard Barthel</i></a><i> and </i><a href="/members/4054656" target="_blank"><i>Errol Leon</i></a><i>.</i></p>
<p>In applications such as position sensors, data-acquisition systems and resistance temperature detectors (RTDs), it is important to design with high precision in mind. In many cases, designing with precision integrated circuits (ICs) reduces signal-chain complexity, lowers the external component count, and minimizes board space and bill of materials (BOM) costs. The inaccuracies of one device may propagate through with the inaccuracies of another device, resulting in undesirable and unpredictable errors. In the case of a buffer-configured <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">operational amplifier</a> (op amp) at the output of a <a href="http://www.ti.com/lsds/ti/data-converters/dacs/dacs-overview.page" target="_blank">digital-to-analog converter</a> (DAC), it’s crucial that your DAC and your op amp are precision devices for an accurate output.</p>
<p>A traditional rail-to-rail complementary metal-oxide semiconductor<b> </b>(CMOS) amplifier architecture includes two differential pairs, PMOS (blue) and NMOS (red), shown in Figure 1. Together, these two transistor pairs span the entire input common-mode voltage range. When one transistor pair takes over from the other, however, a unique and nonlinear phenomenon known as “input crossover distortion” occurs due to the intrinsic input offset voltage of each of the two input differential pairs, shown in Figure 2.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/7725.Zero-drift_5F00_Figure-1.png" target="_blank"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/7725.Zero-drift_5F00_Figure-1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: Traditional rail-to-rail CMOS amplifier architecture</b></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/1440.Zero-drift_5F00_Figure-2.png" target="_blank"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/1440.Zero-drift_5F00_Figure-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Input offset voltage vs. common-mode voltage</b></p>
<p>When you connect a traditional rail-to-rail CMOS op amp at the output of a high-precision DAC, the crossover distortion will introduce an error and result in a drastic increase in integral nonlinearity (INL). This may cause the signal to deviate several least significant bits (LSBs) from its ideal value.</p>
<p>Now, what does 1LSB mean? Equation 1 is a simple equation to calculate LSB:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4263.Zero-drift_5F00_Equation-1.PNG" target="_blank"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4263.Zero-drift_5F00_Equation-1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>where N is the DAC’s number of bits.</p>
<p>The DAC8830 is a 16-bit DAC. If the voltage reference is V<sub>ref</sub> = 5V, then:</p>
<p style="text-align:center;"><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3515.Zero-drift_5F00_Equation-2.PNG" target="_blank"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3515.Zero-drift_5F00_Equation-2.PNG" alt=" " /></a></p>
<p>So to deviate more than 1LSB means that you can have more than 76.3µV of error at your output. This can be detrimental to many precision applications, like critical systems where failure has the potential to negatively impact customers’ end products.</p>
<p>So how do you fix this? Enter zero crossover!</p>
<p>You can span the entire input common-mode voltage range by using a zero-crossover op amp such as the <a href="http://www.ti.com/product/OPA388" target="_blank">OPA388</a>. The zero-crossover topology uses an internal regulated voltage charge pump to increase the positive supply voltage and thus achieve linear operation with input common-mode voltages all the way to its rails with a single input transistor pair, shown in Figure 3. This results in true rail-to-rail input operation without a crossover region, and thus no crossover distortion. If you were to connect this kind of op amp at the output of a DAC, the op amp does not introduce an error within the common-mode region (1V to 2V below the positive rail) like a traditional rail-to-rail CMOS device.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/6330.Zero-drift_5F00_Figure-3.png" target="_blank"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/6330.Zero-drift_5F00_Figure-3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: Zero-crossover amplifier architecture</b></p>
<p>In Figure 4, the black curve describes the output of a traditional rail-to-rail CMOS op amp (OPA340) at the output of a DAC (<a href="http://www.ti.com/product/DAC8830" target="_blank">DAC8830</a>), while the red curve describes the output of a <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-products.page#~p1342=Zero%20Crossover" target="_blank">zero-crossover op amp</a> (OPA388) with the same DAC8830. As you can see, the output of the DAC8830 + OPA388 does not suffer from the distortion that is easily visible in the DAC8830 + OPA340 output curve. The <a href="http://www.ti.com.cn/tool/TIDA-01402" target="_blank">High-Precision Reference Design for Buffering a DAC Signal</a> describes this output in greater detail.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5672.Zero-drift_5F00_Figure-4.png" target="_blank"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5672.Zero-drift_5F00_Figure-4.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 4: INL comparison of rail-to-rail CMOS</b> <b>OPA340 and zero-crossover OPA388</b></p>
<p>Let’s put this reference design into perspective and use it in an application such as an MRI machine. An MRI uses a powerful magnetic field to produce detailed 2-D and 3-D pictures of the human body to diagnose and/or monitor several health conditions. Unacceptably distorted signals that exceed the error budget in any way can potentially impair the quality of the images.</p>
<p>The OPA388 is the industry’s first op amp to employ zero-crossover and zero-drift technology. Zero-drift op amps have an internal self-correcting circuit that produces ultra-low input offset voltage (V<sub>OS</sub>) and near-zero input offset voltage drift over time and temperature (dV<sub>OS</sub>/dT). The technology also delivers other advantages, including no 1/f noise (flicker noise), low broadband noise (white noise) and low output distortion, which can help increase system reliability in harsh environments. Take a swimming pool for instance – pH pool testers and monitoring systems must withstand changes in the environment’s temperature to correctly determine the deficit or excess of chlorine. Since most pools are placed outside, the environment’s temperature can vary many degrees between a cold winter’s day and a hot summer’s day. Offset voltage will change with temperature deviations, introducing error, so it is crucial to select an op amp with low offset voltage drift to support system reliability through these changes.</p>
<p>To assure high performance, high precision and high accuracy, carefully select parts for your design. Make sure that you understand your system and what you can afford in terms of error, and only then sift through <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">TI’s diverse portfolio</a> for your ultimate solution.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Download the <a href="http://www.ti.com/lit/an/sboa182a/sboa182a.pdf" target="_blank">zero-drift</a> and <a href="http://www.ti.com/lit/an/sboa181a/sboa181a.pdf" target="_blank">zero-crossover</a> Tech Notes.</li>
<li>Visit the <a href="http://www.ti.com/lit/ds/symlink/opa388.pdf" target="_blank">OPA388</a> and <a href="http://www.ti.com/lit/ds/symlink/dac8830.pdf" target="_blank">DAC8830</a> datasheets to learn more.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669541&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Tamara M Alanihttps://e2e.ti.com:443/members/4381529How to enable high-accuracy CW Doppler through discrete, precision data convertershttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-enable-high-accuracy-cw-doppler-through-discrete-precision-data-converters2017-03-24T16:00:00Z2017-03-24T16:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/ADS8900B" class="internal-link folder product" title="Link to Product Folder" target="_blank">ADS8900B</a></div><p>Medical ultrasound is a noninvasive method of imaging the body’s internal structures (like organs) by transmitting high-frequency waves and measuring the reflections that occur at various boundaries within, such as between bone and muscle. There are different types of ultrasound, such as B-mode, F-mode (also known as pulsed-waveform) and continuous waveform Doppler (CW Doppler). Each has benefits and drawbacks, including what can be imaged and penetration depth.</p>
<p>In this post, I’ll be taking a closer look at CW Doppler and how a high-accuracy signal chain enables accurate measurement of blood flow deep within the body.</p>
<p>Figure 1 shows the concept of using CW Doppler to measure the rate of blood flow in a vein.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasoud_5F00_1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasoud_5F00_1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: CW Doppler measurement of blood flow</b></p>
<p>In CW Doppler, half of the transducer array (transmitter/receiver array) continuously transmits a sine wave (denoted by Tx), which the blood cells flowing within a vein then reflect, or receive (denoted by Rx). The reflected signal has a different frequency than the transmitted effect, due to the velocity of the blood (this is known as the Doppler effect). The received signal is fed through a mixer and low-noise summer to demodulate the I and Q signals (the real and imaginary portions of the signal), which when simultaneously measured determine the phase change between the Tx and Rx signals. This phase change is used to calculate the velocity of the blood within the target vein.</p>
<p>Figure 2 shows a block diagram of the Rx portion of the CW Doppler signal chain.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_2.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_2.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Block diagram of a CW Doppler signal-conditioning circuit (replicated for both I and Q measurement)</b></p>
<p>Figure 2 highlights the Rx signal chain because of the difficulty in measuring the received signal. While the Tx waveform can be anywhere from 1 to 15MHz with an amplitude of ±2.5V to ±100V, the received signal can be as low as ±10µV and as large as ±500mV. The range of Rx depends on the speed of the blood flow, the angle of the transducer relative to the direction of blood flow and the depth of the vein.</p>
<p>A high-precision analog-to-digital converter (ADC) accurately measures the reflected signal. For CW Doppler, a successive approximation register (SAR) ADC is preferable because it provides the raw conversion (no digital filtering) and has low latency and excellent AC performance. Depending on the desired accuracy and how deep the target veins are expected to be, a high resolution ADC is necessary. The ADS8900B family is an excellent choice because it meets the required performance and throughput, and offers the wide input range necessary for measuring the dynamic signals created by CW Doppler. You can easily design a simultaneous sampling system using two ADS8900B devices to measure the I and Q channels. Table 1 shows several of the key specifications for the ADS8900B family.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_table1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_table1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Table 1: ADS8900B family key specifications</b></p>
<p>While the high performance of the ADS8900B family alone enables it to accurately measure blood flow, another advantage that it offers is an internal reference buffer. The voltage reference circuit is a crucial aspect of the signal chain, as it provides a point of reference for the ADC. Any deviation in the voltage reference can result in an inaccurate conversion. Figure 3 shows an external vs. internal reference buffer configuration.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_3.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ultrasound_5F00_3.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: External vs. internal voltage reference buffer</b></p>
<p>During a signal conversion cycle, the ADC will draw current from the reference to charge a switched capacitor, and compare this voltage to the voltage of the input signal. While drawing current, this reference voltage is prone to voltage droop on the output if not supplied with sufficient current (again, possibly resulting in inaccurate conversion). The voltage reference buffer supplies adequate current to the ADC so as to avoid a voltage droop. Integrating the voltage reference buffer into the ADC not only reduces the overall system size – which is critical for portable or high-channel-count CW Doppler systems – but the buffer is designed specifically for the ADC, further improving overall system performance. The integrated buffer enables the use of a single voltage reference with multiple ADCs, further reducing board space and cost for multichannel systems.</p>
<p>I hope that I have explained how the high precision and integration of the ADS8900B enables CW Doppler ultrasound systems. If you enjoyed this post, be sure to sign in and subscribe to Precision Hub to get similar posts delivered right to your inbox.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Explore TI’s selection of <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page" target="_blank">precision ADCs</a>.</li>
<li>Check out the CW Doppler reference design, “<a href="http://www.ti.com/tool/tida-01351" target="_blank">High-Resolution, High-SNR True Raw Data Conversion Reference Design for Ultrasound CW Doppler</a>.”</li>
<li>Learn more about designing with the ADC8900B in these blog posts:
<ul>
<li>“<a href="/blogs_/b/precisionhub/archive/2016/12/16/enabling-higher-performance-benchtop-test-equipment" target="_blank">How to achieve higher-precision data acquisition in benchtop test equipment</a>.”</li>
<li>“<a href="/blogs_/b/precisionhub/archive/2017/02/17/detecting-pesky-failing-batteries-before-they-cause-a-problem" target="_blank">Detecting pesky failing batteries before they cause a problem</a>.”</li>
</ul>
<p style="padding:0;margin:0;"></p>
</li>
</ul><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669495&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Evan Sawyerhttps://e2e.ti.com:443/members/3474348How new integrated DACs increase efficiency and reduce board space in analog output moduleshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/dacs-enable-performance-in-analog-output-modules2017-03-08T03:24:00Z2017-03-08T03:24:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/DAC8775" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC8775</a></div><p>Industry 4.0 has revolutionized the manufacturing industry, changing how factories are designed and implemented. In factory automation and process-control applications, Industry 4.0’s impact comes down to two fundamental concepts: the proliferation of decentralized systems and smart deterministic systems. Decentralized systems inherently need to be modular and flexible. Efficient, low-power and thermally optimized designs are the key enablers for such systems. Smart deterministic systems are modules that can detect faults early and increase reliability.</p>
<p>In <a href="http://www.ti.com/lsds/ti/applications/industrial/factory-automation/overview.page" target="_blank"><span>factory automation and process-control applications</span></a>, Digital-to-Analog Converters (DACs) are most often found in analog outputs used for both programmable logic controllers (PLCs) and sensor transmitters. In both cases, the <a href="http://www.ti.com/lsds/ti/data-converters/digital-to-analog-converter-overview.page" target="_blank"><span>DAC </span></a>can be used to deliver either a voltage output or current output.</p>
<p>The <a href="http://www.ti.com/product/DAC8775" target="_blank"><span>DAC8775</span></a> is TI’s newest <a href="http://www.ti.com/lsds/ti/data-converters/dacs/precision-dacs-overview.page" target="_blank"><span>high-precision DAC</span></a>, that is the most integrated in the industry by including 4-20mA driver, voltage output and on chip adaptive power management. In this post, I’ll provide examples of design techniques as they relate to the DAC8775 and explore how you can design for the current trends in this industry.</p>
<p>Many system controllers handle hundreds of Input/Output (I/O) points due to the increased number of sensors. This poses a challenge for designers to fit more I/O channels into a small form factor, increasing the need for thermally optimized and highly efficient systems. Most <a href="http://www.ti.com/solution/plcdcs_io_module_analog_output" target="_blank"><span>analog output module</span></a> 4-20mA driver circuits employ a high-side voltage-to-current conversion circuitry with a gain stage. Figure 1 shows a typical architecture.</p>
<p>The loop established by Amplifier A1 converts the DAC output voltage into a current. Through negative feedback, Amplifier A1 will set the voltage across R<sub>SET</sub> to be equal to the DAC output. This voltage drop across R<sub>SET</sub> will set the current flowing through the first stage, I<sub>M</sub>. (I am assuming an ideal case where I<sub>RSET</sub> is equal to I<sub>M</sub>.) This generated current, I<sub>M</sub>, is further gained up by the use of a loop established by the combination of Amplifier A2 and the R<sub>MIRROR</sub> and R<sub>SENSE</sub> resistor pairs. The amplifier A2 will force the voltage across R<sub>SENSE</sub> to be equal to V<sub>MIRROR</sub> .This generates a load current that is gained up from I<sub>M</sub> by a factor proportional to the ratio of R<sub>MIRROR</sub> and R<sub>SENSE</sub>. R<sub>LOAD</sub>, shown in Figure 1, usually represents a linear actuator load, as is the case with PLC systems. Since the current passing through R<sub>MIRROR</sub> is not supplying the load, it will directly reduce the system’s efficiency. A good design practice is to minimize this current – setting it to less than 1% of the output current. For calculation purposes, let’s ignore I<sub>M</sub>, assuming a high ratio (>1-to-100) between R<sub>MIRROR</sub> and R<sub>SENSE</sub>.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: High-side voltage-to-current converter</b></p>
<p>In a typical case, the V<sub>POS</sub> voltage can vary anywhere between 12-36V. R<sub>LOAD </sub>can also vary from a short to 1kΩ. To illustrate the point, consider as our first example the case of V<sub>POS</sub> equal to 36V and R<sub>LOAD</sub> equal to 1Ω. When a valve is set to full scale, the controller will drive 20mA through the load. This means that the power consumed by the load is P<sub>LOAD </sub>= I<sup>2</sup>R = 0.4mW.</p>
<p>The total power generated is P<sub>generated</sub> = VI = 0.72W. From this example, it’s evident that the voltage-to-current conversion circuitry dissipates the rest of the power: 0.72W-0.4mW = 0.7196W. This is a highly inefficient system and will cause an unnecessary increase in system temperature.</p>
<p>Consider a second example where the load impedance is higher, at 1kΩ. In that case, P<sub>LOAD</sub> = I<sup>2</sup>R = 0.4W. The total power generated is P<sub>generated</sub> = VI = 0.72W. The voltage-to-current conversion circuitry dissipates the rest of the power: 0.72W-0.4W = 0.32W.</p>
<p>You can imagine that adding more channels in such a small space becomes unsustainable if there is a vast amount of power loss, which directly increases system temperature, reduces reliability and increases failures. The examples I’ve given show the power loss just for a single-channel design. In the case of four channels, the power losses in the first and second examples would be close to 2.8W and 1.2W, respectively.</p>
<p>Because power loss increases dramatically with the use of even higher channel count modules, one possible solution is to adaptively change the V<sub>POS</sub> supply depending on the load. You can do this by adding a simple feedback network and employing a buck/boost converter to supply just the necessary power to the load. Such a system would look like the block diagram shown in Figure 2.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_2.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_2.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: High-side voltage-to-current converter with a buck/boost converter</b></p>
<p>In this design technique a buck/boost converter will sense the drain-to-source voltage of the output FET driving the load and generate an internal proportional error current. Through a complex state machine algorithm the device will make a decision to buck or boost the supply. This technique is utilized in the quad Channel <a href="http://ti.com/product/dac8775" target="_blank"><span>DAC8775</span></a> to achieve higher efficiency.</p>
<p>If you use the same values as the first example, when the load is 1Ω, the buck/boost converter would buck the supply to the DAC down so that it gets the minimum supply needed. In the case of the DAC8775, that would as low as 4.5V.</p>
<p>As in the first example, P<sub>LOAD </sub>= I<sup>2</sup>R = 0.4mW. The total power generated is P<sub>generated</sub> = VI = 0.09W. The voltage-to-current conversion circuitry dissipates the rest of the power: 0.09W-0.4mW = 89.6mW. Therefore, the power consumption is improved by a factor of eight when compared with example 1.</p>
<p>For the 1kΩ load case, P<sub>LOAD</sub> = I<sup>2</sup>R = 0.4W. The total power generated is P<sub>generated</sub> = VI = 0.46W, since the buck/boost converter will set V<sub>POS</sub> to 23V. The voltage-to-current conversion circuitry dissipates the rest of the power: 0.46W-0.4W = 0.06W. Therefore, the power consumption is improved by a factor of five when compared to the design with no buck/boost converter feedback.</p>
<p>The DAC8775’s efficiency also results in a much more thermally optimized system. Comparing the junction temperature of the die in a four-channel design with and without adaptive power-feedback circuitry shows a significant improvement in die temperature. Figures 3 and 4 show measurement results for the DAC8775, comparing the die temperature with and without the use of a buck/boost converter for both 1Ω and 1kΩ R<sub>LOAD </sub>cases. As you can see from Figure 3, this technique can improve the junction temperature by up to 36°C.</p>
<p>When squeezing more and more channels into a smaller space, thermal optimization becomes a critical performance parameter that differentiates module capability. In thermally unoptimized modules, system failures are common and performance degradation occurs due to large temperature drift. The DAC8775 addresses both of these challenges due to its high integration and high efficiency and achieves excellent DC and drift performance.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_3.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p style="text-align:center;"><b>Figure 3: Die temperature for R<sub>LOAD </sub>of 1Ω</b></p>
<p style="text-align:center;"><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_4.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/DAC8775_5F00_4.png" alt=" " /></a></p>
<p align="center"></p>
<p align="center"><b>Figure 4: Die temperature for R<sub>LOAD </sub>of 1kΩ</b></p>
<p>In case the die temperature goes above 150°C, the DAC8775 offers an over-temperature alarm, part of an extensive set of smart diagnostic features that help detect faults early. These include open load, short circuit, cyclic redundancy check (CRC), watchdog timer and compliance voltage. In addition to fault alerts, the device allows you to choose pre-set actions that facilitate reliable system operation. You can tell the device to do nothing, shut down or go to a pre-programmed safe code.</p>
<p>TI’s wide portfolio of signal-chain offerings enables you to design modules that are efficient, thermally optimized and smarter. Learn more by checking out TI’s broad <a href="http://www.ti.com/lsds/ti/data-converters/dacs/precision-dacs-overview.page" target="_blank"><span>precision DAC</span></a> portfolio or the <a href="http://www.ti.com/product/dac8775?keyMatch=dac8775&tisearch=Search-EN-Everything" target="_blank"><span>DAC8775</span></a>. </p>
<p><b>Additional resources</b></p>
<ul>
<li>Download the <a href="http://www.ti.com/product/DAC8775/datasheet" target="_blank"><span style="text-decoration:underline;">DAC8775 datasheet</span></a>.</li>
<li>Evaluate the DAC8775 with the <a href="http://www.ti.com/tool/dac8775evm" target="_blank"><span style="text-decoration:underline;">DAC8775 evaluation module</span></a>.</li>
<li>Download the reference design, “<a href="http://www.ti.com/tool/tipd216" target="_blank"><span style="text-decoration:underline;">EMC/EMI tested Quad Channel Industrial Voltage and Current output driver reference design</span></a>.”</li>
<li>Download the reference design, “<a href="http://www.ti.com/tool/tipd215" target="_blank"><span style="text-decoration:underline;">Less than 1W, Quad Channel Analog Output Module with adaptive power management reference design</span></a>.”</li>
<li>Learn more about TI’s <a href="http://www.ti.com/lsds/ti/data-converters/dacs/precision-dacs-overview.page" target="_blank"><span style="text-decoration:underline;">precision data converter portfolio</span></a>.</li>
<li style="text-align:left;">Watch this <a href="https://training.ti.com/node/1128104" target="_blank"><span style="text-decoration:underline;">video about the DAC8775 features and uses</span></a>.</li>
</ul><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669429&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Tsedeniya Abrahamhttps://e2e.ti.com:443/members/346497Detecting pesky failing batteries before they cause a problemhttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/detecting-pesky-failing-batteries-before-they-cause-a-problem2017-02-17T16:30:00Z2017-02-17T16:30:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/ADS8900B" class="internal-link folder product" title="Link to Product Folder" target="_blank">ADS8900B</a></div><p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_small2.jpeg"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_small2.jpeg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>As battery-powered systems become more common, quickly identifying a failing battery so that it can be replaced is becoming increasingly important. From an individual battery powering a mobile phone to a bank of batteries used to store renewable energy, a faulty battery can lead to system downtime. At the heart of battery analyzers, which determine the health of a battery, is a <a href="http://www.ti.com/lsds/ti/data-converters/adcs/precision-adcs-overview.page" target="_blank">precision analog-to-digital converter (ADC).</a></p>
<p>In this post, I will explore how key specifications of these ADCs, including speed, resolution and latency, enable a more inclusive analysis of a battery’s health. To better understand the importance of the ADC’s performance in a battery analyzer, let’s look at Randles’ model of a lead-acid battery, shown in Figure 1.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: Randles’ model of a lead-acid battery</b></p>
<p>In Figure 1, R1 is the active electrolyte resistance, R2 is the charge transfer resistance and C is the double layer capacity. Together, they create a simplified equivalent circuit of a lead-acid battery. By measuring all three components and comparing them to the expected/known values, it is possible to generate an approximation of the battery’s “health,” which includes its cold cranking amps (CCA), state of charge and capacity.</p>
<p>While there are a range of battery test methods, such as a discharge/charge cycle, DC load and AC testing, electrochemical impedance spectroscopy (EIS) is considered to be the most accurate by leading battery health researchers. EIS is preferred over other methods because of its capability to quickly measure CCA, SOC and battery capacity. The process involves drawing a range of small, low-frequency signals from the battery and measuring the corresponding current across a shunt resistor as well as the DC voltage of the battery. These measurements can determine R1, R2 and C, which in turn are compared with expected values to determine a battery’s health.</p>
<p>Depending on the health of the battery as well as the type of battery being tested, the measured current and voltage can range from very small to quite large. As such, the ADC chosen to convert the measurements must be capable of accurately measuring small changes to the input signal, across a wide range of inputs.</p>
<p>In many cases, a successive approximation register (SAR) ADC is the preferred converter due to its dynamic range, speed, resolution and low latency. A high-resolution SAR ADC can precisely measure low-speed signals (DC to several megahertz), which can then be oversampled and digitally filtered by a host processor (e.g. FPGA) to increase system accuracy. Alternatives include delta-sigma ADCs (which are not as well-suited for measuring a range of input frequencies) and pipeline ADCs (which offer higher speed at the cost of resolution). Additionally, the low latency of a SAR ADC shortens the time required to take a measurement without sacrificing measurement accuracy.</p>
<p>In the case of a battery analyzer, it can be difficult to measure current (ranging from low milliamps to high amps) or voltage (ranging from several volts to tens of volts) with high accuracy across the entire range. To do so, a high-resolution SAR ADC with a wide dynamic range (input range) and at least several hundred kilo samples per second (kSPS) takes multiple measurements of each input signal, which the host processor then digitally filters to improve measurement accuracy. Figure 2 shows a simplified diagram of a battery tester system.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_2.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_2.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Diagram of a battery analyzer measuring current and voltage</b></p>
<p>In Figure 2, the load is varied to draw a range of AC currents from the battery, resulting in an AC voltage across a small, high-accuracy sense resistor. A high-precision data-acquisition system designed for minimal signal distortion typically amplifies and then measures the voltage created across the resistor. In the case of measuring the DC voltage of the battery, this input is often scaled down by an amplifier to enable an ADC to measure a wide range of voltages. In both cases, the ADC chosen to digitize the signal must have high-enough resolution to enable it to detect small changes to the input signal.</p>
<p>While there are many SAR ADCs that you can select to measure this voltage, the <a href="http://www.ti.com/product/ads8900B" target="_blank">ADS8900B</a> family shown in Table 1 offers several unique advantages, including high resolution, a fast sampling rate, and excellent AC and DC performance. These features are critical for measuring the wide dynamic-range signals encountered in battery health analysis while maintaining accuracy across the input range. </p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_table-1.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_table-1.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Table 1: ADS8900B family key specifications</b></p>
<p>These devices also feature an internal reference buffer that further increases system accuracy and reduces its size, which is especially important for portable battery analyzers. Figure 3 shows an external vs. internal reference buffer in a data-acquisition system.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_3.PNG"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ADS8900B-battery_5F00_3.PNG" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: External vs. internal voltage reference buffer</b></p>
<p>The reference voltage circuit is critical in precision data-acquisition systems, as it provides a point of reference for the data converter to compare against an input signal. Any error in the reference voltage will result in inaccurate measurements of the input signal. During each conversion cycle, the ADC will draw considerable current from the reference due to the internal switched-capacitor architecture of the converter. A reference buffer minimizes the voltage droop created during conversion. In the case of the ADS8900B family, the internal reference buffer is optimized to drive the ADC’s reference pin, maximizing AC and DC performance and resulting in a higher precision system than one using an external reference buffer.</p>
<p>I hope I’ve explained how the ADS8900B is enabling battery analyzers to more accurately measure battery health, although any system requiring precise measurement of a small and/or dynamic signal can realize the benefits that this device has to offer. Stay tuned for a future post, where I’ll show how you can use a pair of discrete ADCs to simultaneously sample inputs and how new ADCs are reducing the headaches of digital design for high-speed, high-resolution data-acquisition systems. Be sure to sign in and subscribe to Precision Hub to get these posts delivered right to your inbox.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Explore TI’s selection of <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page" target="_blank">precision ADCs</a>.</li>
<li>Read more about designing with the<a href="http://e2e.ti.com/blogs_/b/precisionhub/archive/2016/12/16/enabling-higher-performance-benchtop-test-equipment" target="_blank"> ADC8900B</a>. </li>
<li>Learn more about the TI Design, “<a href="http://www.ti.com/tool/TIPD211" target="_blank">20-bit, 1-MSPS, 4-Ch Small Form Factor Design for Test and Measurement Applications Reference Design</a>.” </li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669421&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Evan Sawyerhttps://e2e.ti.com:443/members/3474348Designing a discrete wide-bandwidth, cost-sensitive instrumentation amplifierhttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/designing-a-discrete-wide-bandwidth-cost-sensitive-instrumentation-amplifier2017-02-03T16:00:00Z2017-02-03T16:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/tool/TINA-TI" class="internal-link folder tool" title="Link to Tool Folder" target="_blank">TINA-TI</a></div><p>In this post, I’ll show how to design a cost-optimized, discrete, wide-bandwidth instrumentation amplifier using the <a href="http://www.ti.com/product/TLV3544">TLV3544</a>. <a href="http://www.ti.com/lsds/ti/amplifiers/instrumentation-amplifiers/instrumentation-amplifiers-overview.page" target="_blank">Instrumentation amplifiers</a> are used for their high input impedance and ability to convert differential voltages to single-ended voltages. Fast current sensing, precision data acquisition, vibration analysis, microphone pre-amplification, ADC drivers and medical instrumentation are all applications that need instrumentation amplifiers with wide bandwidth.</p>
<p><a href="http://www.ti.com/lit/ds/symlink/tlv3544.pdf">TLV354x</a> devices provide a rail-to-rail input/output, 200MHz unity gain bandwidth (GBW) and 150V/µs slew rate, which is designed for the applications I just mentioned. Figure 1 shows the standard three-<a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">operational amplifier</a> (op amp) topology.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/three-op-amp-topology-1.png"><img src="/resized-image/__size/700x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/three-op-amp-topology-1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 1: Discrete three-op-amp topology using the <a href="http://www.ti.com/product/TLV3544">TLV3544</a></strong></p>
<p>The input stage uses dual noninverting amplifiers that enable high impedance at both inputs, whose gain is defined by RF1 = RF2 and RG1. The output stage consists of a difference amplifier with a low impedance output, whose gain is set by R2 = R4 and R1 = R3. The reference voltage, input stage gain and output stage gain define the output voltage, shown in Equation 1:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2248.eq1.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2248.eq1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Note that the tolerance of the resistors in the instrumentation amplifier will negatively affect the CMRR and gain error of the circuit. That is why there is a cost and performance trade-off between discrete and integrated instrumentation amplifiers.</p>
<p>The bandwidth of instrumentation amplifiers is bounded by three characteristics: the open-loop gain (Aol) of the op amp, the noise gain (Gn) and filtering. Both Aol and Gn are covered in the <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/precision-op-amps-precision-labs.page" target="_blank">TI Precision Labs training series</a> on bandwidth (see part 3, “<a href="https://training.ti.com/ti-precision-labs-op-amps-bandwidth-3?cu=14685" target="_blank">TI Precision Labs – Op Amps: Bandwidth 3</a>” – viewing requires a myTI login). The output-stage difference amplifier’s noise gain determines the circuit’s bandwidth.</p>
<p>The <a href="http://www.ti.com/product/TLV3544">TLV3544</a>, like many high-speed amplifiers, will have stability issues if the feedback resistors are too large. Figure 2 simulates the effects of 200Ω feedback resistors versus 500Ω feedback resistors. Note that 45 degrees of phase margin (PM) is necessary for stable operation. Increasing the resistor values allows for lower power consumption and larger RG1 values to set the gain.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/tina-ti-2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/tina-ti-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p></p>
<p align="center"><strong>Figure 2: TINA-TI™ software frequency response showing AC peaking, PM and bandwidth with 200Ω (54 degrees PM) versus 500Ω (26 degrees PM) feedback resistors</strong></p>
<p>Since the instrumentation amplifier is discrete, you have access to the feedback paths, which allows you to compensate for AC peaking. By placing capacitors in the feedback path, you introduce multiple poles that create a low-pass filter and attenuate the peaking. Equation 2 calculates the -3dB frequency (f<sub>p</sub>) of this low-pass filter:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2656.eq2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2656.eq2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Figure 3 shows the schematic and frequency response. Remember that adding a filter will affect the overall bandwidth of the circuit.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3-tina-ti.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3-tina-ti.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 3: TINA-TI software schematic and frequency response of compensated feedback paths</strong></p>
<p>Since this instrumentation amplifier topology only requires three op amps, the fourth op amp provided by the <a href="http://www.ti.com/product/TLV3544">TLV3544</a> can serve as a reference buffer for single-supply systems or as an integrator for high-pass filtering of the input.</p>
<p>Thus, you can use the <a href="http://www.ti.com/product/TLV3544">TLV3544</a> to make a discrete wide-bandwidth instrumentation amplifier that is ideal for cost-optimized, high-precision, wide-bandwidth applications.</p>
<p>Have questions or comments about other design considerations for discrete instrumentation amplifiers? Log in and leave a comment.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Watch more than 40 on-demand videos about topics such as <a href="https://training.ti.com/ti-precision-labs-op-amps-bandwidth-1?cu=14685" target="_blank">bandwidth</a> and <a href="https://training.ti.com/ti-precision-labs-op-amps-stability-1?cu=14685" target="_blank">stability</a> on <a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page" target="_blank">TI Precision Labs</a>.</li>
<li>Read Pete Semig’s blog series on V<sub>OUT</sub> vs. V<sub>CM</sub> limitations, <a href="http://www.edn.com/design/analog/4437848/Instrumentation-amplifier-VCM-vs-VOUT-plots--Part-1" target="_blank">“Instrumentation amplifier V<sub>CM</sub> vs. V<sub>OUT</sub> plots: part 1</a>, <a href="http://www.edn.com/design/analog/4437922/Instrumentation-amplifier-VCM-vs--VOUT-plots--part-2">part 2</a>, <a href="http://www.edn.com/design/analog/4438001/Instrumentation-amplifier-VCM-vs--VOUT-plots--Part-3">part 3</a>,” to avoid common pitfalls when using instrumentation amplifiers.</li>
<li>See TI’s <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-products.page" target="_blank">portfolio of performance op amps</a> for cost-conscious applications.</li>
<li>Find commonly used analog design formulas in our wildly popular and free <a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-support-community.page?keyMatch=pocket%20reference&tisearch=Search-EN-Everything#pocketref" target="_blank"><i>Analog Engineer’s Pocket Reference</i> e-book</a>.</li>
<li>Read more <a href="/search?q=Precision%20amplifier%20blog" target="_blank">blogs about precision amplifiers</a>.</li>
<li>Learn about TI’s entire portfolio of <a href="http://www.ti.com/lsds/ti/amplifiers/amplifiers-overview.page" target="_blank">amplifier ICs</a><span style="font-size:12px;"> and explore technical resources.</span></li>
</ul><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669383&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Cole Maciashttps://e2e.ti.com:443/members/4719008Achieve big board-size reductions with tiny, precision op ampshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/achieve-big-board-size-reductions-with-tiny-precision-op-amps2017-01-20T16:27:00Z2017-01-20T16:27:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/OPA2134" class="internal-link folder product" title="Link to Product Folder" target="_blank">OPA2134</a>, <a href="https://www.ti.com/product/OPA1652" class="internal-link folder product" title="Link to Product Folder" target="_blank">OPA1652</a></div><p>Electronics such as smartphones, tablets, notebooks and wearable products are becoming more multifunctional, smaller and slimmer. Achieving higher functionality in smaller form factors requires extremely tiny ICs. Many times, different package types help reduce size and solve various design challenges. Take <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">operational amplifiers</a> (op amps), for example; wafer chip-scale packages (WCSP) typically enable the smallest possible footprint for optical modules and wearables, while flat no-lead packages such as quad flat no-lead (QFN) or dual flat no-lead (DFN) inspire differentiated audio functionality in personal electronics where small size, high performance, and easy testing and tuning are essential. Small-outline transistor (SOT) packages like SOT553 are suitable for emerging industrial applications such as field transmitters, which require a wide supply but a small footprint in a user-friendly leaded package.</p>
<p>The rapid evolution of mobile devices is driving package technology innovations, but the smallest possible package will always be the size of the die itself. The WCSP is a type of package that can be almost as small as the die, so it’s desirable in applications like medical diagnostics, fitness monitoring and handheld electronic devices. The <a href="http://www.ti.com/product/opa2376">OPA2376</a> in WCSP (1.11mm by 2.15mm by 0.625mm) is a device commonly used for signal conditioning purpose in the areas mentioned above.</p>
<p>Audio in portable devices is another example where tiny packages are inspired by the consumer needs. Traditionally in the professional audio world, audiophiles like to use op amps in the dual inline package (DIP), which is more DIY-friendly for audio designers. The OPA2134 from the legacy Burr-Brown<sup>TM </sup>audio portfolio, is such an example with great audio reputation. However, with the increased demand of high-quality audio in portable devices, tiny-package high-performance audio ICs are required in order to enable the high-fidelity sound in a space-constraint design. With the implementation of the DFN package on the <a href="http://www.ti.com/product/OPA1652" target="_blank">OPA1652</a> (Figure 1), a performance upgrade to the OPA2134, you will find a high-performance current-to-voltage converter with great sound quality for cost-optimized portable audio equipment, smartphones and gaming motherboards. Figure 2 shows the total harmonics distortion and noise (THD+N) performance of the OPA1652 and OPA2134 with common-mode impedance mismatching.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3051.1.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3051.1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 1: OPA1652 (DFN) versus OPA2134 (PDIP)</strong></p>
<p align="center"><strong> </strong></p>
<p align="center"><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5127.2.png"><img src="/resized-image/__size/800x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5127.2.png" alt=" " /></a></p>
<p align="center"><strong>Figure 2: THD+N test in a gain of 100: the OPA1652 vs. the OPA2134</strong></p>
<p>A tiny package may not be the only consideration for wearable devices. These types of applications typically spend most of their time in sleep mode until they are needed to measure biometric data. This requirement has inspired op amps with a shutdown feature in tiny QFN packages. For applications where power consumption is a vital concern, tiny packaging helps enable product innovation, because you can achieve both low power and a small form factor. The <a href="http://www.ti.com/product/opa2316" target="_blank">OPA2316S</a> in a X2QFN (1.5mm by 2mm by 0.4mm) operates down to 1.8V with a wide bandwidth of 10MHz, as well as rail-to-rail performance, making it suitable for battery-powered designs.</p>
<p>Some industrial factory automation applications also require a tiny package. Often, designers prefer devices with small-outline lead packages that are easy to prototype, layout and swap with pin-to-pin-compatible ICs. The <a href="http://www.ti.com/product/opa171" target="_blank">OPA171</a> is one of the first micropower 36V op amps offered in both a single SOT553 (1.6mm x 1.6 mm) package and a dual, very-thin shrink small outline package (VSSOP) (2.0mm x 3.1mm), providing an optimized combination of low cost and performance for applications such as tracking amplifiers in power modules, transducer amplifiers and battery-powered instruments.As a fundamental building block for a signal chain, op amps must keep pace with the emerging electronic design trends. TI has developed products in some of the smallest packages available, including WCSP, QFN/DFN and SOT553. With innovations in design and technologies, more tiny-package, low-power precision op amps are on the way. Browse TI’s <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/precision-op-amps-products.page#p1811=1.5;9&p480=1;4http://www.ti.com/lit/an/slyt595/slyt595.pdf" target="_blank">tiny op amp</a> portfolio to find the best one for your next project. </p>
<p><b>Additional resources</b></p>
<ul>
<li>Read the Analog Applications Journal article, “<a href="http://www.ti.com/lit/an/slyt595/slyt595.pdf" target="_blank">Distortion and source impedance in JFET-input op amps</a>.”</li>
<li>Learn more about the TI Design, “<a href="http://www.ti.com/tool/tipd177" target="_blank">A High-Fidelity Headphone Amplifier for Current Output Audio DACs Reference Design</a>”</li>
<li>Visit the “<a href="http://www.ti.com/tool/tipd189" target="_blank">Headphone Amplifier for Voltage-Output Audio DACs Reference Design</a>”</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669367&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Ying ZHOUhttps://e2e.ti.com:443/members/3936803How to achieve higher-precision data acquisition in benchtop test equipmenthttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/enabling-higher-performance-benchtop-test-equipment2016-12-16T16:20:00Z2016-12-16T16:20:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/ADS8900B" class="internal-link folder product" title="Link to Product Folder" target="_blank">ADS8900B</a></div><p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/digital-multimeter.jpg"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/digital-multimeter.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>The <a href="http://www.ti.com/solution/digital_multimeter_handheld" target="_blank">digital multimeter (DMM)</a> is a centerpiece in any electronics lab, but as the precision of electronics continues to increase, so does the need for DMMs that can quickly and accurately measure current, voltage, resistance and other parameters. Thus, there is a constant need to improve the data-acquisition system within a DMM, enabling higher accuracy measurements. At the heart of the data-acquisition system is the <a href="http://www.ti.com/lsds/ti/data-converters/analog-to-digital-converter-overview.page" target="_blank">analog-to-digital converter</a> (ADC), which digitizes the input signal and sends the data to a host processor.</p>
<p>In this post, I will outline the most important features of an ADC as it pertains to DMMs, specifically benchtop systems in comparison to handheld version. Figure 1 shows the block diagram for the data-acquisition system of a typical DMM.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adc.png"><img src="/resized-image/__size/700x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adc.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: Block diagram of DMM input signal conditioning</b></p>
<p>While you can use many different ADC architectures in a DMM, one of the most common is the <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page" target="_blank">successive approximation register (SAR) ADC</a> because of its speed, resolution and configurability. While <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page">delta-sigma</a> ADCs can offer higher resolution and <a href="http://www.ti.com/lsds/ti/data-converters/high-speed-adc-greater-10msps-products.page" target="_blank">pipeline ADCs</a> offer the advantage of faster sampling rates, the combination of high resolution AND fast response time of a SAR ADC enables a DMM to make quick and precise measurements. Since today’s SAR ADCs continue to become higher resolution, they have become a suitable alternative to delta-sigma ADCs for use in higher-precision systems.</p>
<p>Today, DMMs typically use an ADC with at least 16 bits of resolution and a sampling rate of at least 100kSPS (100,000 samples-per-second). As the need for higher-accuracy DMMs increases, so will the need for ADCs with higher speed and resolution. That is because while the resolution increases accuracy, it is much more susceptible to noise. You can mitigate the effects of noise on system performance by sampling at a higher rate and performing an average of multiple measurements (oversampling) and/or digitally filtering.</p>
<p>For systems requiring higher resolution and speed, TI offers the <a href="http://www.ti.com/product/ads8900B" target="_blank">ADS8900B</a> family of precision SAR ADCs with high resolution, fast sample rate and excellent AC/DC performance. Table 1 shows the specifications for this family.</p>
<div align="center"><br />
<table border="1" cellspacing="0" cellpadding="0">
<tbody>
<tr>
<td width="139" valign="top">
<p align="center"> </p>
</td>
<td width="140" valign="top">
<p align="center"><b>ADS890xB</b></p>
</td>
<td width="140" valign="top">
<p align="center"><b>ADS891xB</b></p>
</td>
<td width="140" valign="top">
<p align="center"><b>ADS892xB</b></p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Resolution</p>
</td>
<td width="140" valign="top">
<p align="center">20 bits</p>
</td>
<td width="140" valign="top">
<p align="center">18 bits</p>
</td>
<td width="140" valign="top">
<p align="center">16 bits</p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Speed</p>
</td>
<td width="140" valign="top">
<p align="center">Up to 1MSPS</p>
</td>
<td width="140" valign="top">
<p align="center">Up to 1MSPS</p>
</td>
<td width="140" valign="top">
<p align="center">Up to 1MSPS</p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Integral nonlinearity (typ)</p>
</td>
<td width="140" valign="top">
<p align="center">±1ppm</p>
</td>
<td width="140" valign="top">
<p align="center">±0.5LSB</p>
</td>
<td width="140" valign="top">
<p align="center">±0.5LSB</p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Signal-to-noise ratio (typ)</p>
</td>
<td width="140" valign="top">
<p align="center">104.5dB</p>
</td>
<td width="140" valign="top">
<p align="center">102.5dB</p>
</td>
<td width="140" valign="top">
<p align="center">96.8dB</p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Total harmonic distortion (typ)</p>
</td>
<td width="140" valign="top">
<p align="center">-125dB</p>
</td>
<td width="140" valign="top">
<p align="center">-125dB</p>
</td>
<td width="140" valign="top">
<p align="center">-125dB</p>
</td>
</tr>
<tr>
<td width="139" valign="top">
<p align="center">Package</p>
</td>
<td width="140" valign="top">
<p align="center">4 mm x 4 mm QFN</p>
</td>
<td width="140" valign="top">
<p align="center">4 mm x 4 mm QFN</p>
</td>
<td width="140" valign="top">
<p align="center">4 mm x 4 mm QFN</p>
</td>
</tr>
</tbody>
</table>
<p align="center"><b>Table 1: ADS8900B performance</b></p>
<p style="text-align:left;">In addition to being a high-performance ADC, the ADS8900B has an integrated operational amplifier buffer for the voltage reference that provides two distinct advantages over an external reference buffer. Figure 2 compares an external to internal reference buffer in a data-acquisition system.</p>
<p style="text-align:center;"><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adc-2.png"><img src="/resized-image/__size/700x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adc-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a><b style="font-size:12px;">Figure 2: External vs. internal voltage reference buffer</b></p>
</div>
<p>The first advantage that you can get from an ADC with an integrated buffer is that it increases system performance. An ADC requires a high precision voltage reference, such as the <a href="http://www.ti.com/product/ref5050" target="_blank">REF5050</a>, which is compared to the input signal to determine the signal voltage. Because the output current of a voltage reference is typically only several milliamps (mA), the reference buffer is used to supply the current required by the ADC during a conversion cycle while minimizing droop in the voltage reference. By integrating the reference buffer into the same chip as the ADC, the buffer is optimized to drive the ADC with lower distortion to the voltage reference, enabling higher performance data acquisition than those using an external buffer. The second advantage is that packaging the ADC and buffer together results in a smaller footprint than an discrete solution, resulting in a smaller system.</p>
<p>Although the ADS8900B family is helping enable the next generation of digital multimeters, there are numerous other applications for this SAR ADC. In my future posts, I’ll explain how it can reduce the size and power consumption of data acquisition systems that require multiple ADCs and how it can reduce or eliminate the headache of digital design for high-speed, high-resolution data acquisition systems. Be sure to sign in and subscribe to Precision Hub to get these posts delivered right to your inbox.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Download the <a href="http://www.ti.com/tool/tida-01012" target="_blank">TI Designs Wireless IoT, <i>Bluetooth</i>® Low Energy, 4 ½ Digit, 100kHz True RMS Digital Multimeter Reference Design</a>.</li>
<li>Explore TI’s selection of <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page" target="_blank">precision ADCs</a> deliver the most integration and lowest power consumption for demanding systems.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669294&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Evan Sawyerhttps://e2e.ti.com:443/members/3474348Multiplexers: not so simplehttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/multiplexers-not-so-simple2016-11-18T16:00:00Z2016-11-18T16:00:00Z<p>It’s simple to design a multiplexer (or mux for short) into a signal chain, right? After all, the device simply funnels multiple signals into a data converter.</p>
<p>In reality, a mux can significantly impact the performance of a signal chain in a variety of ways. For example, the on-capacitance can cause crosstalk between channels. Signal- and temperature-dependent variations in the on-resistance can introduce signal distortion. Together, the capacitance and resistance of the mux can limit signal bandwidth. Charge injection can introduce transient errors when the mux switches channels and impact settling time at the output.</p>
<p>To optimize signal-chain performance, it is important to understand these examples as well as numerous other ways that a mux can impact a signal, especially because multiplexers are optimized for different performance characteristics, and thus for different applications. Figure 1 is an example circuit containing a mux with its output connected to an inverting operation amplifier (<a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">op amp</a>). </p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-1.png"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: A mux connected to an inverting amplifier introduces gain error</b></p>
<p>This circuit is one of many common mux configurations in a signal chain, but as we will discover, this design will lead to significant signal gain error. Assuming that the op amp is ideal (no offset, bias current, input/output limitations, etc.), Equation 1 expresses the signal gain as:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-figure-1.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-figure-1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Because the <a href="http://www.ti.com/product/mux36s08">MUX36S08</a> is not an ideal mux and has internal capacitance as well as an on-resistance of 125Ω, Equation 2 expresses the effective gain of the system as:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/multiplexer-eq-2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/multiplexer-eq-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>The calculated signal gain in Equation 2 poses a huge problem if the output of the op amp is connected to a data converter designed to receive the full gain, as nearly 40% of the converter’s range would not be utilized. This equation doesn’t even take into account the on-resistance variation that occurs from changes in temperature, signal voltage or the voltage applied to the supplies.</p>
<p>Figure 2 shows one of the on-resistance curves for the <a href="http://www.ti.com/product/mux36s08">MUX36S08</a>. You can see that the resistance changes based on the temperature as well as the applied signal (the source or drain voltage). The curve that results from changing the signal voltage is known as on-resistance flatness, which can introduce nonlinearity and gain variation. Subjecting the circuit in Figure 1 to a full ±18V sinusoidal signal and temperatures from -40°C to 125°C, the on-resistance of the mux can vary from approximately 75Ω to 250Ω, resulting in an effective gain ranging from -0.44 to -0.73.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/on-resistance-2.png"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/on-resistance-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: On-resistance vs. source or drain voltage</b></p>
<p>Luckily, you can effectively ignore the on-resistance of the <a href="http://www.ti.com/lsds/ti/switches-multiplexers/multiplexer-demultiplexer-overview.page" target="_blank">multiplexer</a> through very simple design precautions. Figure 3 shows the output of the mux connected to an op amp configured as a buffer. The high input impedance of the op amp eliminates any gain error the system would otherwise experience.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-3.png"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/mux-3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: A mux connected to a buffer effectively eliminates gain error caused by the mux on-resistance</b></p>
<p>As a reminder, the effect of on-resistance on signal gain is only one of the many ways that a multiplexer can impact the performance of a system. When you’re ready to learn about the other ways a mux can add error and distortion to a signal as well as how to mitigate the impact a mux has on signal chain performance, check out the new <a href="https://training.ti.com/ti-precision-labs-op-amps-basics-of-multiplexers-1?cu=14685" target="_blank">TI Precision Labs training series covering multiplexers.</a></p>
<p><b>Additional resources</b></p>
<ul>
<li>Explore more than 40 hands-on trainings and lab videos from <a href="https://training.ti.com/ti-precision-labs-op-amps" target="_blank">TI Precision Labs</a>.</li>
<li>Read a blog about whether a <a href="/blogs_/b/precisionhub/archive/2016/02/19/does-a-low-leakage-multiplexer-really-matter-in-a-high-impedance-plc-system" target="_blank">low-leakage multiplexer</a> really matters in a high-impedance PLC system.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669229&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Evan Sawyerhttps://e2e.ti.com:443/members/3474348How to build a monitor and control solution for voltage regulatorshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-build-a-monitor-and-control-solution-for-voltage-regulators2016-10-28T16:00:00Z2016-10-28T16:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/AMC7891" class="internal-link folder product" title="Link to Product Folder" target="_blank">AMC7891</a></div><p>In <a href="/blogs_/b/precisionhub/archive/2016/09/02/give-your-voltage-regulator-the-margin-it-deserves" target="_blank">my last post</a>, I talked about how to use a <a href="http://www.ti.com/lsds/ti/data-converters/precision-dac-less-10msps-overview.page" target="_blank">precision digital-to-analog converter</a> (DAC) to margin a voltage regulator like a low-dropout regulator (LDO) or switch mode power supply (SMPS), providing the ability to either precisely tune the output or allow it to swing over a wide range of voltages.</p>
<p>In this post, I will expand upon that idea to build a closed-loop system that, alongside the compute power of a microprocessor, creates an all-in-one analog monitor and control solution for voltage regulators. Let’s return to the example circuit in Figure 1 that I used last time with an LDO and a DAC.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-1.png"><img src="/resized-image/__size/700x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: Voltage regulator margining circuit</b></p>
<p>The DAC shown controls the regulator circuit by sinking or sourcing current – thereby raising and lowering the voltage output of the LDO. You can add monitoring to the circuit by using a <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-overview.page" target="_blank">precision analog-to-digital converter</a> (ADC) to sample the voltage at the output of the LDO. Additionally, many regulators have an enable pin that you may want to control as well. You can do this by using a general purpose I/O GPIO from the microcontroller. Figure 2 shows these monitor and control devices in the system surrounding the LDO.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-2.png"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Voltage regulator monitor and control system</b></p>
<p>What would be very helpful is if you could use one device to accomplish the functions of the DAC, ADC and GPIO. Fortunately, TI has a portfolio of <a href="http://www.ti.com/lsds/ti/data-converters/integrated-precision-adc-and-dac-products.page" target="_blank">analog monitor and control (AMC) devices</a> that integrate all three of these discrete devices into one product.</p>
<p>Let’s use an example where you need to monitor and control four power supplies. A device like the AMC7891 would be great for this application because it has four DACs and more than four ADC inputs and GPIOs. Figure 3 shows how the AMC7891 fits into this system.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-3.png"><img src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-regulator-3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: A multiple-rail voltage regulator monitor and control system</b></p>
<p>The AMC7891’s integration enables you to eliminate many discrete devices from the board and centralize the control of the power supplies to just one device.</p>
<p>Here are a few helpful tips when designing this solution into your system:</p>
<ul>
<li>SMPS outputs are inherently noisy with the voltage ripple from the switch. Take multiple samples of the output voltage with the ADC and average the samples before changing the DAC code to compensate.</li>
<li>If your regulator output voltage exceeds the ADC input voltage, you will need to use an external amplifier to add a fractional gain to the output voltage to get the signal within range.</li>
<li>Put your ADC trace as close as possible to the downstream devices so that you get the most accurate measurement possible at the point of load.</li>
</ul>
<p>You can visit <a href="http://www.ti.com/lsds/ti/data-converters/integrated-precision-adc-and-dac-products.page?DCMP=hpa_dc_general&HQS=amc" target="_blank">ti.com/amc</a> to find more <a href="http://www.ti.com/lsds/ti/data-converters/integrated-precision-adc-and-dac-products.page" target="_blank">integrated precision ADCs and DACs</a> that are similar to AMC7891. TI provides a very broad portfolio of these devices, many having more inputs and outputs for your control system.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Search for solutions, get help and solve problems in the <a href="http://e2e.ti.com/support/data_converters/precision_data_converters/" target="_blank">TI E2E™ Precision Data Converter forum</a>.<b></b></li>
<li>For more resources about working with <a href="http://www.ti.com/lsds/ti/data-converters/precision-dac-less-10msps-products.page" target="_blank">precision DACs</a>, visit the <a href="http://www.ti.com/lsds/ti/data-converters/precision-dac-less-10msps-learning-center.page?HQS=hpa-pa-null-pdlearningcenter-vanity-lp-pdlc-wwe" target="_blank">Precision DAC Learning Center</a>.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669162&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Matthew Poolehttps://e2e.ti.com:443/members/4114736How to layout a PCB for an instrumentation amplifierhttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-layout-a-pcb-for-an-instrumentation-amplifier2016-10-14T20:00:00Z2016-10-14T20:00:00Z<p>In my <a href="/blogs_/b/precisionhub/archive/2016/01/08/the-basics-how-to-layout-a-pcb-for-an-op-amp" target="_blank">previous post</a>, I discussed the proper way to layout a printed circuit board (PCB) for an <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/op-amps-overview.page" target="_blank">operational amplifier</a> (op amp) and provided a list of good layout practices to follow. In this post, I will discuss common mistakes when laying out a PCB for an <a href="http://www.ti.com/lsds/ti/amplifiers/instrumentation-amplifiers/instrumentation-amplifiers-overview.page" target="_blank">instrumentation amplifier</a> (INA) and then show an example of a proper layout for an INA.</p>
<p>INAs are used in applications that require the amplification of a differential voltage, such as when measuring the voltage across a shunt resistor in a high-side current-sensing application. Figure 1 shows the schematic of a typical single-supply high-side current-sensing circuit.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/high-side.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/700x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/high-side.png" /></a></p>
<p align="center"><strong>Figure 1: High-side current-sensing schematic</strong></p>
<p>In Figure 1, a differential voltage is measured across R<sub>SHUNT</sub>, with R1, R2, C1, C2, and C3 providing input common-mode and differential-mode filtering. R3 and C4 provide output filtering for the INA, U1. U2 buffers the reference pin of the INA. R4 and C5 form a low pass filter that minimizes noise that the op amp introduces to the reference pin of the INA.</p>
<p>While the layout for the schematic in Figure 1 seems straightforward, it is easy to make mistakes in the PCB layout that might degrade circuit performance. Figure 2 shows a PCB layout with three mistakes we at TI commonly see when reviewing INA layouts.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/pcb-2.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/pcb-2.png" /></a></p>
<p align="center"><strong>Figure 2: Common PCB layout for an INA</strong></p>
<p>The <b>first mistake is how the differential voltage is measured across the resistor</b>, Rshunt. Notice that the trace from Rshunt to R2 is much shorter, and therefore has less resistance than the trace from Rshunt to R1. This difference in trace impedance may create a differential voltage at the input of U1 due to the input bias current of the INA. Since an INA’s job is to amplify a differential voltage, having unbalanced traces at the input can cause an error. Therefore, keep the input traces of an INA as balanced and as short as possible.</p>
<p>The <b>second mistake is related to the gain-setting resistor of the INA</b>, Rgain. The traces from the pins of U1 to the pads of Rgain are longer than necessary, which create additional resistance and capacitance. Having additional resistance may introduce error in the desired gain of the INA, since the gain depends on the resistance between the INA’s gain-setting pins, pins 1 and 8. Additional capacitance may cause stability issues because the gain-setting pins of the INA connect to the feedback node inside the INA. Therefore, keep the traces connected to the gain-setting resistor as short as possible.</p>
<p><b>Finally, the positioning of the reference pin buffer circuit may need improving</b>. The reference pin buffer circuit is positioned far from the reference pin, which increases the resistance connected to the reference pin and opens up the possibility for noise and other signals to couple onto the trace. Additional resistance on the reference pin will degrade the high common-mode rejection ratio (CMRR) that most INAs provide. Therefore, position the reference pin buffer circuit as close to the reference pin of the INA as possible.</p>
<p>Figure 3 shows a layout that corrects these three mistakes.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ina-pcb-3.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/650x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ina-pcb-3.png" /></a></p>
<p>In Figure 3, you can see that the traces from the shunt resistor to R1 and R2 are equal lengths and have a kelvin connection. The traces from the gain-setting resistor to the pins of the INA are as short as possible, and the reference buffer circuit is as close to the reference pin as possible.</p>
<p>The next time you lay out a PCB for an INA, be sure to follow these guidelines:</p>
<ul>
<li>Keep all traces on the input perfectly balanced.</li>
<li>Reduce trace length and minimize capacitance on the gain-setting pins.</li>
<li>Position the reference buffer circuit as close to the reference pin of the INA.</li>
<li>Place decoupling capacitors as close to the supply pins as possible.</li>
<li>Pour at least one solid ground plane.</li>
<li>Do not sacrifice good layout to label a component with silkscreen.</li>
<li>Follow the guidelines in my <a href="/blogs_/b/precisionhub/archive/2016/01/08/the-basics-how-to-layout-a-pcb-for-an-op-amp" target="_blank">previous post.</a></li>
</ul>
<p><b>Additional resources</b></p>
<ul>
<li>Find commonly used analog design formulas in the <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/precision-op-amps-support-training.page?keyMatch=pocket%20reference&tisearch=Search-EN-Everything#pocketref" target="_blank"><i>Analog Engineer’s Pocket Reference</i></a> by Art Kay and Tim Green.</li>
<li>Check out the <a href="http://www.ti.com/tool/TIPD135" target="_blank">TI Designs 10µA-100mA, 0.05% Error, High-Side Current Sensing Solution Reference Design (TIPD135)</a>.</li>
<li>Explore TI’s entire portfolio of <a href="http://www.ti.com/lsds/ti/amplifiers/amplifiers-overview.page" target="_blank">amplifier ICs</a> and find technical resources to help with your design.</li>
<li>Review all of TI’s available precision <a href="http://www.ti.com/lsds/ti/amplifiers/instrumentation-amplifiers/instrumentation-amplifiers-overview.page"><span style="color:#0000ff;">Instrumentation Amplifiers</span></a></li>
<li>Use TI’s <a href="http://www.ti.com/tool/inaevm"><span style="color:#0000ff;">Universal Instrumentation Amplifier Evaluation Module</span></a> to test your instrumentation amp sample</li>
<li>Read up on tips and tricks for <a href="http://www.ti.com/lit/an/slyt226/slyt226.pdf"><span style="color:#0000ff;">getting the most out of your instrumentation amplifier design</span></a><span style="font-family:Times New Roman;font-size:medium;"> </span></li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669089&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Tim Claycombhttps://e2e.ti.com:443/members/3718404How to fix your simulations when the macromodel’s voltage noise doesn’t match the datasheethttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-fix-your-simulations-when-the-macromodels-voltage-noise-doesnt-match-the-datasheet2016-09-30T17:00:00Z2016-09-30T17:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/OPA2333" class="internal-link folder product" title="Link to Product Folder" target="_blank">OPA2333</a>, <a href="https://www.ti.com/tool/TINA-TI" class="internal-link folder tool" title="Link to Tool Folder" target="_blank">TINA-TI</a></div><p>When responding to questions posted on TI <a href="/" target="_blank">E2E™ Community forums</a>, we frequently run simulations using <a href="http://www.ti.com/tool/tina-ti" target="_blank">TINA-TI</a>™ software, a SPICE-based simulation program. Since we are always in the process of updating our simulation models, we sometimes run across SPICE models that are old, outdated or incorrect when modeling performance parameters.</p>
<p>One recent example involves the voltage-noise density of the <a href="http://www.ti.com/product/OPA2333" target="_blank">OPA2333</a> macromodel. Unfortunately, we found that the model’s voltage-noise density curve was less than that given in the data sheet. So in this blog post, I will show you how to verify an operational amplifier’s (op amp) voltage-noise density curve and correct it if necessary.</p>
<p>First, you need to know how to generate a voltage-noise density curve using TINA-TI software. In this example, I will use the OPA2333 macromodel and schematic shown in Figure 1.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-1.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-1.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 1: TINA-TI test bench for voltage-noise density</strong></p>
<p>The output noise in this configuration uses the op amp with no gain, filtering or other factors that would change the voltage noise over frequency.</p>
<p>To simulate the output noise, select Analysis > Noise Analysis, and tick the Output Noise check box shown in Figure 2.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-2.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-2.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 2: How to find noise analysis for output noise</strong></p>
<p align="center"><strong><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/7455.3.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/7455.3.png" style="float:left;" alt=" " /></a><br /></strong></p>
<p></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-3.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-3.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 3: Simulated OPA2333 voltage noise</strong></p>
<p align="center"><strong><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-4.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-4.jpg" alt=" " /></a><br /></strong></p>
<p align="center">Figure 4: OPA2333 voltage noise according to the data sheet</p>
<p> </p>
<p>You can add noise to the macromodel by inserting a voltage-noise source in the schematic.</p>
<p>To get the voltage-noise source, go to File > Open Examples. Select the Noise Sources folder and open the TINA Noise Sources.TSC file shown in Figure 5.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-5.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-5.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 5: Finding the voltage-noise source and equivalent op amp noise model</strong></p>
<p>Now, copy and paste the voltage-noise source from the noise-source schematic into the testing schematic and add it to the noninverting input, as shown in Figure 6.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-6.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-6.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 6: Testing schematic for input-voltage noise with a voltage-noise source</strong></p>
<p>Double-click on the noise source and select Enter Macro. A tab will open showing the netlist of the voltage source; see Figure 8.</p>
<p>Since the <a href="http://www.ti.com/product/OPA333">OPA333</a> is a chopper op amp and has no flicker noise, you do not want to add flicker noise to the voltage source. Find the parameters NLF and FLW and change them to 0 and 0.1, respectively. This sets the flicker noise to 0 nV/ √ Hz at 0.1Hz. You will also need to adjust the broadband noise so that the root sum square (RSS) noise is equal to the value you want. In the netlist, this value is represented by the parameter NVR.</p>
<p>In Figure 7, I solve for the desired broadband voltage. Figure 8 shows the updated parameters inside the netlist. </p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-7.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-7.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 7: Solving for the broadband voltage-noise value</strong></p>
<p align="center"><strong><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-8.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-8.jpg" alt=" " /></a><br /></strong></p>
<p align="center"><strong>Figure 8: Netlist of voltage-noise source</strong></p>
<p>Running the noise analysis again (Figure 9), you can see that the voltage noise is now the same value shown in Figure 4.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-9.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/simulation-9.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 9: Corrected op amp voltage noise</strong></p>
<p>Note that this process only works when your macromodel’s voltage-noise density curve is less than that given in the data sheet because RSS noise can only be added. If your macromodel’s voltage-noise density curve is more than that given in the datasheet, the TI Precision Labs on-demand training series includes a short video on how to create your own accurate macromodel for noise simulations. You can find the <a href="https://training.ti.com/ti-precision-labs-op-amps-noise-6?cu=1468" target="_blank">video here</a>*.</p>
<p>Remember, trust your SPICE models, but verify that they are correct. Simulations can take you far, but they need to simulate correctly to produce meaningful results.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Learn more about <a href="https://training.ti.com/ti-precision-labs-op-amps-noise-1?cu=14685">noise</a>, how to calculate it and how to simulate it correctly with our online training series, <a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page" target="_blank">TI Precision Labs</a> – Op Amps.</li>
<li>Learn how to trust and verify your SPICE macromodels in Tim Green’s blog post, “<a href="/blogs_/b/precisionhub/archive/2015/09/25/spice-op-amp-macromodels-trust-but-verify" target="_blank">SPICE op amp macromodels: ‘Trust but verify’</a>.”</li>
<li>Find commonly used analog design formulas in our wildly popular and free “<a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-support-community.page?keyMatch=pocket%20reference&tisearch=Search-EN-Everything#pocketref" target="_blank">Analog Engineer’s Pocket Reference” e-book</a>*.</li>
<li>Read more <a href="/search?q=Precision%20amplifier%20blog" target="_blank">blogs about precision amplifiers.</a></li>
<li>Learn about TI’s entire portfolio of <a href="http://www.ti.com/lsds/ti/amplifiers/amplifiers-overview.page" target="_blank">amplifier ICs</a> and explore technical resources.</li>
</ul>
<p><i>*This requires a myTI log-in. </i></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=669068&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Cole Maciashttps://e2e.ti.com:443/members/4719008Why is it so challenging to design a voltage reference circuit for an ADC?https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/why-is-it-so-challenging-to-design-a-voltage-reference-circuit-for-an-adc2016-09-16T15:00:00Z2016-09-16T15:00:00Z<p><em>This technical article was updated on July 23, 2020.</em></p>
<p>High-precision data-acquisition systems are designed to minimize errors from various system components, like those introduced by switching transients on the reference input of a data converter. In the case of a successive approximation register analog-to-digital converter (<a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-products.page#p89=SAR" target="_blank">SAR ADC</a>), circuitry inside the data converter as it connects and disconnects different capacitive loads throughout the conversion cycle causes switching transients. Other data converters, such as <a href="http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-products.page#p89=Delta-Sigma" target="_blank">delta-sigma ADCs</a> and <a href="http://www.ti.com/lsds/ti/data-converters/digital-to-analog-converter-overview.page" target="_blank">digital-to-analog converters</a> (DACs), can also impose switching transients on the reference pin.</p>
<p>A simplified representation of the SAR ADC architecture is shown in Figure 1. During operation, switches S1 and S2 inside the ADC control the acquisition and conversion cycles. When S1 closes and S2 opens, a transient condition occurs on the input because of an impedance change. There are detailed technical resources that discuss how to optimize the input circuitry to minimize the impact of the input transient, such as the user guide for the <a href="http://www.ti.com/tool/tipd173" target="_blank">TI Design TIPD173, which showcases a 16-Bit 1MSPS Data Acquisition Reference Design for Single-Ended Multiplexed Applications</a>. But in this post, I’d like to focus on the transients generated on the reference voltage input pin (V<sub>REF</sub>) since these transients and their effect on system performance are often overlooked in system-level design.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/sarADC1.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/sarADC1.png" /></a></p>
<p align="center"><b>Figure 1: Simplified SAR ADC internal architecture</b></p>
<p>The V<sub>REF</sub> pin of a SAR ADC is internally connected to a capacitive DAC (CDAC), highlighted in red in Figure 1. Figure 2 provides additional detail on a simplified CDAC structure. The CDAC is a binary weighted capacitor array that determines the digital value that best matches the input voltage in comparison to a reference voltage. The key point is that the reference input pin connects to the binary weighted capacitor array, which can cause variations in the reference voltage applied to the V<sub>REF</sub> pin during a conversion cycle. The capacitors in the array will not be at the same potential as the reference, so there will be a large, fast spike of in-rush current when connecting the capacitors to the external reference.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/cdac2.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/cdac2.png" /></a></p>
<p align="center"><b>Figure 2: The internal CDAC architecture results in a switched capacitor load</b></p>
<p>Figure 3 shows the spikes in reference input current that occur throughout the conversion cycle, which can be as large as 10mA and very short in duration (nanoseconds).</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/switching_5F00_transients3.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/switching_5F00_transients3.png" /></a></p>
<p align="center"><b>Figure 3: Switching transients on the V<sub>REF</sub> pin of a SAR ADC</b></p>
<p>For optimal accuracy, the voltage reference connected to the SAR input needs to respond to the large, fast current spikes. These fast-switching current transients can cause a voltage drop across the high output impedance of the voltage reference. This voltage drop directly affects the output voltage of the reference and therefore the input voltage to the V<sub>REF</sub> pin of the ADC, resulting in erroneous conversion of the input signal by the ADC.</p>
<p>To minimize the error introduced by these switching transients, the voltage reference should resettle to the desired output voltage between each current spike. A stand-alone voltage reference is designed to deliver a very accurate and stable voltage, given that the load is very light and slow-moving. Since these current spikes are very short in duration and large in magnitude, the reference is often buffered with a <a href="http://www.ti.com/lsds/ti/amplifiers/op-amps/high-speed-op-amps-overview.page" target="_blank">high-speed operational amplifier</a> (op amp) (see Figure 4). In addition, placing a capacitor at the pin can provide the total instantaneous current needed.</p>
<p>Although high-speed op amps are good from a transient perspective, they generally are not optimized for DC accuracy, such as offset voltage, linearity and drift. Thus, it can be challenging to find a buffer that meets the DC accuracy requirement but also has good transient behavior. In some cases, an amplifier topology containing two amplifiers will achieve this challenging objective. The <a href="http://www.ti.com/tool/tipd173" target="_blank">data acquisition reference design user guide</a> that I mentioned earlier explains this topology in more detail and covers the selection of the voltage reference, buffer amplifiers and associated filter components.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-reference4.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/voltage-reference4.png" /></a></p>
<p align="center"><b>Figure 4: Voltage reference circuit using a high-speed amplifier</b></p>
<p>In order to simplify the system-level design efforts required to minimize the effects of switching transients on the reference pin, TI’s <a href="http://www.ti.com/product/ref6025" target="_blank">REF6000</a> voltage reference family integrates the reference buffer with the voltage reference. Figure 5 shows this integration in a simplified data-acquisition system. The internal buffer is optimized to respond well to the types of transients generated on the reference pin of a data converter and is also optimized for DC performance. In addition, this combination reduces circuit board area, as it combines the voltage reference and reference buffer.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/reference-buffer5.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/reference-buffer5.png" /></a></p>
<p align="center"><b>Figure 5: Voltage reference circuit using an integrated voltage reference and reference buffer</b></p>
<p>With this integrated approach, the performance of the <a href="http://www.ti.com/lsds/ti/data-converters/analog-to-digital-converter-overview.page" target="_blank">ADC </a>improves by providing a high-bandwidth, low-output impedance, DC-optimized solution for the input to the V<sub>REF</sub> pin. Table 1 compares the noise and distortion performance of an ideal ADC to ADCs with different voltage reference circuit configurations. You can see that the case without a reference driving buffer has degraded performance. Comparing the integrated reference buffer to the external buffer, the reference with the integrated buffer performs best.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ideal-DAC.png"><img style="display:block;margin-left:auto;margin-right:auto;" alt=" " src="/resized-image/__size/750x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ideal-DAC.png" /></a></p>
<p align="center"><b>Table 1: ADC performance of various buffer configurations with an 18-bit ADC sampling at 1MSPS and 10kHz input frequency</b></p>
<p>It’s important to consider the design of the voltage reference circuit when designing a high-precision data-acquisition system. One way to improve overall system performance is to optimize the driving buffer that handles the fast switching transients of the data converter in order to reduce distortion and error. We’ve taken care of that for you by integrating the reference buffer and voltage reference with TI’s <a href="http://www.ti.com/product/ref6025" target="_blank">REF6000</a> family of voltage references.</p>
<p>Be sure to subscribe to Precision Hub by clicking that option in the upper right corner of this page to receive <a href="http://e2e.ti.com/tags/voltage%2breference%2bseries" target="_blank">design advice about voltage references</a> and more right to your inbox.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Read the white paper, “<a href="http://www.ti.com/lit/wp/slyy097/slyy097.pdf" target="_blank">Voltage-reference impact on total harmonic distortion</a>,” for a more detailed discussion about the impact of reference loading on distortion.</li>
<li>See TI’s broad range of <a href="http://www.ti.com/lsds/ti/power-management/voltage-reference-overview.page" target="_blank">voltage references</a>, such as the series voltage reference portfolio featuring the REF6000 family.</li>
<li>Learn more about the <a href="http://www.ti.com/product/REF6025" target="_blank">REF6025</a> 2.5V output high-precision voltage reference with integrated high-bandwidth buffer and family of related output voltage variants.</li>
<li>Learn about TI’s <a href="http://www.ti.com/lsds/ti/analog/dataconverters/data_converter.page">data converter</a> portfolio and find technical resources.</li>
</ul>
<p style="padding:0;margin:0;"></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=668995&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Peggy Liskahttps://e2e.ti.com:443/members/1141124Give your voltage regulator the margin it deserveshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/give-your-voltage-regulator-the-margin-it-deserves2016-09-02T19:00:00Z2016-09-02T19:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/tool/TINA-TI" class="internal-link folder tool" title="Link to Tool Folder" target="_blank">TINA-TI</a>, <a href="https://www.ti.com/product/DAC5311" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC5311</a>, <a href="https://www.ti.com/product/DAC7311" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC7311</a>, <a href="https://www.ti.com/product/DAC6311" class="internal-link folder product" title="Link to Product Folder" target="_blank">DAC6311</a></div><p>Do you feel like your adjustable voltage regulator deserves some margin? I do, and I have some good news: all it takes is one DAC and one resistor!</p>
<p>The traditional feedback system for a <a href="http://www.ti.com/lsds/ti/power-management/linear-regulators-ldo-overview.page" target="_blank">linear regulator</a> (LDO), shown in Figure 1, has a resistor divider network from the voltage output to the feedback pin and then to ground. Choosing these resistor values determines the output of the regulator.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adjustable_5F00_ldo1.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/adjustable_5F00_ldo1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 1: Common implementation of adjustable LDO feedback</b></p>
<p>Figure 2 shows the topology of a typical adjustable LDO. Notice that the internal reference of the LDO feeds to the non-inverting terminal of the amplifier, which drives the gate of the pass FET. Hooking up the inverting input of the amplifier to the feedback (FB) or adjust (ADJ) pin of the LDO then creates a feedback loop. This causes the amplifier to drive the gate of the pass FET in such a way that the resistor divider of R1 and R2 develops a voltage at the FB pin equal to the LDO’s internal reference voltage.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_topology2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_topology2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 2: Example LDO topology, <a href="http://www.ti.com/product/TPS7A7002" target="_blank">TPS7A7002</a></b></p>
<p>However, this is a static solution. If your circuit encounters ohmic losses on a high-current dynamic load or if you’re implementing dynamic voltage scaling, for instance, your circuit might require “reprogramming” of the output voltage from the regulator. Traditionally, this isn’t possible without physically replacing the resistors. Adding a DAC and resistor to the circuit, however, as shown in Figure 3, will allow you to raise and lower the output voltage of the regulator on the fly by programming the DAC voltage.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_3.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Figure 3: Adjustable LDO feedback with a precision DAC</b></p>
<p>After choosing the regulator and DAC that you would like to use, there are a few pieces of information you need to gather or select before designing this circuit, described in Table 1.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_table.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/ldo_5F00_table.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Table 1: Circuit design parameters</b></p>
<p>Consider the currents going in and out of the V<sub>FB</sub> node shown in Figure 3, which is connected to the ADJ pin of the LDO. Almost no current flows in or out the device through the ADJ pin (on the order of 0.01µA). As I previously mentioned, the output voltage of the LDO is always produced such that the voltage at the ADJ pin – and therefore the V<sub>FB</sub> node – is equal to the LDO’s internal reference voltage. Thus, the current through R2 is constant. It follows that any sourcing or sinking of current by the DAC through R3 is reflected as a proportional voltage increase or decrease at V<sub>OUT</sub> to compensate for the changing current that must flow through R1. This helps write the relationship between the two remaining unknowns, R<sub>1</sub> and R<sub>3</sub>, based on Figure 3 and expressed as Equation 1:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2185.eq1.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2185.eq1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Equation 1 shows that the total amount of current change through R3 due to the DAC output voltage has to be equal to current change through R1 by varying the output voltage of the regulator. Equation 2 is the sum of the currents in and out of the V<sub>FB</sub> node based on Figure 3:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4251.eq2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4251.eq2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Here, V<sub>DAC</sub> is the output voltage of the DAC reflected in Figure 3. To have V<sub>NOM</sub> as the regulator output when the DAC voltage is at V<sub>DACN</sub>, you can substitute these design parameters from Table 1 in for V<sub>OUT</sub> and V<sub>DAC</sub>, respectively. The V<sub>FB</sub> node will always be regulated to the voltage of the LDO internal reference, so you can substitute V<sub>REF</sub> for V<sub>FB</sub> to get Equation 3:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/6327.eq3.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/6327.eq3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>You can use Equations 1 and 3 to calculate the remaining two resistor values. As an example, I’ve chosen the values shown in Table 2 based on a DAC with an output range of 0V to 2.5V and an LDO with an internal reference of 1.246V.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/table2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/table2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Table 2: Example circuit design parameters</b></p>
<p>Plugging values from Table 2 into Equation 1 yields the following relationship between R<sub>1</sub> and R<sub>3</sub>:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2117.1.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2117.1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Substituting 2.5R<sub>1</sub> for R<sub>3</sub> into Equation 3 – along with the other circuit design parameters that you have chosen – and then solving will give you the value of R<sub>1</sub>:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5722.2.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/5722.2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Finally, you can reuse the result to find the value of R<sub>3</sub> from R<sub>1</sub>:</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4274.3.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4274.3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>Now your design is done. Building the results of this exercise in TINA-TI™ software confirms that the design is working as expected. At mid scale of the DAC, the voltage is at the nominal 5V, while at the full and zero scale the voltage is at 4.5V and 5.5V, respectively. Figure 4 shows the schematic used, while Figure 5 shows the simulation results.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/TINA_5F00_TI4.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/TINA_5F00_TI4.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p style="text-align:center;"><b>Figure 4: TINA-TI™ schematic of circuit design</b></p>
<p style="text-align:center;"><b><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/poole.png"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/poole.png" alt=" " /></a><br /></b></p>
<p align="center"><b>Figure 5: Simulation results of circuit design</b></p>
<p>TI’s DAC5311, DAC6311 and DAC7311 are 8-/10-/12-bit voltage output DACs that have output buffers and are very low power. Be careful not to choose an unbuffered DAC. The output amplifier needs to both sink and source current.</p>
<p>For the purpose of this explanation, I used an LDO as the example. The same margining principles apply for margining a switching regulator as well.</p>
<p>In my next post, I will talk about how to effectively margin, monitor and control a whole system of regulators with one device. To be notified about my next post and other technical how-to posts on Precision Hub, click the subscribe button on this post to log in and subscribe.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Need more advice on <a href="/search?q=ldo&category=blog" target="_blank">designing with LDOs</a>? Check out posts on the Power House Blog.</li>
<li>Search for solutions, get help and solve problems in the TI E2E™ <a href="/support/data_converters/precision_data_converters/" target="_blank">Precision Data Converter Forum</a>.</li>
<li>For more resources about working with <a href="http://www.ti.com/lsds/ti/data-converters/precision-dac-less-10msps-products.page" target="_blank">Precision DACs</a>, visit the <a href="http://www.ti.com/pdlc" target="_blank">Precision DAC Learning Center</a>.</li>
</ul>
<p></p><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=668960&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Matthew Poolehttps://e2e.ti.com:443/members/4114736How to design cost-sensitive DC instrumentation circuitshttps://e2e.ti.com/blogs_/archives/b/precisionhub/posts/how-to-design-cost-sensitive-dc-instrumentation-circuits2016-08-19T15:00:00Z2016-08-19T15:00:00Z<div><b>Other Parts Discussed in Post: </b><a href="https://www.ti.com/product/TLV333" class="internal-link folder product" title="Link to Product Folder" target="_blank">TLV333</a>, <a href="https://www.ti.com/product/TLV2333" class="internal-link folder product" title="Link to Product Folder" target="_blank">TLV2333</a>, <a href="https://www.ti.com/product/TLV4333" class="internal-link folder product" title="Link to Product Folder" target="_blank">TLV4333</a></div><p>Many sensors produce low-level DC outputs that require a high input-impedance amplification stage to increase the signal amplitude. Sensors used in personal and portable electronics require operational amplifier (op amp) circuits that provide high input impedance and DC precision, while also being low power and cost-effective.</p>
<p>In this post, I’ll explain how to design a few cost-optimized low-power DC-accurate circuits using <a href="http://www.ti.com/product/tlv333" target="_blank">TLVx333</a> op amps in different circuit configurations. These devices provide high levels of DC accuracy with maximum input offset voltages (V<sub>OS</sub>) less than 15µV, and a typical V<sub>OS</sub> drift of 0.02µV/°C. The 0.1Hz to 10Hz low-frequency noise specification is only 1.1µVpp and the 0.01 – 1Hz noise specification is only 0.3 µVpp. Table 1 shows the key performance metrics for TLVx333 family.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2330.t1.jpg"><img src="/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2330.t1.jpg" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><b>Table </b><b>1</b><b>: Key specifications for the TLV333</b></p>
<p>Single-ended sensors can interface with standard noninverting amplifier circuits, as shown in Figure 1. The transfer function is shown in Equation 1. Noninverting op amp circuit-offset errors are dominated by the input offset voltage (V<sub>OS</sub>) and the V<sub>OS</sub> temperature drift of the op amp. Additional offset errors come from the CMRR and the input bias current of the op amp. The tolerance and temperature coefficient of the resistors in the feedback network set the gain error and gain-error drift. The circuit shown in Figure 1 is configured for a gain of 500V/V, and the closed-loop bandwidth is 1.14kHz.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e1.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/1.png"><img src="/resized-image/__size/800x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/1.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center">Figure 1: TLV333 used in a noninverting amplifier configuration</p>
<p>Sensors with differential outputs such as bridge sensors and strain gauges require a circuit with differential inputs. One of the simplest options to interface with a differential sensor is the four-resistor difference amplifier circuit shown in Figure 2. If R<sub>1</sub> is set equal to R<sub>3</sub> and R<sub>2</sub> is set equal to R<sub>4</sub>, then the transfer function simplifies to Equation 2. The tolerance of the resistors in the difference amplifier will directly affect the CMRR of the circuit. Selecting 0.1% resistors achieves at least 54dB of CMRR, while 0.01% resistors achieve at least 74dB. Note that discrete difference amplifier designs will typically not match the performance of integrated solutions, but they often offer advantages in flexibility and cost. The circuit in Figure 2 is configured for a gain of 499V/V, with a closed-loop bandwidth of 1.16kHz.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e2.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2.png"><img src="/resized-image/__size/800x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/2.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p align="center"><strong>Figure 2: TLV333 used in a difference amplifier configuration</strong></p>
<p>High-impedance sensors with differential outputs often require circuits with input impedances >1MΩ. Achieving input impedances >1MΩ is often not practically possible using a discrete difference amplifier topology. Large resistors will increase the DC errors from input bias current, increase circuit intrinsic noise, increase susceptibility to extrinsic noise and will likely require stability compensation.</p>
<p>Figure 3 shows a discrete two-op-amp instrumentation amplifier (INA) using a dual-channel TLV2333. The two-op-amp INA presents a high-impedance differential input to the sensor while only requiring two op amps and five precision resistors. Assuming that R<sub>1</sub> is set equal to R<sub>3</sub> and R<sub>2</sub> is set equal to R<sub>4</sub>, Equation 3 shows the transfer function. The circuit in Figure 3 is configured for a gain of 500V/V, with a closed-loop bandwidth of 1.02kHz.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e3.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3.png"><img src="/resized-image/__size/800x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/3.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>You can also construct a discrete three-op-amp INA using a dual-channel op amp, a single-channel op amp and seven precision resistors. Equation 4 shows the transfer function for the three-op-amp INA. INA designs often require a buffer for a high-impedance reference or an op amp used as an integrator to high-pass filter the input signal. Figure 4 shows a TLV4333 used to create a three-op-amp INA with a reference buffer. The circuit in Figure 4 is configured for a gain of 500V/V and has a closed-loop bandwidth of 1.16kHz.</p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e4.png"><img src="/resized-image/__size/600x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/e4.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p><a href="/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4.png"><img src="/resized-image/__size/800x0/__key/communityserver-blogs-components-weblogfiles/00-00-00-09-30/4.png" style="display:block;margin-left:auto;margin-right:auto;" alt=" " /></a></p>
<p>You can use the TLVx333 family of devices in several ways to create DC-accurate circuits that are ideal for cost-optimized precision-sensor acquisition and precision-instrumentation applications.</p>
<p>Have questions about other op-amp designs? Log in and leave a comment.</p>
<p><b>Additional resources</b></p>
<ul>
<li>Read Pete Semig’s articles on V<sub>OUT</sub> vs. V<sub>CM</sub>limitations to avoid common pitfalls when using INAs:
<ul>
<li>“<a href="http://www.edn.com/design/analog/4437848/Instrumentation-amplifier-VCM-vs-VOUT-plots--Part-1" target="_blank">Instrumentation Amplifier V<sub>CM</sub> vs. V<sub>OUT</sub> Plots”: Part 1</a>, <a href="http://www.edn.com/design/analog/4437922/Instrumentation-amplifier-VCM-vs--VOUT-plots--part-2" target="_blank">Part 2</a>, <a href="http://www.edn.com/design/analog/4438001/Instrumentation-amplifier-VCM-vs--VOUT-plots--Part-3" target="_blank">Part 3</a>.</li>
<li>“<a href="http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slyt647&fileType=pdf&keyMatch=SLYT647&tisearch=Search-EN-TechDocs" target="_blank">V<sub>CM</sub> vs. V<sub>OUT</sub> plots for instrumentation amplifiers with two op amps</a>.”</li>
<li>See TI’s portfolio of <a href="http://www.ti.com/lsds/ti/amplifiers-linear/operational-amplifier-op-amp-products.page#~p1342=Cost%20Optimized" target="_blank">performance op amps for cost-conscious applications</a>.<b></b></li>
<li>Watch more than 40 on-demand precision amplifier training videos in our <a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page" target="_blank">TI Precision Labs – Op Amps</a> series.</li>
</ul>
<p style="padding:0;margin:0;"></p>
</li>
</ul>
<ul>
<li>Find commonly used analog design formulas in the <a href="http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-support-community.page#pocketref" target="_blank"><i>Analog Engineer’s Pocket Reference</i></a> e-book.</li>
<li>Learn about TI’s entire portfolio of <a href="http://www.ti.com/lsds/ti/amplifiers/amplifiers-overview.page">amplifier ICs</a> and find technical resources.</li>
</ul><div style="clear:both;"></div><img src="https://e2e.ti.com/aggbug?PostID=668925&AppID=930&AppType=Weblog&ContentType=0" width="1" height="1">Collin Wellshttps://e2e.ti.com:443/members/1017918