Analog Wire

High speed data converter clocking for JESD204B

2017-07-07T15:00:00Z

The JESD204B standard for data-converter clocking simplifies board routing, but puts some requirements on the device that clocks the data converter. One such requirement is the SysRef pulse that is sent to multiple data converters in order to mark a specific clock edge. The data converters use this pulse in order to line up their data streams. The SysRef pulse needs to occur away from the rising edges of the clock to ensure that it is clear to the data converter which edge it is marking.

Figure 1: SysRef Pulse Edge Placement

The placement of this SysRef pulse becomes more challenging at higher clock frequencies as the period shortens. For instance, let’s say you were clocking multiple high-speed 9GHz digital-to-analog converters (DACs) with 50ps setup and hold times. A 9GHz clock has a period of 111ps, so this only leaves a 11ps window to place the SysRef. So you can see the challenge of precisely positioning the SysRef pulse for a high-frequency clock.

Single vs. multiple clocking device approach

Figure 2 shows the most intuitive approach to clocking multiple data converters.

Figure 2: Single clocking device approach

This method offers excellent control of the skew between SysRef and the clock, but it may be more challenging if the data converters are far from the clocking device and the clock is high frequency.

Figure 3 shows a multiple clocking device approach if the data converters are far apart and the traces are long. In this case, the skew between the re-clocked SysRef and high-frequency clock is better. The SysRef is sent from the master clocking device and retimed through the slave clocking device. This approach allows the high-frequency clock to have short traces and the re-clocked SysRef to have more precise control when the data converters are far apart on the board, or even on separate cards. There will be more skew between the clocks with this approach, but some devices allow you to adjust the skew and tune this out, and maintain the same adjustment settings between power-up cycles.

Figure 3: Multiple clocking device approach

The Multi-Channel JESD204B 15GHz Clocking Reference Design for DSO, Radar and 5G Wireless Testers illustrates the multiple clocking device approach with the LMX2594. In this case, the LMK04828 is the master clocking device and the LMX2594 is the slave clock. The LMX2594 has a sync feature that enables consistent delay through the device, even through power cycles. The fact that the delay is consistent means that it’s correctable either in software or with the LMX2594’s global delay adjustment, which is also consistent even through power cycles. The LMX2594 enables you to adjust the SysRef delay with about 10ps resolution. Figure 4 shows the approach that was used.

Figure 4: High-speed clocking example

Figure 5 shows two 3GHz clocks generated from two LMX2594 devices. As you can see, both the devices and their corresponding SysRef signals are in alignment. You can use this setup for any clock frequency from 10MHz to 15GHz using the LMX2594.

Figure 5: Measured output

As JESD204B data converter clock rates increase, the timing of the SysRef pulse becomes more demanding and needs the clocking device to have fine delay adjustment. The choice of devices that can do this becomes more limited, but there are a few devices that can do this.

Additional resources

Read the blog post, “Timing is everything: JESD204B subclass 1 clocking timing requirements.”
LMX2594 Datasheet

Where are ultrasonic sensors used? – Part 1

2017-06-29T13:10:00Z

Ultrasonic sensors have been used in passenger vehicles for many years in applications like ultrasonic park assist, which help vehicles detect objects at low speeds when parking. However, kick-to-open liftgates and intrusion detection alarms are two other emerging applications for ultrasonic sensors; see Figure 1. In this post, I will explain why – and how – all three applications use ultrasonic sensors.

Figure 1: Ultrasonic sensors in passenger vehicles

Ultrasonic park assist

Ultrasonic park assist is also known as a parking assist system, parking guidance system and reverse park assist. These systems vary from simply detecting an object’s presence and alerting the driver with a noise to autonomously parking the car with little to no driver interaction. Typically, these systems have between four and 16 sensors placed strategically around the car to provide the desired detection coverage, as shown in Figure 2.

Figure 2: Ultrasonic park assist star configuration using the PGA460-Q1

Engineers designing these types of applications should seek out integrated circuits (ICs) driving an ultrasonic transducer (transmitter) while receiving, conditioning and processing the ultrasonic echo that determines the distance of an object from the vehicle. For example, the PGA460-Q1 can reliably detect an International Organization for Standardization (ISO) pole (polyvinyl chloride [PVC] pipe used in ultrasonic park assist as a performance standard) up to 5m away. The device has also passed stringent electrostatic discharge (ESD) and bulk-current injection (BCI) testing – common tests performed during ultrasonic park assist system developments.

Ultrasonic park assist cost pressures will continue to increase over the next several years as original equipment manufacturers (OEMs) face the need to add more ultrasonic sensors per vehicle. The PGA460-Q1 supports a competitive cost structure for high-volume Tier-1 suppliers.

Common requirements in ultrasonic park assist modules include:

Object detection from 30cm to 5m.
Time command interface (TCI) or Local Interconnect Network (LIN) communication from the module to a local electronic control unit (star configuration) or directly to the body control module (BCM) (bus configuration).

In order to meet the needs of autonomous vehicles, short- and long-distance object-detection standards will become more stringent. Beginning around 2025, ultrasonic modules will have to detect objects from 10cm to 7m away. Improvements in analog front end (AFE) sensitivity and drive methods by semiconductor suppliers will be crucial in meeting these distance requirements.

TCI and LIN are the two most common communication interfaces in ultrasonic park assist systems today. However, as vehicles advance in their advanced driver-assistance system (ADAS) vision-processing abilities, expect the use of higher-speed protocols like Peripheral Sensor Interface (PSI) 5, Distributed Systems Interface (DSI) 3 or Controller Area Network (CAN) to communicate larger amounts of ultrasonic echo data.

Kick-to-open liftgates

A kick-to-open liftgate is also known as a smart trunk opener. This feature enables vehicle owners to place their foot under the back bumper in a kicking motion to open the trunk of the vehicle without using their hands.

Traditional kick-to-open liftgate systems used capacitive sensing strips located across the bottom of the bumper. However, many automotive Tier-1 suppliers are exploring ultrasonic sensing for this application, with some systems already in mass production. The advantage of ultrasonic sensing vs. capacitive sensing is the latter’s reliability and robustness against environmental factors such as dirt and water compared to capacitive sensing, which is very sensitive to environmental factors and may not work when a car is dirty.

Common requirements in ultrasonic solutions for kick-to-open systems include:

The ability to detect objects from 15cm-1m away.
Low quiescent current.
The ability to operate off of a 12V car battery supply.

Let’s break down each of these requirements.

Object detection from 15cm-1m away

One of the challenges with using ultrasonic sensing in a kick-to-open liftgate application is the close distance detection range. The ability of an ultrasonic sensor to accurately detect near-field objects depends on the quality and specification of the transducer, the driver method and design, and the performance of the receive path (AFE and digital processing).

High-quality transducers such as Murata’s MA58MF14-7N have more stable and reliable decay or “ringing” during transducer excitation. By selecting a high-quality transducer, you can reduce the length of time of the decay and more accurately predict the stability of the decay as well.

The method and design of the transducer driver can also significantly impact the ultrasonic decay period and profile. In kick-to-open applications requiring near-field performance, TI recommends using a transformer drive topology. Figure 3 is an example transformer drive schematic using the PGA460-Q1.

Figure 3: PGA460-Q1 transformer drive schematic

When using a transformer to increase the supply voltage to excite the transducer, the decay profile is more predictable and less “choppy,” resulting in better near-range object-detection performance.

Finally, the performance of the AFE and digital processing affect near and far object detection. For example, the PGA460-Q1 has a low-noise amplifier followed by a programmable time-varying gain stage feeding into a 12-bit successive approximation register analog-to-digital converter. The low-noise amplifier reduces noise from the received signal, and the programmable gain amplifier’s time-varying gain feature enables small gain applied to near-field objects and larger gain for far-field object detection. You can set the gain profile settings in the register for storage in electrically erasable programmable read-only memory (EEPROM).

Low quiescent current

Since kick-to-open ultrasonic sensors must operate with the vehicle off, system quiescent current is critical and specified aggressively by OEMs. The PGA460-Q1 has a ~500µA sleep mode that you can use intermittently to bring the overall system’s current consumption to the necessary levels.

Ability to operate off of a 12V car battery supply

The PGA460-Q1 device is designed to operate from an input voltage supply range from 6V to 28V. In kick-to-open liftgate applications, the PGA460-Q1 device connects directly to a car battery. Proper external component safeguards such as a transient voltage suppression (TVS) diode help protect the device from battery transients and reverse-battery currents.

Intrusion detection alarms

In Europe, an intrusion detection alarm is optional equipment that the consumer can select at the time of purchase or have installed after market. These alarms use ultrasonic sensors to detect any movement inside the vehicle when the car is off and parked. This alarm acts as a backup for the primary alarm system, and will also set off the alarm if children or pets are moving inside the vehicle. Consumers often receive insurance discounts for including this feature in their vehicle depending on the specific region where they live because of the additional anti-theft and safety protection provided.

Most systems use one to two ultrasonic transmitters and one to two receivers. Ultrasonic sensors like the PGA460-Q1 can drive and receive one transmitter and one receiver, so one to two PGA460-Q1 devices may be required.

Conclusion

While these three applications are the most common using ultrasonic sensors, Tier-1 suppliers and OEMs are exploring additional applications such as gull-wing doors, blind-spot detection and forward collision avoidance. Do you know of other automotive applications using ultrasonic sensors? Post a comment below.

Additional resources

Order the PGA460-Q1 evaluation module (EVM).
Watch PGA460-Q1 EVM training video series.
Download the PGA460 frequently asked questions (FAQ) and EVM troubleshooting guide.
Download the PGA460 ultrasonic module hardware and software optimization application report.

Precise constant current regulation helps advance fast-charging

2017-06-15T15:30:00Z

With battery-operated devices now being an essential part of our daily life, the burden of charging these devices is receiving more attention than ever. A good number of new approaches have emerged in the last couple of years to address the long charging times to enable users to get their devices fully charged in minutes rather than hours.

In this post, I’ll highlight trends in fast charging and the essential role that precise constant current (CC) regulation plays to help enable fast, safe and cost-effective solutions to charge devices faster.

Batteries generally go through two phases while charging: constant current (CC) and constant voltage (CV). Figure 1 shows the typical charging curve for a 4.2V lithium-ion (Li-ion) battery. CC is used roughly for the first 67% of charging, when most of the energy transfers from the charger to the battery. CV kicks in during the last 33% of the remaining charging time to help charge the battery fully and maintain a full charge. Some chargers pump small currents (also called trickle charging) during CV to account for discharging currents and keep the battery voltage fully charged. The time it takes for the battery to fully charge depends on its capacity and maximum allowable charging current, which is a function of battery chemistry and ambient temperature. For example, if you have a Li-ion battery with a capacity of 3000mAh and a charge rate of 0.8C (where C refers to the current needed to charge the battery in one hour, which is the default rate battery manufacturers recommend to prolong battery life), the battery will need two to three hours to fully charge.

Figure 1: Typical Li-ion battery charging curve

The latter paragraph describes a typical charging scenario where the charging rate is considered normal with limited handshaking. Newly introduced methods maximize charging time by pushing more energy to the battery during the CC phase. These methods use either proprietary charging algorithms or follow a mainstream standard, like the USB Power Delivery Programmable Power Supplies (PPS) standard. Both the wall charger and the device perform continuous handshaking to intelligently communicate the battery’s needs and maximize charging efficiency.

The two main fast charging methods are high voltage and low current (legacy method), and high current and low voltage (new mainstream trend). The first method uses the existing charging cable and limits the current to about 2A, while increasing the voltage levels up to 15V. The issue with this method is the high heat dissipation from the required voltage-conversion stage at the device side, which decreases both battery life and the maximum allowable energy transferable to the battery.

The second method uses a voltage close to the battery’s voltage and a higher current that can flow to the battery directly. This method is usually known as direct or flash charging. This method enables higher charging rates at cooler temperatures because there is no voltage conversion on the device side. However, flash charging does require special charging cables to enable higher currents to flow. The idea is to try to charge the battery at a rate as close as possible to the maximum allowable rate to minimize charging time.

Given flash charging’s cooler temperature profile; it is becoming more and more popular, with most existing standards adopting it. Figure 2 shows a high-level block diagram of a flash-charging system.

Figure 2: High-level block diagram of a flash-charging solution

As you can see in Figure 2, a precise current-control loop is necessary to enable faster charging times, and to add an extra layer of protection on top of what other blocks like battery chargers and fuel gauges are already using. Although you can integrate the current-sensing function, it’s hard to match the level of accuracy that dedicated current-sensing solutions can provide using small shunt resistances to minimize heat dissipation, and the ability to monitor the current on the high side.

TI offers a variety of dedicated current sensors that fit well in Flash Charging. These solutions include the INA210 family, which offers great accuracy across a wide dynamic range; the INA199, which has a great combination of accuracy and cost; and the new INA181 family, which offers the best value in terms of bandwidth, accuracy and price. In this application, the INA181’s wide 350kHz closed-loop bandwidth enables the detection of the CC signal’s fast ripples – information that you need to maximize the CC by minimizing the guard band for battery protection and safety.

Figure 3 shows a typical flash-charging signal as seen from the output of a wall charger.

Figure 3: Flash-charging current profile example

To sum it all up, the main limitation of today’s fast-charging methods is heat dissipation close to the battery, which limits the maximum allowable energy transferred and thus minimizes the charging time. Also, high temperatures have safety and battery-life degradation concerns. Flash charging is a promising method because it allows a high level of energy transfer at relatively cooler temperatures while maximizing charging efficiency and minimizing charging time. Enabling this high efficiency requires a precise current-control loop, which is best achieved by dedicated current sensors.

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Additional resources

Learn more about TI’s portfolio of current-sense amplifiers.
Get started with the Current Sense Amplifiers video training series.
Read these TI TechNotes:
- “Current Mode Control in Switching Power Supplies.”
- “External Current Sense Amplifiers vs. Integrated On-Board Amplifiers for Current Sensing.”

How to maintain thermal performance when designing with USB Power Delivery

2017-06-12T15:30:00Z

Over the past decade, portable devices like tablets and smartphones have become a more integral part of our daily life. We depend on them to stay in touch with friends and family, get work done and provide directions to unknown places. Personal electronics manufacturers have focused on delivering portable devices that last longer on a single charge and take less time to charge to support our busy lifestyle.

The most common port for portable electronics is USB, and USB protocols have evolved to transfer data faster and deliver more power. For example, the USB 2.0 standard downstream port (SDP) has been around for a long time; it can transfer data and provide up to 500mA of charge current. However, the most recent USB Power Delivery (PD) protocol enables up to 5A and 20V (100W) of power. This higher power output means that USB PD ports can quickly charge small electronics and efficiently charge larger electronics such as laptops. Additionally, automotive manufacturers are installing USB ports in automotive head units, media hubs and rear-seat entertainment consoles.

With the USB PD port providing up to 100W of power, there is a greater need to ensure proper thermal management. An overheating USB PD port can damage critical components like the USB charge controller, or may become unsafe to touch. Personal electronics and automotive designers can help protect against overheating by leveraging a USB PD charge controller with a built-in thermal shutdown. For example, the TPS65981 will shut down the power path when the die temperature exceeds 135°C, thus helping keep the USB PD charge controller from burning up and permanently disabling the USB port. However, the temperature-sensing element inside the USB PD controller is targeted at measuring its own die temperature and is susceptible to errors caused by self-heating. Many designers, specifically automotive designers, will implement the external circuit shown in Figure 1 to have tighter control over the overall USB system temperature.

Figure 1: Thermistor and comparator temperature circuit

The thermistor and comparator in Figure 1 create a switched output that will notify the microcontroller (MCU) when exceeding a pre-determined temperature threshold. The most commonly used thermistor for this type of external temperature protection is the negative temperature coefficient (NTC) thermistor. Most NTCs have their resistance specified at 25°C, a nonlinear response as temperature increases and low sensitivity at high temperatures. It is possible to improve NTC performance by performing thermistor calibration, but that will increase the overall system cost.

Semiconductor-based temperature sensors can now replace discrete temperature circuits like the one shown in Figure 1. For example, the TMP302 and automotive-qualified TMP302-Q1 are both temperature switch products that will send an alert signal when a pre-determined temperature threshold is exceeded. This behavior enables you to replace the thermistor and comparator temperature circuit with temperature switches like the TMP302; see Figure 2. Additionally, the TMP302-Q1 has a more linear response over a wide temperature range for improved accuracy and does not require any device-level calibration. The TI temperature switch portfolio also includes more advanced features like selectable temperature hysteresis and dual threshold detection.

Figure 2: Temperature switch IC vs. discrete temperature switch

Temperature switches with an active low alert signal, like the TMP302 and TMP302-Q1, enable the implementation of USB PD port overtemperature protection without the need for an MCU. This is accomplished by tying the active low alert/temperature switch output to the enable of a USB PD charger like the TPS65981. During normal operation, the temperature switch alert signal will be high, which will keep the TPS65981 enabled. When the temperature switch detects an overtemperature condition, it will pull the alert signal low, which will disable the USB charge controller; see Figure 3.

Figure 3: USB overtemperature protection without an MCU

USB PD ports provide a great way to quickly charge small portable electronics and even replace the charging cables currently used for laptops. With the increased power density and small solution size, concerns about proper thermal management are valid. TI offers several solutions that enable you to fully leverage the power-delivery capabilities of USB PD and maintain a safe operating temperature for both the system and end user.

Additional resources

Check out TI’s temperature switch solutions.
Learn more about temperature sensor integrated circuits and their advantages compared to thermistors.
View the entire temperature sensor portfolio.

Accelerate your ECG design time with delta-sigma ADCs

2017-06-08T18:55:00Z

The electrocardiogram (ECG), which measures electrical signals produced by the heart, has benefited greatly from technological advances. These technological evolutions have resulted in less invasive, more accurate medical procedures and diagnostics and improved patient experiences.

Whereas medical analog front ends (AFEs) were once implemented using discrete components, technological advances enabled manufacturers to integrate most of these components into a single device, making designs simpler and quicker to complete. One of the biggest challenges to designing high-quality ECG systems is acquiring quick and accurate measurements. Portable ECG equipment in consumer and ambulatory applications requires minimal solution size and power consumption, adding to the challenge.

The main function of an AFE is to take electrical signals from the real world and digitize them. However, ECG measurements require several key features that are often designed discretely. Devices that provide a highly integrated solution for a medical AFE include several commonly used features for biopotential measurements. In addition, the architecture of delta-sigma analog-to-digital converters (ADCs) use oversampling and noise shaping to offer high signal-to-noise ratio (SNR), which enable measurements of the smallest of ECG signals. The performance specifications of these devices enable you to design system solutions that can help satisfy International Electrotechnical Commission (IEC) and Association for the Advancement of Medical Instrumentation (AAMI) requirements for medical end equipment.

The ADS1298 family, for example integrates several components with the ADC itself, including a reference voltage, a master clock oscillator and programmable gain amplifier. In addition, these devices incorporate many features that are important when designing ECG systems, including:

Continuous lead-off detection – If an electrode becomes disconnected, the device provides immediate notification. Alternatively, the physician can monitor the contact quality of the electrodes, which may degrade over time.
Right-leg drive circuitry – A right leg circuit biases a patient’s biopotentials relative to the measurement subsystem and drives the patient’s body with an inverse common-mode signal. The common-mode signal increases the common-mode rejection ratio (CMRR). This feature is crucial to reducing interference from power-line mains.
Pace detection – Dedicated pace detection amplifiers enable external processing of pacemaker signals.
Respiration – Devices like the ADS1298R have integrated impedance detection circuitry that enables medical personnel to take respiration measurements.
Channel flexibility – An integrated daisy-chain feature enables designers to easily chain multiple devices together to design high-channel-count, simultaneously sampling systems with fewer components.
Wilson’s central terminal – Wilson’s central terminal (WCT) is an average of the electrical potential in the body calculated from the three primary leads. ADS1298 devices include internal buffers and a summing junction to output the WCT voltage. Single-ended precordial electrodes can then use WCT as the reference.

With high levels of integration and features and exceptional device performance, the ADS1298 device family accelerates the development of precision ECG equipment. Just as the ADS1298 family fits into ECG devices, they also fit into many other types of medical equipment, such as electromyographs (EMGs) and fitness wearables. In future blog posts, I’ll delve into how the high level of integration and device performance of the ADS1298 family enables many other different biopotential measurement applications. To get posts like this delivered to your inbox, sign in and subscribe to Analog Wire.

Additional resources

Download the ADS1298 data sheet.
Learn what TI has to offer for your medical design.

What are the building blocks of high-definition audio systems?

2017-06-06T21:00:00Z

Audio system designers, responding to customer demand for top audio quality, are looking into high-resolution (hi-res) or high-definition (HD) audio as more and more mid-tier system buyers demand the type of HD audio performance that was previously available only in high-end systems. In the past, a 44.1kHz CD-quality sampling frequency may have been sufficient for much of the market, but today (and for the foreseeable future), high-fidelity sound will only continue to increase in popularity.

According to professional and consumer audio equipment companies, a higher sampling frequency captures and reproduces a much wider frequency range. The reproduction of audio frequencies greater than 20kHz, including ultra-high-frequency harmonics, gives the subtle components of a sound (especially acoustic instruments) their character. According to these audio equipment companies, there are technical merits that make it worth moving toward higher sampling frequencies, such as minimizing unwanted side effects due to the steep filters employed during digital-to-analog or analog-to-digital conversion.

Simply picking an off-the-shelf music player will not deliver the promise of hi-res audio, which requires dedicated hardware to really enjoy its richness and subtleties. Of course, not every audio file or medium is recorded in HD audio either.

Designing an HD audio system

As with every high-performance design, taking a systemwide approach to the entire audio signal chain ensures a robust and high-performance solution. Because each link in an audio signal chain is important, it is only as strong as its weakest link; each and every link must be capable of meeting all design target specifications, such as performance, cost, time to market and ease of use. Figure 1 shows the basic blocks inside an HD system.

Figure 1: HD audio system

The power-management block provides the right power levels to the Wi-Fi® speaker circuitry. Assuming a system-powered application (not battery-operated) and depending on the output power, the power supply not only needs to deliver the voltage and current required to match the system’s power requirements; it also needs to provide a clean and stable power rail to prevent any power noise from entering the audio system and degrading the audio quality.

The connectivity block provides wireless communication to the system using Wi-Fi or Bluetooth® from a computer, smartphone, tablet or wireless-enabled product. Standard Bluetooth modules today offer complete solutions for portable audio systems, as they support wireless and wired-in audio natively. But their limited bandwidth poses a potential bottleneck given the higher bandwidths that lossless hi-res audio streams require. New Bluetooth audio-coding technologies promise to increase the bandwidth to support HD audio.

Wi-Fi, on the other hand, includes greater network capacity, robust signaling and an extended radio-frequency range over alternative short-range wireless technologies. Its increased bandwidth capacity and system throughput make it more suitable for HD audio applications.

The processor executes various audio processing functions like decoding and signal equalization while also processing the Wi-Fi and/or Bluetooth communications software stacks. In the past, a CD-quality sampling frequency may have been sufficient. But today’s high-fidelity audio systems place additional demands on the processor in the signal chain, as high-fidelity sound-quality requirements grow to provide sampling rates in the range of 48kHz to 192kHz on a 24-bit music signal stream.

The audio block contains all of the electronics necessary to drive the speakers in the system. Because the signal coming from the connectivity block and processor have both low-voltage and low-current capabilities, an audio amplifier provides the signal with the necessary higher voltage and current capabilities to drive the drivers in the speaker system. This block may include an audio digital-to-analog converter (DAC) to convert the digital audio signal from the processor to analog, and to provide additional audio processing in the digital domain to further enrich the customer experience in higher-end systems. Another key component of the audio block is the audio amplifier.

Audio amplifiers: Class-AB vs. Class-D

You have two choices when selecting the best audio amplifier for your HD audio systems: Class-AB or Class-D. Class-AB audio amplifiers are linear amplifiers that do not require many external electronic components. But they are highly inefficient and require substantial passive or even active thermal management in the form of heat sinks and fans.

On the other hand, Class-D audio amplifiers are highly efficient switching amplifiers that need very little thermal management, but they do require output inductors.

For years, the amplifier of choice in HD audio was a Class-AB amplifier. Audio Class-D amplifiers were considered substandard because they didn’t meet all of the requirements for HD audio; not anymore. TI’s latest generation of Class-D amplifiers boost the overall performance and efficiency of HD audio systems by creating a symbiotic relationship with the digital engine while efficiently transferring enhanced audio quality to the speakers.

Optimizing HD audio design

High-performance HD audio amplifiers with an integrated DAC and processing, like TI’s new TAS5782M, can help audio designers greatly simplify their designs, reduce costs and shorten design cycles. The integrated DAC helps reduce system complexity by providing a way to connect the audio amplifier directly to the processor, ensuring maximum signal integrity and reduced component count.

Integrated audio processing can greatly help reduce the cost and computational requirements of the main applications processor by offloading all of the audio processing into the amplifier itself. You can further reduce cost and size by reducing the cost and size of the main processor. Figure 2 is an optimized block diagram.

Figure 2: Optimized HD audio system

Have you designed an HD audio system? If so, what specifications were most important to you? Log in and leave a comment below, or visit the TI E2E™ Community Audio Amplifiers forum.

Additional resources

Start designing an HD audio system quickly with the TAS5782M evaluation module.
Check out TI.com/audio to browse the entire audio portfolio.
Read the application note, “TAS5782M Process Flows,” which shows how the TAS5782M’s easy-to-use hybrid flows can help you simplify your design cycle time.

System trade-offs for high- and low-side current measurements

2017-06-01T15:30:00Z

Measuring current may sound like a simple task, but it is not as easy as it sounds. Current can’t be sensed directly; however, it is related to other measureable parameters that you can sense directly, such as voltage and magnetic field density.

In general, there are two ways to measure current. The first is by measuring the magnetic field density generated around a current-carrying conductor. This method is suitable when you require noninvasive current measurements, but you’ll need to justify its relative high cost and complex implementation. The second is to use a small shunt resistance and measure the differential voltage across it that results from the current flow, a direct implementation of Ohm’s law. This method is common due to its high accuracy and low implementation costs. Figure 1 shows the two different methods and underlying physics behind each.

Figure 1: Current-sensing methods and their underlying physics

In this blog post, I will discuss the resistive-based method that utilizes Ohm’s law across different implementations and present some of their system benefits and tradeoffs.

Depending on the application and use of the current to be measured (protection, system monitoring or control), you can place the shunt resistor either between the supply and the load or between the load and ground. The first placement is called high-side sensing (sensing the current entering the load), while the second placement is called low-side sensing (sensing the current leaving the load). Table 1 summarizes those two approaches.

	High-side sensing	Low-side sensing
Implementation	Differential input	Single or differential input
Ground disturbance	No	Yes
Common voltage	Close to supply	Close to ground
Common-mode rejection ratio (CMRR) requirements	Higher	Lower
Can detect load shorts?	Yes	No

Table 1: High- vs. low-side sensing

Many engineers choose low-side sensing for cost-sensitive applications. Figure 2a shows the most common approach, using a single-ended operational amplifier (op amp). This approach is easy and inexpensive to implement; however, the main trade-off here is accuracy. With this method, parasitic resistances and the temperature coefficients of the resistive gain network will significantly affect accuracy. You can read more about these errors by looking at the 50mA-20A Single-Supply, Low-Side or High-Side Current Sensing Solution precision reference design and TI TechNote, “External Current Sense Amplifiers vs. Integrated On-Board Amplifiers for Current Sensing.”

Figure 2: Main low-side op amp implementations for current measurements

Another pitfall is that you can’t detect load short faults. You can mitigate the accuracy issues related to ground disturbance and parasitics by using the differential configuration shown in Figure 2b. Accuracy will still be limited by the CMRR and drift of the solution, which are a function of the op amp and the matching of the gain resistors. The better the CMRR and drift, the higher the cost of the solution.

To overcome these issues, TI introduced dedicated current-sensing solutions. The INA180 family of devices is a great example that offers high accuracy for cost-sensitive applications. The INA180 family features a high 350kHz closed-loop bandwidth and a fast 2V/µs slew rate that enable their use in applications such as constant current regulators, power supplies, and motor drives, which benefit from such features. Figure 3 shows the basic application diagram of the INA180 with some key specifications.

Figure 3: INA180 basic application diagram and key features

A current-sensing location varies depending on the application and intended use of the measured current. Low-side sensing is preferable for cost-sensitive applications that can tolerate ground disturbances and load shorts. High-side sensing is preferable when ground disturbances cannot be tolerated and load-short detection is required. Traditional discrete implementations compared to dedicated current sensors tend to have limited accuracy, a bigger solution footprint and a relatively higher cost for the same accuracy level.High-side sensing addresses the issues of load short detection and the need to eliminate ground disturbance. The main challenge is the high common-mode voltage that the amplifier needs to tolerate. This challenge, in addition to the accuracy challenges of discrete implementations and cost trade-offs, pushes design engineers to consider other solutions. Again, dedicated current sensors are the perfect solution because they offer high accuracy, low cost and a high common-mode voltage. The INA180 is another great solution for high-side sensing, as the common-mode voltage of the device can go up to 26V.

The INA180 offers a great combination of performance, solution size and design flexibility at the right price point for cost-sensitive applications that are well suited for the main drivers for new electronic devices – cost, performance and size. To get posts like this delivered to your inbox, sign in and subscribe to Analog Wire.

Additional resources

Learn more about TI’s portfolio of current-sense amplifiers.
Get started with the Current Sense Amplifiers video training series.
Read these TI TechNotes:
- “Precision, Low-Side Current Measurement.”
- “Low-Side Current Sense Circuit Integration.”

How to bias PIR sensors to prolong battery life in wireless motion detectors

2017-05-30T15:00:00Z

Wireless, battery-operated, passive infrared (PIR) motion detectors (represented in Figure 1) are gaining popularity in monitoring systems for smart buildings. In these systems, for a single CR2032 coin-cell battery to power the entire circuit close to the circuit’s lifetime, both the PIR sensor and related analog front end (AFE) must have current consumption below 3μA.

In this blog post, I’ll answer some hypothetical questions about PIR transducers and how careful design can significantly lengthen battery life in wireless motion detectors.

Figure 1: Block diagram of a wireless battery-operated PIR motion-detector system

What are PIR transducers?

PIR motion-detector transducers consist of two or more pyroelectric elements within a package. The PIR transducers generate a signal due to a change in the thermal energy pattern caused by a moving object in the field of view. The wavelength of the infrared energy spectrum falls within 1µm (1µm = 10^-6 meters) to 1,000µm. The human body’s radiated thermal energy wavelength is between 7µm and 15µm (typically 12µm). PIR motion sensors mainly operate in the far IR wavelength range of 4µm to 20µm.

Why not power cycle the PIR sensor to reduce power?

The PIR sensor takes many seconds to ramp from power up to produce a stable output signal. Thus, power cycling the PIR sensor to lower power consumption is not practical in PIR motion-detector applications because motion is not detectable during power up. Therefore, the sensor and related analog interface signal path must be always on.

How can I reduce PIR transducer current consumption?

A simple solution for reducing PIR transducer current consumption is to starve the PIR transducer by using a 3.0V supply (the PIR’s supply range is 2V to 15V) and use a very large value for the bias resistor, R2 (for example, 1.3MΩ) instead of the manufacturer-recommended value of 47kΩ, thus reducing the sensor current consumption from a fraction of a milliamp to about 0.5µA. See Figure 2.

Figure 2: Surface-mount PIR sensor, Fresnel lens, related passive components and output signal

What is the penalty for starving a PIR transducer?

Reducing the PIR transducer comes at the expense of decreased sensitivity and higher voltage noise at the sensor output. This is a fair trade-off for a drastically increased battery lifetime. With proper filtering and additional gain in the analog path, you can minimize the drawbacks of the biasing method.

What type of circuit compensates for these losses?

Figure 3 shows a solution comprising a two-stage active bandpass filter with 3dB corner frequencies of 0.7Hz and 10Hz and a total gain of 94dB. The entire analog path was implemented in the TI TLV8544 quad nanopower operational amplifier (op amp), designed specifically for cost-optimized applications. The values of the components for the PIR interface circuit is provided in Table 1.

Figure 3: The analog stages for filtering and amplifying the sensor signal

Table 1: Passive component values of the PIR interface circuit

What is the total current consumption of the PIR and analog path?

You can find the total consumption of the analog interface plus the sensor with the equation below,

where is the current consumption of the PIR transducer; is the leakage current of the reference generator network to ground and is the current consumption of the TLV8544.

In our solution, the current consumption is 2.16µA, helping to lengthen the lifetime of the CR2032 coin-cell battery.

Conclusion

There is increasing demand to extend the battery life of wireless PIR motion detectors. Power cycling the sensor and related analog interface is not practical, but there are other ways to lower the PIR sensor current consumption drastically. Learn more about PIR design in the application note, “Design of Ultra-Low Power Discrete Signal Conditioning Circuit for Battery-Powered Wireless PIR Motion Detectors.”

Additional resources

Download the TLV8544 nanopower op amp datasheet.
Evaluate nanopower op amp functionality as an AFE in a PIR motion-detector system using the TLV8544 BoosterPack and find additional information in the user’s guide.

How to select a redriver or retimer for HDMI 2.0 jitter cleansing

2017-05-01T15:00:00Z

Signal integrity has always been a priority for high-speed signal interfaces like High-Definition Multimedia Interface (HDMI), which runs at 6Gbps. As the HDMI 2.0 signal travels from a graphics processing unit (GPU), central processing unit (CPU) or Platform Controller Hub (PCH) to an end receiver such as a TV or monitor, several factors can cause jitter on the signal path, resulting in bit error rate (BER) at the receiver and unsatisfying video quality.

There are two different kinds of system jitter: deterministic and random.

Deterministic jitter includes:
- Inter-symbol interference, caused by frequency-dependent attenuation or reflection from the channel.
- Duty-cycle distortion, caused by propagation delay differences between a signal’s rise and fall time.
- Periodic jitter, caused by external noise such as from DC/DC regulators.
- Random jitter is caused by thermal noise, resulting in unpredictable timing distortion in the signal.

To clean up system jitter, correct signal-integrity issues, and reduce or eliminate BER, you can use a signal conditioner such as a redriver or retimer:

A redriver performs signal conditioning through equalization, providing compensation for input channel loss from deterministic jitter such as inter-symbol interference.
A retimer is a mixed-signal device that includes equalization functions plus a clock data recovery (CDR) function to compensate both deterministic and random jitter, and in turn transmit a clean signal downstream.

For HDMI 2.0 interfaces in PCs, DVD players, gaming devices, set-top boxes or TVs, I would consider using a signal conditioner before HDMI signal transmission out of the system, or at the HDMI receiver to ensure compliance and interoperability.

So between the two types of signal conditioners, when do you choose a redriver and when do you choose a retimer?

In video-source applications like DVD players, PCs, notebooks and gaming systems, the majority of jitter is caused by printed circuit board (PCB) trace length insertion loss, which is rather predictable. A redriver is a perfect solution for fixing inter-symbol interference jitter such as from long traces or long cables, because the signal will always attenuate by a certain amount when going through a fixed distance.

If additional jitter becomes prevalent from system layout and thermal noise such that phase and timing distortion occurs in the signal, a retimer will be a better solution because the equalizer and CDR inside the retimer can improve signal strength and correct random jitter.

In a video sink-side application such as a high-definition television (HDTV) and monitor, the receiver will typically face an unpredictable signal input caused by different video sources or HDMI cables of different qualities and length, possibly resulting in random jitter. A retimer is always a safe solution because a good one will have adaptive equalization. It can adjust equalizer settings based on the input signal. CDR inside the retimer can help remove random jitter to ensure signal quality.

You could also use a redriver in the sink side if the system setup is rather fixed, and the main input jitter is caused by insertion loss. You can correct this by amplifying the signal through equalization.

Another aspect to keep in mind is that because a retimer has more functionality than a redriver, a retimer can solve the majority of jitter issues in a system, resulting a better performance (with the trade-off of higher cost and higher power consumption).

Texas Instruments has scalable redriver and retimer solutions to address HDMI 2.0 signal-integrity challenges. The TDP158 is a 6Gbps AC-coupled transition-minimized differential signaling (TMDS) to HDMI 2.0 level shifter and redriver, while the SN65DP159 is a 6Gbps AC-coupled TMDS-to-HDMI 2.0 level shifter and retimer. Both devices are pin-to-pin compatible to enable flexible design choices. For example Figure 1 shows in notebook PC, you can place either TDP158 or SN65DP159 at the HDMI connector based on system design needs to ensure HDMI2.0 signal output quality.

Figure 1: Adding TDP158 or DP159 for HDMI2.0 interface to enable better HDMI signal quality

The TDP158 is a cost-effective and power-efficient solution that provides programmable equalization up to 15dB; additionally, it has transmit swing, pre-emphasis and slew-rate control for fine-tuning system performance. The SN65DP159 retimer supports both fixed and adaptive receiver equalization. Using adaptive equalization, the gain will adjust automatically based on the input signal to compensate for variable trace or cable loss. The CDR inside the SN65DP159 can clean up random jitter to deliver a clean signal downstream and help pass system compliance.

By selecting the right redriver or retimer for the HDMI 2.0 interface, you can achieve optimal system performance with the widest interoperability.

Additional resources

Download the TDP158 and SN65DP159 data sheets.
Read these related blog posts:

Quick fixes to common I2C headaches

2017-04-24T15:00:00Z

If you’ve ever programmed anything in your life, you know the feeling of relief that comes with writing your code and having it work with no errors on your first try. Back in the days of Computer Science 101, this was much more feasible. Unfortunately, it rarely happens in the real world, although trial and error can sometimes be how we learn.

Take designing an Inter-Integrated Circuit (I²C) bus, for example. Many designers use I²C for its simplicity – however, the specification is still 64 pages of rules on how to communicate with the protocol. When your first printed circuit board (PCB) comes back and your I²C bus isn’t communicating properly, you’re in for a long night and a huge headache. So in this post, I’ll identify two very common I²C bus problems and introduce solutions to address them.

Headache No. 1: static voltage offset mismatch

I²C slave devices send a signal to the I²C master when they have data to transmit, and they do this by pulling the bus to a low state. A common headache with I²C bus design is when the output low voltage (V_OL) of one device is not low enough to meet the threshold of another device’s input low voltage (V_IL) requirement. The I²C standard states that a “low” is 30% of the V_CC value, and anything above 70% of V_CC is a “high.” This leaves an undefined region between the two thresholds, as shown in Figure 1.

Figure 1: I²C voltage-level thresholds

Ideally, your V_OL spec should be as close to 0V as possible, in order to eliminate any potential to hit the undefined range after considering other parasitics such as series resistance or high current loads. Previously, most I²C devices (such as the TCA9517) had a static voltage offset, which results in a V_OL of ~0.5V. However, TI has developed a new architecture that eliminates the requirement for a static voltage offset, resulting in a V_OL of ~0.2V for the TCA9802, one variant of this new family of TCA980x products.

Headache No. 2: rise-time violations

One benefit of I²C is that it can communicate with multiple devices on the same bus, as long as you limit the capacitance to 400pF. However, additional considerations come into play here as well. Headache No. 2 is often associated with rise-time issues caused by systems with complex I²C buses, either caused by having many devices on the bus or long board traces. This type of heavily loaded bus will result in large capacitance with traditional I²C buffer devices, which will in turn slow down the rising edge of your signal. The new series of I²C buffer devices uses an integrated current source to drive the bus, rather than pull-up resistor-based devices (shown in Figure 2).

Figure 2: Architecture of a traditional I²C buffer (a) vs. the TCA980x buffer (b)

This constant current drive enables a quicker rise time and a more controlled falling edge, resulting in much stronger signal integrity, as shown in Figure 3. The light blue signal in Figure 3 shows the new current source-based architecture compared with a competitor solution shown in dark blue. This new architecture helps prevent ringing on the falling edge, which will also prevent noise reflections on the bus.

Figure 3: TCA980x rise-time benefit vs. the competition

TI’s new architecture takes these common design headaches into consideration and provides simple solutions to prevent them. I²C is a standard that intends to simplify designs, which is exactly what the new TCA980x family helps do.

For more details about solving problems with this device family, read the application report, “Advantages and Design Considerations of the TCA980x Family” or see the full portfolio.

Additional resources

Learn more about TI’s portfolio of I²C buffered translators, I/O expanders and switches.
Find the right device for your application with the application report, “Choosing the Correct I²C Device for New Designs.”
Watch this video which provides additional details about the the TCA980x family benefits.

Three reasons why a bidirectional I/O will simplify your next 4K video design

2017-04-18T18:00:00Z

As 4K video becomes the norm for the professional video industry, high-quality serial digital interface (SDI) components and meticulous board layout are imperative for a high-performance end product. In addition, flexibility, scalability and cost savings are necessary to maximize design reuse, whether you are looking to expand your 12G product portfolio or aiming to transition from 3G-SDI to 12G-SDI.

A bidirectional input/output (I/O) addresses these critical needs. A bidirectional I/O is a device that you can configure as either a receive cable equalizer or a transmit cable driver through the same port. Let’s look at how this flexible device simplifies 4K video interfaces.

Reason No. 1: To enable design flexibility

Traditionally, SDI designs have a fixed number of input and output ports. Since cable equalizers and cable drivers are not interchangeable at the same port, designing a new system is necessary whenever you require a different combination of inputs and outputs. With a bidirectional I/O, you can easily support multiple configurations of inputs and outputs with the same design, as illustrated in Figure 1.

Figure 1: With a bidirectional I/O, a single design supports multiple input and output port configurations

TI’s latest 12G bidirectional I/O, the LMH1297, also enables dynamic port provisioning, meaning that end users can configure the port as an input or output on the fly. This design flexibility and scalability reduces both overall development time and the cost of stocking unique boards to support each port-configuration combination.

Reason No. 2: To minimize board space and bill-of-materials cost

In a traditional design, two ports support input and output functionality, resulting in a four-chip solution. A bidirectional I/O minimizes board space by reducing the overall number of ports. With a bidirectional I/O, you only need one port, and thus a single-chip solution. Comparing these two design approaches in Figure 2, a bidirectional I/O significantly reduces the number of board components.

Figure 2: TI’s bidirectional I/O reduces the overall number of ports, while its on-chip integration reduces the number of external passive components required.

TI’s new 12G bidirectional I/O takes minimization and cost savings a step further. The LMH1297 has an integrated reclocker, return loss network and terminations. The integrated reclocker ensures a clean output signal with minimal jitter. Meanwhile, the integrated return loss network and terminations eliminate the need for an external return loss network, not to mention time spent fine-tuning these network parameters.

The LMH1297 integrates an additional 75Ω loop-through cable driver output and a 100Ω loopback printed circuit board (PCB) driver output. You can use these additional outputs to expand signal distribution efficiently and improve system diagnosis capability, without extra cable drivers or reclockers to support the same system functionality. Applications for the additional loop-through and loopback driver are shown in Figure 3.

Figure 3: The additional driver outputs in the LMH1297 simplify signal distribution when the I/O is configured as either an input (EQ Mode) or output (CD Mode).

Furthermore, TI’s bidirectional I/O is implemented on a single-die solution. Traditionally, bidirectional I/Os are designed as a multichip module (MCM), which adversely affects overall performance compared to a stand-alone cable equalizer or cable driver. With the single-die approach, the LMH1297 achieves performance equivalent to or exceeding that of many other stand-alone cable equalizers and drivers. These features are offered in a 5mm-by-5mm very thin quad flat no-lead (WQFN) package.

Reason No. 3: To provide an easy upgrade path

Before taking your first steps to designing with a bidirectional I/O, it is worth considering whether alternative upgrade options are available to prepare current designs for the next generation. As the SDI community trends upward from 3G-SDI to 12G-SDI, having a pin-compatible upgrade path makes sense to minimize board redesigns and future-proof your products.

The LMH1297 comes with several pin-compatible alternatives in an identical package for easy upgrade. These alternatives are also software compatible. As shown in Figure 4, the LMH0397 is a 3G bidirectional I/O with an integrated reclocker, while the LMH1228 and LMH1208 are 12G dual cable drivers.

Figure 4: LMH1297 pin and software compatible portfolio

Why re-invent the wheel for each project? Maximize your efficiency and simplify your next SDI design with a bidirectional I/O.

TI will be exhibiting at the National Association of Broadcasters (NAB) Show in Las Vegas April 24-27. To learn more about TI’s SDI portfolio and talk to experts developing groundbreaking new interface products, stop by booth N3527 at the NAB Show and get an edge in your 4K transition. Or log in to post a comment below or talk with other engineers in the TI E2E™ Community High Speed Interface forum.

Additional resources

Learn about TI’s latest featured 3G-SDI and 12G-SDI products.
Find out more about a 4K field programmable gate array (FPGA) mezzanine card (FMC) development module supporting the LMH1218 and LMH1219.
Check out the blog post, “Three things to consider when upgrading to 4K Ultra HD 12G-SDI interfaces.”
Read The Broadcast Bridge article, “What You Need To Know About Return Loss.”

TI resolver interface offers integration benefits in industrial drives and HEV/EV

2017-04-10T13:32:00Z

Many motor-drive applications (see Figure 1) typically need a position/speed feedback loop to ensure efficient system performance. For users to meet their accuracy needs there are many different technologies for position/speed sensing on the market today. In this blog post, I’ll discuss one of these technologies – the resolver sensor – as well as its related applications.

Figure 1: Basic schematic of a motor-drive system

What is a resolver?

A resolver is a well-known technology for position sensing that functions somewhat like a transformer. It consists of a stator and rotator (as shown in Figure 2) where the rotator is fixed to the motor shaft. Because a resolver does not have any built-in electronics, it is suitable for harsh environments with dusty, high-temperature and high-speed conditions.

Figure 2: A resolver comprises a stator and rotator

How does a resolver work?

Again, much like a transformer, a resolver’s stator windings receive an excitation voltage on the primary side. The rotator windings then generate a voltage through electromagnetic coupling on the secondary side. The output voltage amplitude then shows the sine-cosine correlated with the rotator’s angular displacement, as shown in Figure 3. You can find the angular displacement by converting the output signals to digital and calculating the arc-tangent.

Figure 3: The principles of resolvers are similar to the principles of transformers

Surrounding a resolver sensor are also several other necessary functions, as shown in Figure 4.

To power the resolver, the sensor needs a power stage and excitation amplifier. The power stage supplies power to the excitation amplifier, and often to the sensor itself. The exciter amplifier generates the input sine wave that the resolver sensor requires to generate the sine and cosine values.

The outputs from the resolver are analog signals that require an analog front end (AFE) to clean and condition the signals for the microcontroller (MCU) or control unit doing the angle calculation. A resolver-to-digital converter (RDC) translates the sine and cosine waves into the digital domain for processing. These outputs can communicate the angle and velocity information to the MCU through several different options, such as serial peripheral interface (SPI), parallel, emulated encoder and analog. The system also benefits from multiple protection and fault-diagnostic functions between the sensor and electrical components, such as overcurrent, overvoltage, short circuit and thermal protection.

Figure 4: The topology of resolver peripheral circuits

Where should you use a resolver?

As I mentioned, because there are no electrical components inside a resolver, the sensor works well in high-temperature, dusty, high-speed (8000rpm), high-vibration conditions. This lack of electrical components also enables the resolver to have a longer life span than other sensor technologies.

Resolvers are often chosen for industrial and automotive applications such as those shown in Figure 5. Resolvers are used in servo control systems (such as elevators), industrial robots, AC inverter drives, plastic compression systems, spinning systems and metallurgical systems. In automotive, resolvers are used in hybrid electric vehicle/electric vehicle (HEV/EV) traction inverters; heating, ventilation and air conditioning (HVAC) systems; stop-start alternators; and power-steering systems.

Figure 5: Resolver applications

Existing resolver solutions

So far, existing solutions are mainly discrete, with a topology like that shown in Figure 6. These solutions have high bill-of-materials (BOM) costs, large printed circuit board (PCB) areas and weak electromagnetic compatibility (EMC) performance. A discrete design also adds development time, especially between new platforms, because users need to edit the design to adjust to different resolvers.

Figure 6: Resolver interface discrete solution topology

TI has released the PGA411-Q1 family to help customers developing industry drives and EVs/HEVs, shorten development cycles, reduce cost and improve system reliability. The PGA411-Q1 integrates an exciter operational amplifier, boost DC/DC, AFE, RDC, and multiple protection and fault-diagnostic functions into one chip. With the PGA411-Q1, the system is simply three parts, as shown in Figure 7: the sensor, the PGA411-Q1 and the MCU.

Beyond integration, the device can provide the following benefits:

System simplification: High levels of integration reduce the total system cost through BOM and PCB savings.
Reduced development time: Engineers can adjust the output signals by modifying registers, with no need to change the peripheral circuits. It also has scalability across development platforms and resolver sensor flexibility.
SafeTI™ design packages for functional safety: Components of these design packages are certified by an independent third party up to Automotive Safety Integrity Level D (ASIL D) (International Organization for Standardization [ISO] 26262). This helps customers developing their functional safety systems get to market more quickly and easily.

Figure 7: TI resolver interface solution topology

PGA411 + C2000™ MCU combination

The PGA411-Q1, when used in combination with the C2000 MCU, offers a full TI solution. In addition to the PGA411-Q1’s functional safety standards, the C2000 Delfino™ MCU is also designed to meet compliance to SafeTI®-QM (ASIL-B/SIL-2 MCU certification is currently under assessment by and independent third party). Reference designs with the PGA411-Q1 and C2000 MCU include the EMC Compliant Single-Chip Resolver-to-Digital Converter (RDC) Reference Design for industrial applications and the Automotive Resolver-to-Digital Converter Reference Design for Safety Applications. Both reference designs have plug-and-play compatibility with the InstaSPIN-Motion (and InstaSPIN-FOC)-enabled C2000 Piccolo LaunchPad, and provide faster and simpler development.

Digital signal processing in RF sampling DACs – part 2

2017-04-04T15:00:00Z

In this blog post, I will describe mixers and numerically controlled oscillators (NCOs), which are the basic building blocks used to implement digital up converters (DUCs) in the RF sampling digital-to-analog converter (DAC) signal chain.

In the previous installment of this series, I explained the use of interpolation filters to increase the data rate of the input signal to more than twice the desired output RF signal frequency. If the final data rate after interpolation is much higher than twice the desired RF output frequency, you can easily filter out the image of the desired signal in the second Nyquist zone.

Mixers and NCOs perform the important function of moving a signal from one frequency location to another (also known as frequency translation) within the first Nyquist zone. Frequency translation occurs when a lower-frequency signal modulates a higher-frequency signal in the time domain. Common names used for the high and low frequency signals are the local oscillator (or carrier) frequency and the baseband signal frequency, respectively.

To understand how frequency translation works, consider the effect of multiplying two pure sinusoids with equal amplitude (A), equal phase (Φ) and frequencies at fc and fbb, as shown by Equation 1:

where fc and fbb are the carrier frequency and baseband frequency, respectively.

Figure 1 illustrates a case where A = 1 and Φ = 0.

Figure 1: Implementation of a real mixer

To simplify, I will only focus on a case where the desired RF signal frequency is higher than the carrier frequency, known as low-side injection. Low-side injection has an advantage in that the RF spectrum is not a mirrored image of the original baseband signal spectrum.

From Equation 1, the output of the multiplier has two signals with the same energy located at frequencies fc+fbb (this is the desired signal) and fc-fbb (this is an unwanted image). This multiplier also describes a real mixer since the carrier and baseband signals are both real valued. A significant drawback of real mixing is that both the desired signal and unwanted image have equal energy and are separated by twice the baseband signal frequency. It is not practical to filter out the unwanted image in a real mixer because the baseband signal frequency is usually low. Consequently, complex valued baseband and carrier signals when used in complex mixers produce an output at one frequency (fc+fbb) only. Equation 2 expresses the output of a complex mixer as:

Complex mixing results in a perfect cancellation of the unwanted sideband image if the gain of the real (I_IN(t)) and imaginary (Q_IN(t)) inputs to the mixer are perfectly matched and their phase offset is exactly 90 degrees. Any deviation from this requirement creates a sideband image at frequency fc-fbb, with energy proportional to the magnitude of the gain and phase imbalance. This requirement is difficult to achieve in an analog mixer where gain and phase can vary randomly with process, voltage and temperature (PVT) variations and limits the usefulness of analog complex mixers to high intermediate frequency (IF) heterodyne transmitters. However, RF sampling DACs like the DAC38RF83 use digital complex mixers for frequency translation and are able to achieve perfect cancellation of the sideband image independent of PVT variations. This makes RF sampling DACs like the DAC38RF83 a suitable solution for low or zero IF transmitters.

Although the complex mixer in Figure 2 is implemented with multipliers, at certain carrier frequencies, you can implement the mixer with inverters and by swapping the I and Q inputs to save power. This mixing mode is known as coarse mixing and is suitable for carrier frequencies that are multiples of a quarter of the sampling rate (or n*Fs/4 where n is an integer). For example, you can implement an Fs/2 mixer by simply inverting every other I and Q sample.

Figure 2: The implementation of a complex mixer

Up to four complex mixers are implemented in the DAC38RF83 to enable multiband support. These four DUCs can be used to aggregate multiple carriers (e.g. 4x200MHz) carriers into one contiguous carrier (eg 800MHz carrier), suitable for bandwidth-demanding applications like 5G wireless networks. The input to each mixer has an optional divide by 2 to reduce the input power to the mixer by 6dB. This feature is useful to avoid saturating the mixer output when the real and imaginary inputs can be full scale simultaneously.

In the next and final installment of this series, I will continue the discussion with NCOs and inverse sinc filters.

Additional resources

Download the application report, “RF Sampling DAC with 800 MHz of IBW LTE.”
Read more about designing with RF sampling data converters.

RS-232: Use this old dog for new tricks

2017-03-30T16:00:00Z

The other day, as my dad was guiding me on a tour of his metal manufacturing shop, we stopped at a computer numerical control (CNC) machine, like that seen in Figure 1 that’s just about as old as I am. After unscrewing a latch, he revealed none other than an RS-232 interface port. We wandered to another CNC machine that he had purchased just two years ago. He unscrewed its latch, and what do you know – another RS-232 port. “Some have a USB port, some don’t,” he said. “But they all have RS-232. I don’t understand why they don’t update to USB.”

Figure 1: CNC machine

This seems to be a common question. Why is RS-232 still being used and why should you care? Being one of the first serial interfaces, RS-232 has a wide, established base. Once it’s designed in, it doesn’t cost any extra time or money to upgrade. Aside from its incumbent status, RS-232 has also earned ubiquitous use because of its reliability. While newer interfaces like USB require complex hardware and software to function correctly, RS-232 requires simple hardware and minimal software for communication. And in some situations, RS-232 can solve problems that other interfaces can’t - like if your processor is hung, you need to debug a low level issue, or you need to update the firmware without risk. All of these are things that you simply cannot do with a USB interface. For that reason alone, I don’t see the RS-232 interface disappearing anytime soon.

RS-232 also offers simplicity in implementation. It is software agnostic, meaning that it can enable serial communication between systems with their own proprietary code, such as those in my dad’s CNC machines. This protocol’s flexibility opens up a cornucopia of possibilities to interface with many different systems. For those in the DIY community looking for a cost-effective, simple method of communication prototyping, RS-232 is your answer. With RS-232, you can wander into your garage or attic, pick up an old electronic device, and give it new life.

I myself had a little fun with the RS-232 interface recently, aiming to bring older devices into the 21st century IoT world. I started with prototyping how I could connect an offline device to the online world. As you can see in Figure 2, I used TI’s MSP-EXP430G2 LaunchPad™ development kit (the “offline device”) to communicate with the SimpleLink™ Wi-Fi® CC3200 LaunchPad kit (the “online connection”) via RS-232. The RS-232 Transceiver BoosterPack™ plug-in module made prototyping much easier. For example, the INVALID function is a simple jumper and line of code away. Now, I can monitor the RS-232 connection and send an alert via text if it becomes disconnected.

Figure 2: RS-232 Transceiver BoosterPack™ kit prototype set-up

This is just one use case among many for the RS-232 interface. RS-232 can be like a “master key” of communication, opening up an endless world of new possibilities with legacy devices. What are your exciting new ideas for using RS-232? Please let me know in the comments below. And check out our RS-232 portfolio today to find the right device for your design.

Additional resources

Get started on your own RS-232 project.
Engage in TI’s online RS-232 forum.
Read more about RS-232 power consumption.
Check out an isolated RS-232 to UART converter reference design.
Try the RS-232 BoosterPack with the new MSP-EXP430FR4133 LaunchPad kit featuring the MSP430™ ultra-low power (ULP), FRAM-based microcontroller (MCU) platform.

Advancements in X-ray imaging

2017-03-28T15:00:00Z

Diagnostic imaging technologies like X-ray systems are continuously evolving to improve not just the quality of patient diagnostics but also clinical efficiency. The ability to accurately and cost-effectively capture detailed images of the human body to achieve these goals is becoming increasingly critical across small to large modern medical practices, due to the market becoming more competitive. Additionally as the market becomes more competitive, there erupts numerous advancements in the technology.

Introduced in the mid-1990s, X-ray systems capture higher-quality images at higher resolutions than ever before, all while requiring less time to capture. Digital X-ray systems have revolutionized diagnostic radiology. Many of the improvements in X-ray systems are a direct result of the advantages of digital capture over traditional film capture.

Digital X-ray systems have a higher dynamic range than film, which allows for clearer, more detailed images. Since digital X-ray systems do not require extensive processing, there is a significant reduction in the time it takes to deliver images to radiologists and ultimately patients. Digital capture also improves image-processing capabilities such as spatial zooming and contrast enhancement. Finally, there are more convenient storage options for digital images than film, and digital storage reduces the amount of polluting waste products.

Given all of the improvements over the years, X-ray systems have become very complex and require an analog front end (AFE), a digital signal processor (DSP), an LCD, power supply design, sensor controls and a few other external components. The block diagram in Figure 1 shows the readout electronics required for direct imaging to convert a charge in a flat panel detector (FPD) to digital data. There are two chains: the acquisition chain and the biasing chain. At the start of the acquisition chain, an AFE circuit (sometimes called the readout integrated circuit) digitizes the charges from the array of pixels on the FPD. The biasing chain generates bias voltages for the thin film transistor (TFT) array through intermediate bias and gate control circuitry. A DSP, field-programmable gate array (FPGA), application-specific integrated circuit (ASIC) or a combination of these applies signal conditioning. These processors also manage high-speed serial communications with the external image-processing unit through a high-speed interface.

Figure 1: X-ray system

Devices like the AFE2256 AFE are specifically designed for FPD-based digital X-ray systems. This device can be used for static or dynamic imaging and also features multiple sleep and power-down modes that are especially useful for battery-powered systems. The AFE2256 has 256 channels and an onboard 16-bit ADC.

With the evolution of X-ray systems, there is an increasing demand for clearer, more detailed images, faster scan times, better storage options and simpler power-supply schemes. The ability to accurately and cost-effectively achieve these goals becomes easier with a device like the AFE2256. There are a few key differentiators of this device to others on the market. The AFE2256 is very easy to use; there is a simple power supply management scheme enabling smaller board space and less external circuitry. This device also has low power consumption, especially at low integration times, which enables longer battery life. There is also low noise: 750 electrons RMS with 1.2pC input charge range, which enables lower dosage and dynamic imaging resulting in clearer images. Faster scan times (less than 20µs) also allow for more efficient image captures.

TI has a long history and continually invests in digital X-ray technology. To learn more, check out our entire X-ray portfolio, including the AFE2256.

Additional resources

Download the AFE2256 data sheet.