Amplifiers

Forum Post: OPA836 linear output range question

Gerard Visser — Fri, 24 May 2013 01:55:00 GMT

I note two facts about the OPA836:

- The output range is only specified in the datasheet at a gain of +5

- The PSPICE simulation of an inverting amplifier at low gains seems to show saturation effects well within the datasheet linear range (0.2 to 4.75 at 5V supply). [In particular, I am looking at G = -1.65, VCC = 5V, Vin+ = 2.728V, Rf=1.65k, Rg=1k, Vsource=4.238 V +Vac, then the output AC gain is less than expected 1.65,]

My real question is, is the datasheet guaranteed linear output range spec actually something I can count on at an inverting gain of 1 or a bit more??? Or is there some kind of intermediate-stage signal swing limitation in the OPA836 that I need to worry about.

Of course, I will build a prototype and see. SPICE is just for picking the general form of the circuit and which parts to use. But, I'd like to have some more confidence that I am going in the right direction by picking the OPA836.

If anybody can confirm the 0.2 to 4.75V output range for 5V supply with G=-1 (or a bit more like I need), I would appreciate to hear from you. Thanks!

[p.s. In case you are wondering about the DC bias point mentioned above, this is for a unipolar pulse application.]

Forum Post: RE: High Speed Current Sense

Mayrim Verdejo — Fri, 24 May 2013 00:01:00 GMT

Hi Eli,

Thanks for reaching us to find a solution for your application. As we anticipated before, there is no current shunt monitor in our product line for the 10MHz speed requested.

Although, looking at our applications notes we found a High-Side Current Monitor circuit that, probably, you can use as a guide to create your own current monitor with the specs you are looking for. (You can find this circuit in OPA333 datasheet, Page 10, Figure 23) http://www.ti.com/lit/ds/symlink/opa333.pdf

For this circuit you will need a High Speed Amplifier. Looking at our products we found OPA836 (http://www.ti.com/product/opa836 ) which have: Supply Voltage: 2.5V to 5.5V, Quiescent Current: 1mA, high speed. This is just a general idea, hope it can help or guide you in a good direction.

- Mayrim

Forum Post: amplifier 1Hz - 10 MHz

Prit Patel — Thu, 23 May 2013 21:02:00 GMT

Hi,
I am working on a project and i want an amplifier ranging from 1 Hz - 10 MHz and a power of 1W. I don't wont an op amp, either i need whole amplifier or an evaluation kit. Can you please suggest me some of them.

Thank you

Regards,
Prit Patel

Forum Post: RE: OPA2188

Bruce Trump — Thu, 23 May 2013 20:43:00 GMT

Daniel,

The OPA2188 has diode clamps across the input terminals. See this link for a discussion of how this can affect use as a comparator...

Op Amps used as Comparators—is it okay?

These clamps will behave as a diodes with a series resistance of approximately 1k-ohm. If you limit the differential input voltage to a few hundred millivolts, it could be used as a comparator and you probably would not notice the clamps. If you have enough differential voltage to really turn on the clamp diodes, expect that pretty high currents could flow. I suggest that the differential input voltage should be limited to 5V or less at which point you would have several milliamps of input bias current.

Regards, Bruce.

Forum Post: RE: OPA129 and LMC6001 offset voltage

Bruce Trump — Thu, 23 May 2013 19:33:00 GMT

Roxana,

Both of these op amps have extremely low input bias current that should make their contribution to offset negligible.

The op amps may be oscillating. Have you looked at the output with an o-scope? Connect a 100pF capacitor across the feedback resistor. This should stabilize the circuit with virtually any photodiode. Do you have bypass capacitors on the power supplies?

Still have too much offset? To eliminate the possibility of a damaged or leaky photodiode, disconnect it. What is the offset voltage now?

Still have too much offset? Please post a complete schematic showing all the connections and biasing of the photodiode, information on photodiode type and power supply voltages.

Regards, Bruce.

Forum Post: RE: How to stabilize OPA551/OPA551 with capacitive load?

Tim Green1 — Thu, 23 May 2013 18:25:00 GMT

In general for Gains less than 5 use the OPA551 which is unity gain stable. Due to your cap load and the new recommended compensation you can use OPA552. See attached for new compensation for stability. Inside the PowerPoint are embedded TINA SPICE files you can open and run with TI free SPICE simulator by link included in attached. In addition I reference in attached an op amp stability article series that will give you all of the theory and techniques you need to understand this problem and future ones.

Forum Post: RE: LM139 Output Failure Mode

Nonis — Thu, 23 May 2013 18:04:00 GMT

Ronald,

I'm using LM139 to monitor the current of one circuit and in overcurrent it turns off the circuit by a relay. After, a microprocessor from another circuit can turn on the relay again. I need the output of one failed LM139 remains low, so that the processor can operate the relay without problem.

There is a pull-up resistor on LM139 output and I can't see another way to design it, ensuring the low output in case of LM139 failure.

Forum Post: RE: 24V Battery operated system (INA114, XTR110) picking up noise

John Caldwell — Thu, 23 May 2013 17:09:00 GMT

Jayant,

The INA333 is a 5V part and will not work on the 10V supplies that you are providing to the INA114. Also, RFI/EMI filtering at the inputs is really meant to deal with higher frequency noise and won't be effective for low frequency problems. Talking to Bruce I realized that we were a bit unsure of the nature of interference you are experiencing. Is this high frequency noise, transients, low frequency mains hum, or something else? I would really recommend doing some careful investigation of your system before investing in a new PCB. One thing to try would be to remove the load cell and cable and replace it temporarily with a resistor divider directly at the input of the INA114 to simulate the output of the load cell, does the effect persist? This would help determine if interference is coupling into the circuit through the load cell and cable, or somewhere else in the signal path. The shield driver does not need to be a high precision device. An OPA171 would be suitable as long as a resistor is placed in series with the output to isolate the cable capacitance from the op amp as this can cause oscillation.

Forum Post: RE: OPA227 - Offset trimming

Pablo Martin — Thu, 23 May 2013 14:34:00 GMT

Thanks for the quick answer Bruce.

Regards,

Pablo

Forum Post: Comparator LMV7219

Vladimir Naumenkov — Thu, 23 May 2013 13:52:00 GMT

Comparator needed as a zero-crossing detector.

In the beginning was the circuit 1. Comparator never switched.

Then I did the circuit 2. Comparator works, at the 3 input of the comparator is Um = 50mV. Watched broadband oscilloscope.

The generator should have the load 10 kohm±20%. Otherwise, not required frequency stability is guaranteed! The output signal (sine wave) is symmetric with respect to zero (the output is connected through a capacitor, which is inside the generator). The signal amplitude of generator is 0.5 V.

Question: How do I estimate the value of the generator load ?

TIA

Vladimir Naumenkov www.agat.by

(Please visit the site to view this file)

Forum Post: LM48580: amplifier input resistance

Ivan Pivovarov — Thu, 23 May 2013 10:03:00 GMT

Hello!

I'm going use LM48580 connected to 2-channel DAC in parallel with some other load. DAC will have two modes:
- generating audio signal;
- generating some other signal with audio amplifier switched off.

Is it even possible to power LM48580 audio inputs while it's switched off? If it's possible, what is the value of input resistance for amplifier switched off? Maybe I should better place some resistors in series to input capacitors?

With best regards, Ivan.

Forum Post: RE: LM4675 driving headphone question.

peter zhu2 — Thu, 23 May 2013 09:31:00 GMT

Don, what's the difference between the speaker and headphone? Can i think all speakers are BTL? I have thought headphone is also a speak, which has low volume.

Is there any white paper?

Peter

Forum Post: LM7321 Unity Gain

1886332 — Thu, 23 May 2013 09:23:00 GMT

Hi,

I designed the LM7321 as a unity gain amp and got a reduced output of 15mV.

After 20 cascaded amps it is summed to 300mV.

How can I reduce it to not more than 2mV?

Thanks in advance.

Forum Post: RE: Can INA331 be used in the signal amplification of the microphone electret capsule(with built-in FET)?

Christoph Neuhaus — Thu, 23 May 2013 06:39:00 GMT

Hi,

I had cited from NationalSem Datasheet DS009063 from August 1999. Maybe You can find that on alldatashets.com.

The actual Rev_V Datasheet "SLVS044V, revised" of TI doesn´t show the diagram any more, but still it specs the Vi-Vo-differential at the top of page 3.

Also the schenatic may give a hint, in that the pass transistor is a NPN-Darlington style, which alone accounts for two diode drops Vi-Vo.

A positive voltage lowdropout-reg would feature PNPs or PFETs as pass transistors.

Btw: wouldn´t one of the newer lownoise LDO-regs with 3.3V (or even 3.0V) fixed output be feasable for the 3V supplies? Less parts counts, and superior parameters.

regards

Chris

Forum Post: RE: TAS5731 / How to calculate output level

Ravi Singh — Thu, 23 May 2013 05:38:00 GMT

Hi Satoshi-san,

Here is an approximate level estimator (checked on the TAS5731EVM)..

(Please visit the site to view this file)

Note that this is only an approximation and does not take into account second & higher order effects (like increasing RDson with thermal etc..).

The blue & yellow cells are user inputs..the green cell shows the estimated level.....Give it a try and let me know if it works out well.

-Ravi

Forum Post: RE: OPA735 Inverting amplifier problem

Saravanakumar Shanmugam — Thu, 23 May 2013 04:50:00 GMT

Marek,

one thing i noticed in my circuit, if i isolate this inverting amplifier section from next stage power supply circuit, and i tested with providing negative supply externally, in this case its working properly, but if its get connected with power supply regulator feed back pin its not working properly,

please find the complete schematic below

Wiki Page: ECG (Heart) Signal Acquisition

Bruce Trump — Thu, 23 May 2013 01:59:00 GMT

We get many requests for assistance on the design of ECG (heart signal) monitoring. Please understand that, though we have some familiarity with these designs, we are not authorities on the subject. We have never designed production equipment of this type.

Requirements for ECG signal acquisition can vary widely. Acquisition of a signal to detect heart rate, for example, may be very simple. Input circuitry for diagnostic equipment must be very high performance and is more complex and demanding. There is a whole range of applications between these extremes. We find it very difficult to judge the scale and needs of your project.

There are many tradeoffs that can dramatically affect the design and amplifier selection:

Single power supply or dual (±) power supply
Supply voltage
Current consumption requirements
Size or package type restrictions
Galvanic isolation requirements
Type of electrodes and leads
Temperature range
Expected usage and noise environment

There is no one “best” amplifier for ECG signal acquisition. The right selection may be any one of perhaps 30 different types depending on the factors listed above (and others that we can’t guess). Very good input stages can be made with general purpose op amps. One general recommendation, however, may save you some time and grief—FET or CMOS amplifiers are generally provide better signals. Their very low input bias current reduces signal artifacts produced by the varying electrode contact resistance when the subject moves.

Single supply circuitry may seem simple and appealing but these designs can be more challenging to implement. Dual (±) power supply designs are generally easier.

Optimization of tricky parts of the circuit (such as right-leg drive) can be difficult for experienced circuit designers working with hands directly on the equipment. Much depends on your electrode and lead configuration and other factors that are difficult to predict or replicate. These issues can be virtually impossible for us to diagnose from afar.

Our assistance is best limited to help with specific circuit problems once a design approach is selected. Questions such as “can you recommend a circuit” leave us bewildered. We can help with amplifier selection if we know something of the influencing factors (the list above). Otherwise, we’re just guessing.

Note to Students—

ECG signal acquisition appears to be a popular class assignment or research project. We understand why. There is much theory to be learned and practical application experience to be gained from this type of project. We don’t want to deprive you of an important learning opportunity or discourage innovation by providing a “canned solution.” Furthermore, a good solution will vary greatly, depending on the goals of the project and factors mentioned above. Good luck.

Resources:

(Please visit the site to view this file) Presentation with general background on ECG signal acquisition

(Please visit the site to view this file) An ECG Cardiotachometer Design Example

ADS1298 ECG Data Acquisition System Component, product page

INA333 product data sheet. See figures 36 and 37.

Forum Post: RE: Inqury regarding LMV358 Output Voltage vs.Output Sink Current

Atsushi Okui — Thu, 23 May 2013 01:10:00 GMT

Hello, thank you for your reply.

The requirement of 25mA sink current is transient.

We are going to select TLV342 this time.

Best regards,

Forum Post: TAS2505 Support on Codec Control SW - slac366

Miguel — Thu, 23 May 2013 00:54:00 GMT

Hi,

I downloaded the latest slac366 Codec Control SW (http://www.ti.com/lit/zip/slac366) and it does not support the TAS2505. Also, I tried the SW in this post (http://e2e.ti.com/support/data_converters/audio_converters/f/64/t/84881.aspx) and the same thing.

Could you share the correct version please?

Thank you,

Miguel

Forum Post: RE: Troubleshooting OPA445 - 5Vpp noise on protoboard

Anik Duttaroy — Wed, 22 May 2013 21:31:00 GMT

Thanks for your prompt help with this issue Bruce. The op amp is functioning correctly now on the protoboard.

best,

Anik