OPA348: Abnormal offset from the output of the amplifier

Why starting a new thread?

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1075302/opa348-offset-voltage-vs-common-mode-voltage

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1072548/opa348-settling-time-of-opa348

Kai

Hello Kai

Thanks! I start a new thread as I think it is different issue by OPA348

Hello,

Is this behavior consistent across multiple devices? Also, do you have a more granular set of data like an oscilloscope measurement that you can share?

Best,
Jerry

Hello

I don't try it on different Op. It seems the changing of offset do not link with the input signal level. So if the input signal is small, the impact is bigger. If the signal is large, the impact is less.

I do not use a oscilloscope as it could not tell the tiny signal.

I use a R/C before the amplifier to reduce the signal. The RC is 470000ohm and 200nf.

The input signal before the RC is 17mV, 25mV, 50V by a signal generator. After the RC, it is 0.5757mV, 0.8466mV, 1.693mV. Then with the amplifier for 12 times gain.

The crest factor of these data is 1.5257, 1.4907, 1.4874. Obviously, for bigger signal, the impact is less.

Detail data attached. The reference voltage is 3V with a 16 bit ADC.

I need TI to give suggestion to avoid this situation.

Thanks in advance!

New Microsoft Excel Worksheet (3).xlsx

Hello User,

The amplifier will amplify the input offset error of the op amp by 12. (R3/R4)+1. If your signal source has a DC error, that will also get amplified. 

Hello Ron

The abnormal thing is it seems the offset is change with the signal. When the signal in positive half, the offset is positive. When the signal in negative half, the offset is negative. For the signal source, I don't see a DC error

Hello Kai

I don't think think the error is from ADC. When I input 0V signal, the error disappear. And I will try you way to amplifier the signal and let you know the status.

Thanks!

Hello Kai

I don't think think the error is from ADC. When I input 0V signal, the error disappear. And I will try you way to amplifier the signal and let you know the status.

Thanks!

Can you explain in more detail why this is not a good signal?

Hello Ron

This is a very good question.

1. The inject signal is a pure sine wave, so the output should also be a pure sine wave. The crest factor of the sine wave should be sqrt(2). But for this curve, the crest factor is 1.5257, it seems the waveform is distorted

2. When the signal pass the mid-point, it seems it rise very quickly. So I guess when pass the midpoint, an additional offset is added.

I believe the input is not a pure sine. You could try a triangle ramp instead. That will have has a constant slope. A sine wave's slope varies.  

If the result is different then it can't be the op amp; it still passes through center. 

Hello

I check the waveform and believe it is not a problem of amplifier. When I input 0V, I see a background noise which include some 500Hz noise. So it change to to another problem. But it still a problem to me.

Thanks!

What are you trying to do? Can you tell more about your application?

Kai

I am trying to do a simple PCBA include only the necessary circuit and verify the result.

The application is about a current measurement circuit

Hello

I believe the problem is not from the amplifier. I directly inject the signal into the ADC. The problem still exist.

And I change the bias point of the circuit and get the correct result.

I will work with the ADC vendor to know why?

Thanks!