OPA847: Not work for Instrument amplifier

Hello Bohai, 

It is possible you have a common mode oscillation in that first stage, Split R3 into two 25ohm values and put a 0.1uF cap to ground in the middle, This will peak the output CM noise quite a lot - but, the FDA will reject all that. 

We ran into this on the OPA838 as well, check out section 9.1.5 in the OPA838 datasheet, 

Also, why are you using 5% R's in the FDA ckt. That will give you quite a lot of gain imbalance which becomes CM to DM conversion. In some other work lately we have found even 0.1% R's are quite inexpensive in volume these days - that would help your performance over production spread a lot. 

You might try a worst case gain imbalance and drive a CM signal into the FDA stage and see what I am talking about in sim. 

Hi Michael,

 I have see the opa835 datasheet (9.1.5). I know this problem.Next week，I will test it.  Thank you for your help.

Also，I want to simulate  the common mode oscillation. I don't know how to do it. Could you give me any document about it.

Thanks

Bohai

Hi Michael ,

This is my missing. I use the Tina default precision. I forget to modify it.  In fact, I use %1 R‘s in this.

Thanks

Hello Bohai,

The easiest way to see this to look at the output CM noise, as shown here and in the attached file, 

Diff OPA847 CM Noise test.TSC

And then splitting that Rg into 2-25ohm with a 1nF to ground gives a lot of CM noise peaking, but you can tell by how broad it is this is just gain peaking not resonance - the FDA will reject this, pretty cool actually, 

And adding the FDA stage we see another issue at higher F looking at the final output CM noise - I was expecting to see this pretty low, rejecting the peaking out of the diff OPA847 stages. not so, nasty resonance at 400MHz, 

You need to remove those 30pF, not doing anything for your diff signal path and causing this issue, now this is what I expected, complete rejection of the CM noise coming into the FDA stage, 

Diff OPA847 CM Noise test with FDA.TSC

Hi Michael，

I modified the circuit according to your advice.And I test it. I found the circuit is still abnormal. 

So,the noise gain peaking don't lead to the abnormal. Maybe some reason lead to it.

I don't know if the PCB layout lead to it. I plan to re-layout the PCB and test it.

Thanks

Bohai