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| Filter caps in diff amps - yes/no |
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| SiliconWizard:
Good points made in this thread. My personal take on this would be: no. If you need any filtering, as others have said, do this after the differential amp stage. For RFI/EMI filtering, you may in some cases also do this before the diff amp (with common mode filters of some kind for instance.) Although specifically dealing with true in-amps, you can take a look at this note: https://www.analog.com/media/en/training-seminars/tutorials/MT-070.pdf In any cases, I would not do this in the feedback network. Don't think it's worth the trouble. |
| David Hess:
In applications where the highest AC common mode rejection is required, a variable trimmer capacitor, sometimes a butterfly capacitor with independent stators, is used. But this is *not* used for bandwidth control because the adjustment range of variable capacitors is too small. |
| ejeffrey:
So I was thinking about this recently. I have a case where filters are used not in the feeback loop but at the input to a differential amplifier. The justification was that the input is a high speed (~GS/s) DAC with return-to-zero output. So the sample pulses from the DAC far exceed the slew rate and bandwidth of the amplifier. So the concern is that if you don't filter the signal before the amplifier you will increase amplifier distortion even in the signal band you care about. However, I am not sure this argument hold up. |
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