Electronic – Chopper Amplifier feedback loop stability


I have a question regarding the stability of a circuit I plan on building. This is a voltage-controlled current source, with the IN-AMP being used to sense the current through Rsns and provide feedback to the op-amp. I'm trying to use a programmable instrumentation amplifier and turns out most of the ones that fit my requirements are chopper amplifiers.

However, as I understand it this means there will be some delay from when the current through Rsns changes to when the capacitors in the chopper charge and discharge, and then the output of the in-amp is changed. Am I correct in assuming this delay will lead to oscillation? (I don't have the parts yet or I would just build it up). Is it in general a bad idea to introduce delay elements into a feedback loop, or is there a way to use them without instability? Thanks!


For those that would like an update: I built this circuit with a vanilla op-amp and instrumentation amplifier, with the instrumentation amplifier having a G=100, Vin= 1Vpp sinewave at 60Hz, Rsns=1R, and ZL=22R, and I see my 60Hz signal "amplitude modulated", if you will, at an oscillation frequency of 133kHz. Here is the oscilloscope trace across ZL.
oscilloscope trace

Best Answer

Yes, stability will likely be a problem, and the internal construction has little to do with it. Most (modern) chopper amps have many MHz bandwidth and behave similarly to normal op-amps or in-amps aside from really nasty spikes out the inputs and some small noise near the modulation frequency.

However, you are introducing a lag and more gain into the feedback loop and both of those will tend to result in less phase margin and thus potential instability. By keeping the gain of the in-amp low and perhaps introducing some compensation you should be able to make this concept work.