I have a x100 opamp amplifier, which seems to work:
simulate this circuit – Schematic created using CircuitLab
Datasheet for the opamp: here. The diodes are there to protect the inputs of the (expensive) opamp. The opamp is powered from +-250V rails, with 1nF and 100nF decoupling caps right by the pins, and have less noise than I can measure with my scope and probes (<10mVptp).
I built and tested the circuit. It works, but I would like to reduce the noise present at the output. Since the DAC is relatively noisy at about 10kHz and harmonics, I thought I could reduce the bandwidth of the amplifier. I tried adding 15nF in parallel with R3, on the expectation that this would reduce the gain high frequency. Simulating in LTSpice with a generic opamp suggested it should work. But it started oscillating at about 1-2MHz, rail to rail.
I have realised I don't really understand the criteria for an amplifier to oscillate. Why is my amplifier oscillating? And how should I reduce the bandwidth without oscillation?
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Best Answer
Your amp seems to have pins for a deliberate compensation cap. They added those for a reason, implying that the usual capacitor from the output directly to the negative input might not work so well. Looks like you confirmed that. If you want the amp to be more stable, increase C1.
To reduce out of band noise, try adding a little low pass filtering of the input signal. For example, you could split R1 into two halves and put a capacitor to ground from the middle point.