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?