Hello! I've recently been interested in measuring the gain non-linearity of various op-amps and composite op-amp configurations for use in integrating ADCs. Following the circuits and measurement techniques posted in these app notes from ADI and TI, the results I got were unusable:
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problem I found with these circuits is that they are basically noise amplifiers with a high-gain network created by the top feedback resistor and the divider on the inverting input. Instead of a clear line, I get a flood of noise that comes from the resistors and op-amp (the noise waveforms are very characteristic of band-limited noise). I don't understand how the clean lines in the app notes were obtained. I tried both a small capacitor from input to the summing junction as recommended in the TI app note, as well as a film capacitor (10s of nF) across the top resistor on the divider on the inverting input. The latter fix helped remove the noise, but the XY plot was flat even for the incredibly poor LM358. I had a couple other people repeat these measurements for me, and they had similar results. Is there a sane way to measure gain non-linearity that actually works? Regards, NNNI