Characterization of an Amplification/ADC circuit
Henry

Input a low-distortion sinusoid, at some slow frequency, and capture one or more complete sinwaves. Perhaps on your scope. Then take the FFT, and examine TWO things

(1) any harmonics, because these indicate specific flaws in the ADC transfer curve, caused by INL or DNL

(2) the noise floor, because this indicates DNL

Best wishes