Delta Sigma ADC's small signal harmonic distortion

andrew1986

A strange phenomenon appears when I was testing my sigma delta ADC(MASH 2-1 structure, with single bit quantizer at both stages) . The small signal (10mVpeak-peak, 2.9Vpeak-peak as FS value) 1kHz sine wave is used as stimulus to the data converter and 2nd, 3rd, 4th, 5th harmonic distortions can be observed in Audio Precision measurement's output spectrum, what's more, the harmonic distortions are incredibly TIME-VARING which means if I perform an fft now we may get -110dBV at 2kHz, -105dBV at 3kHz, for example, while after 5 seconds I perform FFT again with the same conditions and stimulus we may get -100dBV at 2kHz, and -115dBV at 3kHz for example. They are not like large signal non-linearity distortion that they seldom change as time goes and my ADC's large signal harmonic distortion is stable.


Does anybody know what the reason is for this result?