Sigma delta ADC small signal harmonic distortion problems

[threekingtiger]

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?

 

[Audioguru]

Why are you worrying about such low distortion that nobody can hear? Maybe you can hear distortion as low as -60dB but probably not lower.

 

[threekingtiger]

ths for your reply.

Because I think the -60dBFS input should give rise to noise in audio band not harmonic distortions. So the dynamic range spec, which is the absolute value of thd+n when -60dBFS is input plus 60dB, will be much better if no distortion exists in band.
What interests and bothers me most is that small signal excitation can have time-varying harmonic distortion. This conflicts with my knowledge.

 

[Audioguru]

Maybe your signal level is so low that there is quantization noise or external interference.