intermodulation in sigma-delta modulator

[jiangxb]

hi all

i'm measuring a sigma-delta modulator prototype, when i input a differential 40hz 4Vp-p sine wave, i get a psd plot as below. there are the intermodulation distortions between input signal and 50hz tone(i think), but i don't where they come from. even though i use battery, this case is present.

 

[waster]

is it a continuous time or discrete time model? either your feedback dac must be non-linear or your switches ,or, your opamps may be non-linear .
your out of band noise is folding back into your signal band.

 

[jiangxb]

it's a switched-capacitor sigma-delta modulator chip. the feedback dac is 1-bit. as for the effect of switches and opamps, maybe, i don't know.

you say: out of band noise is folding back into your signal band, what means it? it's due to sampling on those non-linear components? why is 50hz?

 

[jiangxb]

well, i think i have found the cause. using a better clock generator, those intermodulation distortions disappear. 50hz and it's harmonics exist yet, they come from signal source.