Forum Post: RE: How to test ADC’s CMRR and PSRR?

Hi, Bob,

Thank you very much. The reason why I can not use DC common voltage to test CMRR is ADS1248' offset voltgage is not constant, right?

I use a AC common voltage, 2Hz, 1.2V to see the FTT analysis, while I can not see the 2Hz signal. I think maybe ADS1248's CMRR is too high, so the offset comes from AC common voltage is drown by noise, right? see below

But when Vcm amplitude is 1.8V and 0.8V,the 2Hz siganl can be seen only a little higher than noise.

Best Regards,

Nicole