under certain operating conditions we have some noise on the VCC +5V rail of the AD7689. Not pretty, but the ADC should be able to handle it.
The AD7689 is supposed to have > 60 dBe of power supply rejection ratio up to 500 kHz. So much for theory.
I get a PSRR = 20* log10 (16 mVpp / 80 mVpp) = 14.4 dBe at 20 kHz. For simplicity I used the peak peak values, rather than doing a FFT.
My values are that far off that I am either missing something or I do not fully understand the definition of PSRR...
adding PSRR frequency values
[edited by: Bast! at 9:12 AM (GMT -5) on 18 Jan 2021]