ADS8685: PSRR

Hi JH,

Performing an ac PSRR test on an ADC requires injecting an ac signal into the dc supply voltage to the ADC and measuring the output disturbance on the ADC output that is relative to stimulus on the power supply. The 100mVpk ac sine wave is usually used for the testing. The ADC input can be shorted to ground since the ADS8685 is a bipolar input ADC. An analog oscilloscope can be used to check the amplitude of  the ac signal at the supply pin of ADC. If you want to check a particular PSRR at a specific ac frequency, you can simply check the output variation of the ADC by implementing data conversion, then the PSRR can be calculated with the equation PSRR=20log10(ΔVout/ΔVsupply). DC PSRR can be calculated with a same method by changing dc supply +/-100mV directly instead of an ac signal injection. To get a whole PSRR result across frequencies, you have to sweep the ac signal frequency and check either direct ADC output or the error spur at a specified frequency through FFT in frequency domain. The shape of curve is related to the device design. For instance, the PSRR is getting better at a higher frequency than 15kHz because of ADS8685's internal LPF filter. Check the following app note to see more information:

 Fundamentals of Precision ADC Noise Analysis

AN-1261 Power Supply Effects on Noise Performance

Best regards,

Dale