I'm using the high performance AD7177-2 adc in the evaluation board EVAL-AD7177-2SDZ because we need to achieve the highest possible resolution (in terms of dynamic range). I'm evaluating the rms noise when the inputs are shortcut (or shorted to GND); using the SINC3 filter (which has better performance) there are a spurious frequency at 57Hz while it disappear when SINC1+5 filter is used. I've used different power supplies (I've tried even a battery) and the board is inside a metal box (grounded) to exclude that the noise is coming from outside.
The set up is the following:
- Split supply (regulated) - as described in the User Guide (UG-849)
- Ch0 enabled (AIN-=Ain1, AIN+=Ain0 or AIN-=Ain3, AIN+=Ain2) all the other channels are disabled
- All buffers disabled, external reference, external crystal (same results persist with internal reference and internal clock)
Do you have any suggestion how to avoid, or at least reduce, this behavior?