How to avoid distortion or clipping on the ADC inputs for the ADAU144x evaluation board

Discussion created by BrettG on Aug 5, 2011

The evaluation board uses AD1938 converters. You can see in the  AD1938 datasheet that the full-scale differential ADC input is 1.9 Vrms.  However, since the evaluation board uses line inputs that are  single-ended, the input filtering is actually single-ended, and the  filter circuit artificially creates the differential signal.


The single-ended range on this board is actually ~2.8  Vp-p. I would suggest dropping the test input signal down a bit below  2.8 Vp-p and then inching it up until you read about -1 dBFS peak in the  DSP core.


You can easily monitor the input level in the DSP using these methods:

I  would suggest using the peak method. You don’t want to be right at 0  dBFS because harmonic distortion will be a bit high. I would step it  down to -1 dBFS for the best balance of noise and distortion.