ADAU1761 input noise

Hello, I am using ADAU1761 at 48Khz sampling. I have noticed a fairly strong "hiss" type noise when the is no input and levels are all set around 0db or -10db etc.

 if I mute all inputs the noise is still there. if I disable the input mixer, the noise is zero. I have tried lots of things to get rid of the noise. I am using a dynamic vocal mic for the input.

using the Pos-Neg and gnd/shield. I found that when i turned down the ADC level in hardware settings to about -20DB the noise mostly goes away. I can compensate for that by boosting the signal down the line and the noise isn't a factor. This noise problem did not seem to be there on previous revision of my PCB. my question is, what might cause this noise that is present even with all inputs muted, no noise when input mixer is disabled, and noise seems to be effectively suppressed when I reduce the ADC levels in hardware configuration. Is reducing the ADC levels the best solution or could it be something else, like circuit layout or capacitors on the power pins? or some other setting. I did notice that changing the sample rate lower seemed like it was cutting out some of the high end of the noise like a low pass filter.

TIA for any ideas 


update. I found the source of the noise. New to this design is the USB to I2C chip microchip is powered by the same supply 3.3v to the ADAU1761's

The chip in the previous rev powered from 5v usb. I installed a large capacitor across the power and ground at the chip, and the noise went away.

I found the answer to the noise problem. it was Microchip MPC2221A being on the same 3.3v power. I put a 33uf cap across the power leads. noise problem solved. I will experiment with the capacitor values.
[edited by: PaulBKaraoke at 2:09 AM (GMT -5) on 21 Feb 2021]