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ADAU1761 output noise problem

Question asked by rsbonini on Jan 4, 2011
Latest reply on Jan 11, 2011 by rsbonini

We have an ADAU1761 project, which is being booted by an external uC over I2C.  We're using line level, differential inputs to the ADCs, with a passive U-pad to bring input signals in under the ADCs max input level.  The output from the DACs is also differential, the signals pass through a differential analog filter stage which also converts the signals to single-ended and sends it to the output gain and driver stage.  The ADAU1761 internal volume controls are all set to 0dB and for testing purposes the SigmaStudio schematic is simply a straightwire in-out connection through the DSP core.  The problem is that the output noise floor is fairly high; quite audible on an average set of headphones.  The noise seems fairly unaffected by alterations to the input configuration of the CODEC, but in the output configuration, the line out volume control seems to make a big difference, and each mute / volume control further back (closer to the DSP core) seems to affect the noise level less and less.  Is this high noise floor normal for the ADAU1761?  Are there any specific methods (close impedance matching etc.) to reduce it?  Any help / suggestions would be appreciated greatly.

(Also, the analog section when physically bypassing the CODEC (differenital in bridged to differential out) has an extremely low noise floor, and we're positive is not the source of this noise issue.)