When trying to use an SGTL5000 CODEC for audio input, we are getting clean ADC audio from the LINEIN_R input, but noisy audio from LINEIN_L. The average noise power on the bad channel is about -80 dBFS, and the median frequency of the noise is about 7 kHz, though it varies with the absolute DC level on the LINEIN_L pin.
Tantalizingly, this old thread seems to describe the same problem. https://community.nxp.com/message/505328
The old thread explains that the noise is bleed from the unused microphone input, which makes sense, since it affects only one side of the stereo CODEC. Unfortunately, the fix from the thread, a 0.1 uF capacitor to ground from the MIC pin, didn't work for us.
Does anyone have any ideas or insights to offer? We have tried everything we can think of, but nothing seems to affect the noise in any way we can measure.