I have encountered an issue with TLV320AIC3106 in a newly developed custom board with an ARM processor, Linux 3.0.35 and driver tlv320aic3x.
Both LINE and HP outputs exhibit quite a large amount of audible noise. The noise sounds similar to a cassete player noise but is a deal larger. As to my ears, sound itself is reproduced accurately. Noise appears AFTER the DAC is enabled but is not affected by muting the output mixers.
Perhaps the noise origins from a strong ultrasonic comonent at the outputs, which appears at the same time. The component is ~200mVp-p ~530kHz and is OF THE SAME PHASE at both terminals of the differential output port. (attached are the waveforms at the LINE output after a 1st order RC-filter at the input of a D-class amplifier). The component is not dependant on the sampling frequency (44.1/48kHz), master clock frequency (24/12MHz) and master/slave clocking mode, but shows some dependency on the programmed common-mode voltage: it gets a bit lower in amplitude and a bit higher in frequency when VCM changes from 1.35V to 1.8V. Meanwhile the common-mode voltage at the inputs is free of such a disaster. Two boards have been checked to behave similarly.
Power lines show the traces of the same frequency ~10mVp-p. Adding/removing capacitors/ferrite beads to them has no visible effect on the ultrasonic component.
Any help is much appreciated.
Sergey
