SGTL5000 ADC noise L ch. only

[davidthomas]

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.

[reyes]

Hi David,

The easiest way to reduce noise issues you are hearing would be enabling the ADC_HPF_FREEZE bit (bit 1) on the CHIP_ADCDAC_CONTROL 0x000E.

This way the noise would be reduced (but not eliminated), if your application needs more his noise reduction, then an external filter should be implemented.

A general recommendation to reduce noise on audio signals, specifically for differential signal pairs, is that these lines should have the same length and the same impedance. The signal pairs should be routed as closed as possible to each other to ensure that they pick up the same amount of noise. For the SGTL5000 these signals are LineIn_R and LineIn_L, LineOut_R and LineOut_L, and HP_R and HP_L.

Series coupling capacitors on the LineIn and LineOut lines is recommended to reduce AC noise. Please add (in case you don’t have it in your current design) a 1uF or 2.2uF capacitor in series on the linein and lineout lines as in the following image:

 

If this test helps (even if it helps just a little) we can assume that this is the source of the problem and we can add an LC filter in these lines as follows:

 

Hope this helps.

Regards,

Jose

NXP Semiconductor

[davidthomas]

Jose,

Thank you very much for your fast response. While I wasn't able to solve my problem, your suggestions have helped to narrow the scope of the problem and hopefully will eventually lead to a solution.

    Jose> The easiest way to reduce noise issues you are hearing would
    Jose> be enabling the ADC_HPF_FREEZE bit (bit 1) on the
    Jose> CHIP_ADCDAC_CONTROL 0x000E. [...]

I tried this. The freeze function was definitely working because the ADC responded to DC, whereas normally DC is filtered out. Unfortunately, though, the noise did not change.

    Jose> A general recommendation [...]
    Jose> Series coupling capacitors [...]
    Jose> an LC filter [...]

Generally these are good suggestions, but I have found that the noise is still present even when all input drive is removed from the ADC inputs. (A 1 nF capacitor to ground is the only connection to each input.) To me, this demonstrates that the noise is internal to the ADC itself.

Strikingly, only the left input is noisy, which lends credence to the idea in the previously referenced discussion thread (https://community.nxp.com/message/505328) which suggests that the microphone input circuit is involved. One way or another we have two inputs that do not behave identically -- one is noisy, the other is quiet -- and since all input drive has been disconnected, the CODEC itself is implicated... unless I am missing something.

Gratefully and hopefully,
David