SGTL5000 LINE input noise

[Tommy-A]

Our customer use SGTL5000 with i.MX6.

The LINE input pull down to GND and they measure LINE out.(ADC in => DAC out).

It is measured noise on 7kHz at sampling frequency of 44.1kHz, but there is no noise at sampling frequency of 48kHz.

Please tell me the countermeasures of noise at ADC input.

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[art]

The possible cause of the issue is that the system contains or is affected by some source of electromagnetic noise with the main frequency of k*44.1kHz +/- 7kHz , where k is integer number. It might be some clock signal, noise on power rail or something like that. Please check it first.

Have a great day,
Artur

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[jeremysmith]

We were seeing a similar noise in our application.

We were able to narrow it down systematically by turning off portions of the codec and seeing if the noise persisted.  We found that it was coming from the ADC block, and determined that it was bleed through from the MIC input to the ADC. 

Since our design does not utilize the mic input of the SGTL5000, we left it unconnected. Once we identified that the noise was coming in through the codec, we needed to determine if it was something internal to the codec.  We grounded the line-in and saw no difference, but when we grounded the mic-in our unwanted tone went away.  We then routed a 1kHz tone to the line-in and found that the known input signal was amplified as expected when routed through the line in, but when we configured the codec to connect the mic input to the ADC we saw the same 1kHz tone that we were injecting on the linein (but the mic input was not connected to anything, it just had the left line-in pin routed nearby).  It had an amplitude that was nearly equivalent to that coming out of our source.  Once we connected a 1uF cap to ground on the mic input pins, we saw our random tones go away.

[art]

It seems to be the crosstalk between the line-in trace and mic-in input. You have not to leave the mic-in pin floating even if not used. Bypassing it to GND is the correct solution. Also, please follow the schematic and routing recommendations, given in the "Typical Applications" section of the SGTL5000 Rev.6 Data Sheet document, available on the Freescale web site:

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=SGTL5000&fpsp=1&tab=Documentation_Tab

Also, you can use the design files of the SGTL5000 Tower System module:

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=TWR-AUDIO-SGTL&fpsp=1&tab=Design_Tool...

as the reference.

Have a great day,
Artur

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