SGTL5000 Mic In & Line In Noise internally generated by ADC inbuilt inside codec

[adityapandey]

Hi we are using i.MX6 with SGTL5000. We have followed guidelines & application circuit, as recommended in datasheet.

Test Setup :

1.SGTL5000 VDDA=3.3V, VDDIO=1.8V,

VDDD derived internally.

2.Line Out given to external speaker amplifier.

3.Line In capacitive grounded. (Not Used)

4.Mic-In, external electret condenser mic connected (In Use)

5. I.MX6 to SGTL5000 Digital audio interface (I2S)

   a. SCLK :  48Khz

   b. LRCLK : 3Mhz

6. Master Clock to SGTL is given by i.MX6 @:  24 Mhz

Issue Faced :

1. When we are recording through Mic & saving it as .WAV file, or just playing back captured signal from mic in speakers in real time, then we are getting a constant noise in .WAV & speakers both.

We have tried :

1. Removing the mic, & Capacitive grounding mic input, Line In disabled internally.  (.WAV file was noisy)

2. Removing the mic, & Capacitive grounding mic input, Line In enabled internally, Line In floating.  (Same Noise in .WAV file)

3. Removing the mic, & Resistive grounding mic input, Line In disabled internally. (.WAV file was noiseless)

4. Removing the mic, & Resistive grounding mic input, Line In enabled internally, Line In floating (Same Noise in .WAV file)

5. Tried various configuration of Mic gain, Internal Bias resistor, Bias voltages with & without external 2.2K bias resistor, mic connected, Mic Input enabled, Line In disabled . (Same Noise in .WAV file)

6. All above tests were tried with Line In also, with Mic in disabled internally & externally capacitively ground. .WAV file was noisy in all cases except when Line In was Resistively grounded.

7. Tried giving VDDA=2.8V from LDO that receives its input voltage from a single Lithium Ion Cell (which is the cleanest supply one can use).  (Same Noise in .WAV file)

8. Activating internal HPF has completely eliminated the noise, but with noise elimination, it simultaneously eliminated the required speech signal. Some frequencies of speech were audible but that too sounded like noise.

Conclusion Drawn as a result of above tests :

1. If any of the block is disabled, like Line In or Mic In, then disabled block doesn't have any effect on other input channel signal. Eg. If Line In is disabled, then irrespective of whether Line In is floating or is Capacitively or Resisitively ground, it will not reflect any of its effect on Mic Input signal & Vice-Versa.

2. Noise in power rail is not an issue, 100% confirm, as we have used cleanest supply one can practically use.

3. No High speed signal is there in vicinity of Analog inputs of codec in Gerber, therefore negligible possibility of cross-talk.

3. Looks, Noise is getting generated by ADC block.

Please give solution to this issue. I have attached a .wav file & my schematics as a reference.

Note : Mic Preamp in schematics is unmount & Mic Preamp Bypass option is used.

Message was edited by: Aditya Pandey

[pinkesh]

Hi Aditya ,

I am getting the almost same noise ..

Is there work-around to overcome this ?

- Pinkesh

[andrelabelle]

In your test setup you have 5. b. LRCLK as 3MHz. Typo or actually reversed with SCLK?

[igorpadykov]

Hi Aditya

if noise disappears when Line In and Mic are grouned, then

this proves that noise is not produced by sgtl5000.

Noise is produced from external source: mic or Line In and probably

due to grounding/shielding issues. Also it may be useful to look at

AN3663 SGTL5000 Initialization and Programming

http://cache.nxp.com/files/analog/doc/app_note/AN3663.pdf

Best regards

igor

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