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ADAU1361 Noise When Unplugged


Hope all is well.

We use ADAU1361 in our design with two single ended microphones. 

The two mics are connected to negative inputs as pseudo differential and grounded with AUDIO_GND.

When we unplug each microphone, we observe high background noise (not your typical noise floor). 

We have the EVAL-ADAU1361, we do not observe the same using the same settings.

Below is the schematic;

We experimented with capacitor and resistor values but we were not able to find the cause of this floating noise.

Any help is much appreciated.

  • Galenthas,

    I would like to confirm if the red "+" symbols in the schematic indicate an open circuit?  If so, it appears one of the differences between this schematic and the EVB is that there are no 10uF decoupling capacitors on IOVDD or AVDD.  Has any testing been done adding these capacitors, if that is indeed the case?

  • Hello Galenthas,

    I addition to what galvinpjg asked I would also like to ask a few questions.

    Why do you have a voltage divider on the Mic Bias? Normally, you just feed the microphone with a 2K resistor in series with the mic. What kind of microphone is it?

    I suggest you remove the ferrite beads connecting to ground. By using these inductors to ground you create a radiating plane for the high frequency noise. It is best to tie it directly to ground and filter the power feed into the audio circuits. Which goes back to what galvinpjg was asking about.

    I think you may be picking up the noise from the bias current passing through the divider and also just the noise picked up from an open line acting like an antenna. If you are removing the microphone for a test then you need to terminate the input and not leave it floating. Then where you terminate it can also make a difference. If you terminate it to the ground of the system instead of the audio ground then you will be injecting the noise from ground into the mic input. If terminating the input it is best to use the ground very close to the input of the part. The best place is that same point where the LINP pin series cap is tied to.

    Dave T