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PCM2912A: Noise in microphone

Akshit Bhandari
Prodigy 50 points

Part Number: PCM2912A

I am using PCM2912A IC and have made PCB design for the same. I have connected an electret mic same as in design. But I am getting noise in the mic. Sometimes noise is like a hum, sometimes it is like a hiss and most of times noise is like air blowing in the back. I have tried using various things like pop filter in front of mic, changing bias, cleaning bias source, selecting different coupling capacitors etc. but noise remain.

Can anyone suggest a solution?

  • 0
    TI__Mastermind 30240 points

    Akshit, 

    can you share your schematic? 

    is it possible your microphone was damaged when soldering it?  microphones are pretty fragile and we have special instructions for our manufacturers when building EVMs.  for example the mics are soldered after everything else, at lower temperatures, and no cleaning solutions can be used on them. 

    damaged mics can have a fair amount of noise exactly as you describe or just not work at all. 

    best regards,

    -Steve Wilson

  • 0 in reply to Steve-Wilson
    Prodigy 50 points

    I am using the same circuits as described in example in datasheet. Also tried with changing values.

    I have tried various mics. One with 1.2k around impedance, other normal 2.2k impedance mic. Also tried with external preamp mic (used in quadcopters). I would have considered it as IC generated self noise of PCM2912a but sometimes circuit just works properly without any noise (still to figure out how?). Sometimes it just generates static noises like drilling or like wind. I have tried by minimizing the traces of output of mic to pcm2912a, that did reduce the noise a little but still did not remove it completely.

    I will again try by taking extra care while soldering and let know. But the mic I tried with preamp inbuilt was a module with already soldered mic in it. So I guess it is not problem with soldering

  • 0 in reply to Akshit Bhandari
    TI__Mastermind 30240 points

    Akshit, 

    Does the noise occur if the microphone is disconnected?  

    Needless to say this isn't expected or normal behavior for the PCM2912a.  Have you tried using an Anti-aliasing filter on the input? 

    best regards,

    -Steve Wilson

  • 0 in reply to Steve-Wilson
    Prodigy 50 points

    Hi,

    Sorry I am unaware of how anti-aliasing filter works. 

    I have tried to solder my mic output to Vin capacitor pin directly and using bias low pass filter for around 7hz frequency. That have reduced noise. Somehow it looks like traces of pcb were inducing noise although i have separated by analog input output from any other circuit. Is there any specific way the track should go? 

  • 0 in reply to Akshit Bhandari
    Prodigy 50 points

    Noise is reduced heavily by using direct solder at vin.

  • 0 in reply to Akshit Bhandari
    TI__Mastermind 30240 points

    Akshit, 

    Section 12.2 in the Datasheet shows an example layout. 

    You can also check out the EVM user guide,  it has the schematic and layout files shown. 

    Regarding anti-aliasing filter,  you can add a small passive LPF after the AC coupling capacitor. 

    The above image is for the PCM186x,  but same principles apply.  This is one method,  but not the only method. 

    best regards,

    -Steve Wilson

  • 0 in reply to Steve-Wilson
    Prodigy 50 points

    I will try to implement it but I have one doubt regarding this. Won't it break the high pass filter formed by coupling capacitor and ADC (20k impedence as per datasheet)? 

    Also i have implemented pcb traces similar to in datasheet with exception that traces are slightly longer as i am extending them to corner of bigger board because using stereo amplifier at other side for headphones.

  • 0 in reply to Akshit Bhandari
    TI__Mastermind 30240 points

    Akshit, 

    I see your point regarding the HPF.  the coupling capacitor value can of course be reduced,  for example in the example above,  changing to 1uf for the coupling capacitor still results in a good HPF and good anti-aliasing.  the values of the passives can be optimized for your needs.  

    There is also a Digital HPF on the ADC,  This HPF purposefully has a low cutoff frequency,  but it is there and should help a little bit.

    best regards,

    -Steve Wilson