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PCM2903: Microphone Hiss/Noise

Michael
Prodigy 40 points
Part Number: PCM2903
Other Parts Discussed in Thread: LMV796

I'm designing a circuit that is using the PCM2903 Audio Converter with a preamp built from the reference design TIDU765. When recording audio and playing it back there is a noticeable hissing/noise in with the recorded audio. For the Electret Microphone preamp circuit I am using an LMV796 opamp since the circuit is running on 5V (being powered from USB). Originally I thought the noise was coming from the USB port and so I connected the VCC of the mic preamp to the +3.3V output from a TPS73033DBVR LDO, but I'm still getting the noise. 

Any suggestions on how to reduce the noise from the microphone? 

  • 0
    TI__Mastermind 29745 points

    Hi,

    There could be several possible causes of this noise, do you hear the noise if a pure sinewave is used at a start? 

    Is the MBIAS for the ECM filtered etc.? I have attached here an apps. note for common noise in audio below.

    https://www.ti.com/lit/an/slaa749/slaa749.pdf?ts=1646757045123&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTLV320AIC3204

    Though this is with different device, it's still applicable to this PCM part.

    By the way this PCM part is not recommended for new design, there are other USB converter devices in this link you can peruse.

    https://www.ti.com/audio-ic/converters/usb/products.html

    Regards.

  • 0 in reply to pdjuandi
    Prodigy 40 points

    Thank for you for the application note. I will try to separate the planes as indicated on on the note. 

    There is noise being picked up even with the microphone completely removed from the circuit so I think it's amplifying noising being pick injected from the board itself? I can lower the value of feedback resistor and it lowers the gain of the microphone and it sounds like the noise level also decreased. 

    Lastly, I don't think it's high frequency noise that I've head before, but more like that background noise you hear if you turn up the volume on something really really loud with no signal present. I can try to get a recording of it if that'll help. 

    Also, if I remove power from the OpAmp PreAmp, there is no noise....and of course the microphone level is too weak to be picked up from the Audio Converter, so to me it is narrowed down to the PreAmp which is picking up and amplifier noise from the board? Since without the microphone is present. 

  • 0 in reply to Michael
    TI__Mastermind 29745 points

    You might want to filter the preamp supply itself and see that improves besides the grounding.