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PCM1863: Microphone support

Steven Weng1
Expert 2200 points

Part Number: PCM1863

Hi Sir,

Is it suitable that use PCM1863 PGA gain as the preamp gain for the microphone which sensitivity is about -30 ~ -40dB and 60dB SNR, single-ended?

Take a microphone with -35dB sensitivity/60dB SNR to be considered and our target is the digital data up to -6dBFS level.

The -35dB means the mic signal is about 18mV and the single-ended full scale is 2.1V of PCM1863. To achieve -6dBFS digital data, the gain should be 20xlog(1.05/0.018) = 35dB

So the gain of PCM1863 must set to max 32dB for the microphone of sensitivity -35dB.

For this case, how about noise level? is it OK?

I have no sense for ADC SNR, microphone SNR...

Please help me check.

Thank you very much

BR,

Steven

  • 0
    TI__Mastermind 23050 points

    Hi Steven,

    The signal for the mic you have given is going to be about 18mV at 1kHz at 94dBSPL, but is this the maximum pressure level you care about? I'm guessing your mic has a much higher threshold for the 1% and 10% THD points so you may want to scale things depending on the max signal level you care to resolve at whatever frequency your mic is most sensitive to. 

    The noise of your microphone is going to be ~18uVrms, which should be the limiting factor in resolving your signal as the input referred noise of the ADC+PGA is generally around 10uVrms or so. Thus your dynamic range should only degrade by about 1.2dB or so and will be slightly better at higher gains since the PGA of PCM186x is lower noise than the ADC.

    I hope this helps!

    Best,

    Zak

  • 0
    Expert 2200 points

    Hi Zak,

    Thanks for your reply.

    The max gain of PCM1863(32dB) is not enough to generate 0dBFS digital data for the mic of -35dBV sensitivity.

    In our design, the digital data from PCM1863 could be amplified via another digital volume control for more gain applied.

    But I think it's not a good design. It seems pre-amp the microphone signal before going to PCM1863 make sense.

    (1) Could you explain more about 10uVrms derived from? what is "the PGA of PCM186x is lower noise than the ADC" means?

    (2) The SNR of PCM1863 for the single-ended input at 32dB gain is 75dB. Could I say it is good enough for the mic with SNR 60dB?

    (3) For PCM1863, what kind of microphone is suitable without additional preamp circuit? could you give me some suggestions?

    Sorry for many question.

    Thanks.

    BR,

    Steven