Conditioning ADMP404 output for ADC

I am looking into using the ADMP404 MEMS mic as a source for medium quality (100-3,000Hz) voice.  Everything runs off a low power 3v supply.  I need to compress/condition the audio input and also amplify it so that it can be digitized using a 12 bit ADC at 8 Ksps.  Being cost sensitive, I would like to use the minimum number of components.

I am designing for normal operation at 60 to 80 dB-SPL, so with the ADMP404 (-38 dBV sensitivity) I would need a gain of between 72 to 52 dB (4000 to 400).  Prototyping will help select the final gain.

The SSM2167 seems very attractive, but it does not have sufficient gain to swing an ADC.  The SSM2166 is not quite suitable.  Is there a single chip alternative that avoids having a compressor plus op-amp configuration?  If I had to use an op amp, would the AD8542 or AD8506 be appropriate?  I have read (and appreciated) the AN-1165 and AN-1112 design notes.

I am also looking into noise cancelling but could not find any design notes.  I was thinking of two ADMP404, one on each side of the PCB.  Can you suggest any designs or guidelines?



Message was edited by: Rajaram Pejaver (oops, cut & paste error with mic name)

  • Hi Raj,

    Am I correct that you're looking to add 52 dB of gain to the microphone's output so that an 80 dB SPL signal is amplified to 0 dBV (1 Vrms)? Even if you're only intending to capture voice in the typical speech range (60-80 dB SPL), I don't think that you'll want to amplify the signal so that all inputs above 80 dB SPL will be clipped. Even for speech signals with an rms level of 60-80 dB SPL, they may have a crest factor (difference between the rms level and peak level) of 20 dB. This would mean that the peaks of the speech signal could reach 80-100 dB SPL. If you still want to match the peaks of the acoustic signal to 0 dBV, then you'll just need 32 dB of gain. On top of that, if you assume that someone using your design may speak right into the microphone, the input signal could be at an even higher SPL. You may want to configure your design to account for that by using even less gain.

    It does look like the SSM2167 would be a good device to pair with the ADMP404. We have a Circuit from the Lab describing exactly this setup (CN-0262). In a voice capture application, the compression and noise gating that the SSM2167 offers may make your captured sound more intelligible by reducing the dynamic range while still applying a reasonable gain. If you just want to use an op amp in a straight amplification circuit, then either the AD8542 or AD8506 should work fine.

    Could you explain more about what exactly you're looking for with "noise cancelling"? Are you trying to do acoustic noise cancellation, or are you just looking for a "noise cancelling" microphone with a figure-8 pickup pattern?

  • Hi Jerad,

    Thanks for the quick response.  You understand what I am trying to do and your comments on the gain are helpful.  I was expecting to learn all that while playing with (i mean testing) the prototype !!  I had thought I might need up to 72 db of gain, which is more than what the SSM2167 can do.  The specs on page 3 say that the Output Voltage Range is only 700 mV RMS at Vs = 3V.  I wonder if it would be enough to rock the ADC.  I think it needs a signal that is ~1.5 V peak (1.06 V RMS).

    As a simple (and inexpensive) noise cancelling method, I was planning on physically placing 2 mics back to back.  One of them would be the "ambient noise pickup" while the other would be closer to the speaker and pick up voice. I expected to be able to subtract one signal from the other and get "noise cancelled" input.  Of course, if the mics were positioned so that both picked up voice equally, then the voice signal would cancel out and I'll be in big trouble.  Do you think that this a reasonable idea?  You probably need more info about the actual application :-(

    If the idea is reasonable, then what is a decent circuit to wire the two mics?  One idea is to feed the two mic signals to the inverting and noninverting inputs of an unity gain op amp and then feed the output to the SSM2167.  Or I could have a unity gain op amp to invert the signal from one mic and then wire add them.



  • On a basic level, your idea for using two mics in a noise cancelling configuration is how a "noise cancelling" microphone works (see a short Wikipedia article on the topic). The mic that is closer to the desired sound source (voice) will pick up that signal with little cancellation from the rear microphone, because of the difference in level between the two mics. For signals that are far away, there will be little pressure difference between the two mics, so those signals will be more completely cancelled through the subtraction. In order to implement this, you will need to invert the signal from one of the two microphones and then sum them together.

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