Is it possible to attach 4 digital mics on a ADAU1781?

garygru Question

Feb 10, 2013 00:00

Hi there,

the ADAU1781 has two analog microphone inputs LMIC and RMIC which can also be used with two digital microphones, each. My question: Is it possible to use all 4 channels within the processor or am I limited to stereo on input MICD1 or MICD2? If so, is there another sigmaDSP-processor which can handle more than 2 inputs?

thanks,

Gary

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ADIApproved Reply

Feb 12, 2013 00:00

Hi Gary. Good question!

Remember that the term "digital mics" has two meanings, at least with current technology. The first meaning would be "PDM-output mics", and the second meaning would be "I2S-output mics". Here are examples of each:

ADMP521: Ultra-Low Noise Microphone with Bottom Port and PDM Digital Output

ADMP441: Omnidirectional Microphone with Bottom Port and I2S Digital Output

I2S microphones go via the serial port directly to the DSP core, whereas PDM microphones go through the decimation filters on the backside of the ADCs and go to the DSP core.

There are two separate paths, and only two channels on each. You can only use 2 PDM mics because there are only 2 decimation filter channels. Likewise, you can only use 2 I2S microphones because they are configured by default to only be able to use 2 channels. However, if you use a combination (2 PDM + 2 I2S), then you can actually connect 4 microphones to the ADAU1781. That would solve your problem, I think.

As a bonus, the sensitivity on the ADMP521 and ADMP441 are exactly the same (-26 dBFS), so that means you don't need any special gain stages or anything to compensate for the fact that you're using two kinds of microphones. The output levels (and indeed, the frequency responses) of all 4 microphones in your system should match.