Electronic – Calculating bits/sample from a digital MEMS microphone

adcaudiomemsoversampling

I'm thinking of using a digital MEMS microphone in a project – looks like it would save on some parts: no need for an amplifier and ADC. The output from the microphone is PDM and I'd need to convert it PCM for any useful processing.

Say I'd like to work with 16-bit, 48 kHz audio. Seems like the answer is to oversample 64x by clocking the microphone at 3.072 MHz, then low-pass filter and decimate the output. What's the effective number of bits (ENOB) I get out of this?

If the microphone datasheet claims 60 dB SNR (just less than 10 bits), and 64x oversampling gives me 2.5 bits (1 bit for every 4x oversampling), then I'm only getting in the neighborhood of 12-13 bits. Is this calculation right?

FWIW, the parts I was considering are http://www.mouser.com/ds/2/720/DS45-1.01%20AKU240%20Family%20Datasheet-552987.pdf or http://www.mouser.com/ds/2/389/DM00121815-524679.pdf. They seem fairly close in specs.

Best Answer

Ten bit data can be increased in resolution if there is gaussian noise present and oversampling is performed. 4 times over sampling increases the resolution from 10 bits to 11 bits. 4 x 4 times oversampling gets you one more bit and 4 x 4 x 4 (=64) times oversampling gets you another bit. So with 64x oversampling you get 13 bits resolution from a 10 bit source.

See this source from silicon labs entitled AN118, IMPROVING ADC RESOLUTION BY OVERSAMPLING AND AVERAGING