I would like to sample the output of an analog MEMS microphone with an ADC embedded in a uC, sampling rate up to 1MHz.
I don't care too much about signal quality (as I'm more interested in ambient noise level than anything else). I want to absolutely minimize the footprint of the overall solution.
Application notes typically show a DC blocking capacitor at the output of the MEMS microphone, followed by a simple OpAmp stage that provides some gain and sets a suitable DC offset for subsequent ADC sampling.
I would like to do something even simpler and avoid the OpAmp. I would just pass the microphone output signal through a low-pass RC filter (so no DC blocking) and sample that. It seems to me that would exploit correctly the ADC dynamic range, given that:
- My ADC would be set to have a 0-3V conversion range.
- My MEMS microphone (Knowles SPW0430HR5HB – acceptable VDD range 1.5V-3.6V) is specified with a DC output = 0.72V when VDD=1.5V (so DC output=VDD/2); I would use it at VDD=3V so I assume the DC output would then be 1.5V. In which case I can directly sample the microphone output (unchanged DC level) with my ADC without clipping.
Do you see any flaw in that reasoning? One concern I have is whether the DC level at the output of the MEMS is really proportional to VDD.
Best Answer
Your assumption may well be right. Quite possibly the MEMS is biased to 1/2Vdd +/-20%. But the datasheet does not give any guarantees. However a couple of 0603 NP0 capacitors for decoupling and DC rejection and two 0402 resistors are not going to increase your footprint that much.
EDIT1: Also the DC output may depend on your local air-pressure, and as such elevation. It may not be a strong response, or even a linear one, but compared to signal levels one or two mV of drift caused by weather effects could already confuse your measurements if you don't decouple
However, what the datasheet does give, is the following: Sensitivity = -42dBV/Pa This comes down to about 8mV/Pa output signal. In this 1 Pa is about 94dB(SPR). So that's already pretty much talking quite audibly into the microphone and you will only get 8mV of signal. I'm not assuming your uC ADC is measuring over a +/- 10mV range, so I don't even think you are going to get away with no Op-Amp.
That said, again an op-amp comes in SOT23-5 case from most any vendor these days, not to mention some more specialty companies may make them in those smaller SOT323 boxes. Even the SOT23 is hardly going to increase your footprint coming from the microphone, let alone a micro controller.
With small footprint for this set-up it comes down more to good placement and good sourcing rather than shaving of components you can't miss.