The output impedance of an analog MEMS microphone is typically a few hundred ohms. This impedance is higher than the low output impedance typically found in op amps, and the low impedance circuitry behind the microphone attenuates the signal level. For example, some codecs have a programmable gain amplifier (PGA) before the ADC. At high gain settings, the input impedance of the PGA may be only a few thousand ohms. A MEMS microphone with an output impedance of 200Ω followed by a PGA with an input impedance of 2kΩ will attenuate the signal level by nearly 10%.

       The output of the analog MEMS microphone is typically biased to a DC voltage value between ground and the supply voltage. The bias voltage is chosen so that the maximum amplitude of the output signal peak is not clamped by the supply voltage or ground potential limits. The presence of this DC bias voltage also means that the microphone is usually connected to the subsequent amplifier or converter chip via AC coupling. The principle of series capacitor selection is that the high-pass filter circuit formed with the codec or amplifier input impedance does not cause the low-frequency portion of the signal to roll off above the natural low-frequency roll-off of the microphone.

       A microphone with a low frequency -3dB point of 100Hz and a codec or amplifier with an input impedance of 10kΩ (both are common values), even a relatively small 1.0?F capacitor will set the angular frequency of the high pass filter to 16Hz This value goes far beyond the range that can affect the microphone response. An example of such a circuit is shown in Figure 6, where the analog MEMS microphone is connected to an op amp configured in phase.