An analog MEMS microphone's output impedance is typically a few hundred ohms. This is higher than the low output impedance that an op amp typically has, so you need to be aware of the impedance of the stage of the signal chain immediately following the microphone.
A low-impedance stage following the microphone will attenuate the signal level. For example, some codecs have a programmable gain amplifier (PGA) before the ADC. At high gain settings, the PGA's input impedance may be only a couple of kilo ohms. A PGA with a 2 kΩ input impedance following a MEMS microphone with a 200 Ω output impedance will attenuate the signal level by almost 10%.
The output of an analog MEMS mic is usually biased at a dc voltage somewhere between ground and the supply voltage. This bias voltage is chosen so that the peaks of the highest amplitude output signals won't be clipped by either the supply or ground voltage limits. The presence of this dc bias also means that the microphone will usually be ac-coupled to the following amplifier or converter ICs. The series capacitor needs to be selected so that the high-pass filter circuit that's formed with the codec or amplifier's input impedance doesn't roll off the signal's low frequencies above the microphone's natural low-frequency roll-off.
For a microphone with a 100-Hz low-frequency -3-dB point and a codec or amplifier with a 10-kΩ input impedance (both common values), even a relatively-small 1.0-µF capacitor puts the high-pass filter corner at 16 Hz, well out of the range where it will affect the microphone's response. Figure 6 shows an example of this sort of circuit, with an analog MEMS microphone connected to an op amp in a non-inverting configuration.
Figure 6: Analog microphone connection to non-inverting op amp circuit
Digital microphones move the analog-to-digital conversion function from the codec into the microphone, enabling an all-digital audio capture path from the microphone to the processor. Digital MEMS microphones are often used in applications where analog audio signals may be susceptible to interference.
For example, in a tablet computer, the microphone placement may not be near to the ADC, so the signals between these two points may be run across or near Wi-Fi, Bluetooth or cellular antennae. By making these connections digital, they are less prone to picking up this RF interference and producing noise or distortion in the audio signals. This improvement in pickup of unwanted system noise provides greater flexibility in microphone placement in the design.
Digital microphones are also useful in systems that would otherwise only need an analog audio interface to connect to an analog microphone. In a system that only needs audio capture and not playback, like a surveillance camera, a digital-output microphone eliminates the need for a separate codec or audio converter and the microphone can be connected directly to a digital processor.
Of course, good digital design practices must still be applied to a digital microphone's clock and data signals. Small-value (20-100 Ω) source termination resistors are often useful to ensure good digital signal integrity across traces that are often at least a few inches long (Figure 7). For shorter trace lengths, or when running the digital microphone clocks at a lower rate, it is possible that the microphone's pins can be directly connected to the codec or DSP, without the need for any passive components.
Figure 7: PDM microphone connection to codec with source termination
PDM is the most common digital microphone interface; this format allows two microphones to share a common clock and data line. The microphones are each configured to generate their output on a different edge of the clock signal. This keeps the outputs of the two microphones in sync with each other, so the designer can be sure that the data from each of the two channels is captured simultaneously.
At worst, the data captured from the two microphones will be separated in time by a half period of the clock signal. The frequency of this clock is typically about 3 MHz, which would lead to an intrachannel time difference of just 0.16 µs – well below the threshold that a listener will notice. This same synchronization can be extended to systems with more than two PDM microphones by simply ensuring that the microphones are all connected to the same clock source and the data signals are all being filtered and processed together. With analog microphones, this synchronization is left up to the ADC.