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Tackle issues in analogue, digital MEMS mic design

Posted: 26 Apr 2013     Print Version  Bookmark and Share

Keywords:Microphones  MEMS  analogue-to-digital converter  pulse density modulated 

The output of an analogue 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 100Hz 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 16Hz, 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 analogue MEMS microphone connected to an op amp in a non-inverting configuration.

Figure 6: Analogue microphone connection to non-inverting op amp circuit.

Digital microphones
Digital microphones move the analogue-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 analogue 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 analogue audio interface to connect to an analogue 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.

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