AN-000111 – Selecting PDM Microphone Clock Frequencies and Decimation Ratios

AN-000111 – Selecting PDM Microphone Clock Frequencies and Decimation Ratios

A Pulse Density Modulation (PDM) microphone uses a Sigma-Delta modulator to oversample an acoustic signal at a high sampling rate. This digital PDM signal is output from the microphone as a 1-bit data word, where the density of ones and zeros in the data represents the amplitude of the audio signal. An audio codec or DSP cannot typically process this 1-bit PDM signal directly, so the signal needs to be decimated to the baseband audio sampling rate. Signals at this baseband sampling rate are what is processed, stored and played back in digital audio systems.

Selecting the proper PDM clock and baseband sampling frequencies is important for applications using PDM microphones. This application note describes the relationship between these two frequencies and can help a system designer to select the appropriate clocks for an application.

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