55.6.2.1 Description

The PDMIC includes a DSP section containing a decimation filter, a droop compensation filter, a sixth-order low pass filter, a first-order high pass filter and an offset and gain compensation stage. A block diagram of the DSP section is represented in Figure 55-2.

Data processed by the filtering section are two’s complement signals defined on 24 bits.

The filtering of the decimation stage is performed by a fourth-order sinc-based filter whose zeros are placed in order to minimize aliasing effects of the decimation. The decimation ratio of this filter is either 32 or 64. The droop induced by this filter can be compensated by the droop compensation stage.

The sixth-order low pass filter is used to decimate the sinc filter output by a ratio of 2.

An optional first-order high pass filter is implemented in order to eliminate the DC component of the incoming signal.

The overall decimation ratio of this DSP section is either 64 or 128. This fits an audio sampling rate of 48 kHz with a PDM microphone sampling frequency of either 3.072 or 6.144 MHz. The frequency response of the filters optimizes the gain flatness between 0 and 20 kHz (when the droop compensation filter is implemented and the high pass filter is bypassed) and highly reduces the aliasing effects of the decimation.