As shown in the figure below, the power for the microphone is VCC_P3V3, which is filtered by a low-pass filter to reduce noise from the power supply.
Typically, the original voice signal is quite weak, with measured amplitude around 10mV (less than 50mV). So the signal must be amplified and filtered accordingly. To make sure that the signal can be amplified without being distorted, a voltage offset is required.
As shown in the figure below, the original signal is processed in a 4-order Sallen-Key Butterworth low-pass filter. The filter is designed to have a gain of 50 and a cut-off frequency of 4kHz. The amplified signal is connected to the MCU ADC input 11 (pin PB0_AIN11).
The real filter response vs. frequency curve based on measured data is shown in the figure below.
