Components of the microphone module
This section will give an overview of the components of the microphone module.
Microphone
A Vesper VM1010 is chosen because it is a low power microphone with a good performance. The microphone consumes 87 µA in active mode (measured). There is also a mode-pin to enable sleep-mode which reduces the current to approximately 5µA. To enable the sleep mode, one has to supply a low level voltage to the mode pin and vice versa. The sensitivity of the microphone equals -38 dBV while the frequency response curve is flat for the audible frequencies.
High pass filter
The passive high pass filter has a -3dB-frequency of 100 Hz and removes the DC-offset of 0.8 V supplied by the microphone. Only 1 alternation of the sound wave is taken into account. This reduces the component cost and increases the stability of the circuit.
Amplifier
An non-inverting amplifier is dimensioned to amplify the signal with 30 dB. A TLV341 is used as operational amplifier because it’s gain bandwidth product of 2.3 MHz is rather high. The quiescent current measures approximately 70 µA per channel.
Low pass filter
A Sallen-Key low pass filter is dimensioned with a -3dB-frequency of 8 kHz to reduce the amplitude of the unwanted ultrasonic sounds. An amplification of 2.73 dB amplifies the signal to an amplitude which is appropriate for the ADC.
Analog-to-digital converter
The 12 bit ADC of the PIC16F18446 samples the analog signal to 16-bit values with a sample frequency of 20 kHz. Because only 1 alternation is used, the ADC will measure the amplitude and not the peak-to-peak value. The PIC16F18446 will sample 1000 values to make sure the max value is approximated as close as possible to the real signal originating from the analog circuit. Whenever a threshold-value is exceeded, an interrupt will inform the motherboard that a dB value is ready.
Communication
The microphone module communicates with the motherboard via I2C-commands to set the threshold value or timing interval.