A block diagram of the IC that’s used in ST’s MEMS microphones is shown on this slide. The portion in the green box represents the sensor with its variable capacitance. The IC charges the sensor capacitance, measures the change in voltage across the capacitor as the capacitance changes, and then amplifies the signal. In the case shown, a digital microphone, the IC then takes the analog signal and runs it through an A/D converter to produce the output. There are three signals on the digital MEMS microphone: a clock input, a left-right channel select input which allows the user to connect more than one microphone to a single data line, and the output. ST’s digital microphones use pulse-density modulation to encode the audio signal. Pulse-density modulation is similar to the pulse-width modulation that is used on ST’s class D amplifiers. The difference is that with pulse-width modulation there is a fixed frequency and a variable pulse width, whereas with pulse-density modulation there is a fixed pulse width and a variable frequency. It is possible then to measure the change in time between pulses to recover the audio signal.