Basic principles of MEMS microphones
The application of MEMS (microelectro-mechanical systems) technology to microphones has led to the development of small microphones with very high performance. MEMS microphones offer high SNR, low power consumption, good sensitivity, and are available in very small packages that are fully compatible with surface mount assembly processes. MEMS microphones exhibit almost no change in performance after reflow soldering and have excellent temperature characteristics.
Figure 1 Top port and bottom port MEMS microphones
MEMS microphone acoustic sensors
MEMS microphones use acoustic sensors that are fabricated on semiconductor production lines using silicon wafers and highly automated processes. Layers of different materials are deposited on top of a silicon wafer and then the unwanted material is then etched away, creating a moveable membrane and a fixed backplate over a cavity in the base wafer. The sensor backplate is a stiff perforated structure that allows air to move easily through it, while the membrane is a thin solid structure that flexes in response to the change in air pressure caused by sound waves.
Figure 2 Cross-section diagram of a MEMS microphone sensor
Figure 3 A typical MEMS microphone sensor viewed from above
Changes in air pressure created by sound waves cause the thin membrane to flex while the thicker backplate remains stationary as the air moves through its perforations. The movement of the membrane creates a change in the amount of capacitance between the membrane and the backplate, which is translated into an electrical signal by the ASIC.
MEMS microphone ASICs
The ASIC inside a MEMS microphone uses a charge pump to place a fixed charge on the microphone membrane. The ASIC then measures the voltage variations caused when the capacitance between the membrane and the fixed backplate changes due to the motion of the membrane in response to sound waves. Analog MEMS microphones produce an output voltage that is proportional to the instantaneous air pressure level. Analog mics usually only have 3 pins: the output, the power supply voltage (VDD), and ground. Although the interface for analog MEMS microphones is conceptually simple, the analog signal requires careful design of the PCB and cables to avoid picking up noise between the microphone output and the input of the IC receiving the signal. In most applications, a low noise audio ADC is also needed to convert the output of analog microphones into digital format for processing and/or transmission.
As their name implies, digital MEMS microphones have digital outputs that switch between low and high logic levels. Most digital microphones use pulse density modulation (PDM), which produces a highly oversampled single-bit data stream. The density of the pulses on the output of a microphone using pulse density modulation is proportional to the instantaneous air pressure level. Pulse density modulation is similar to the pulse width modulation (PWM) used in class D amplifiers. The difference is that pulse width modulation uses a constant time between pulses and encodes the signal in the pulse width, while pulse density modulation uses a constant pulse width and encodes the signal in the time between pulses.
In addition to the output, ground, and VDD pins found on analog mics, most digital mics also have inputs for a clock and a L/R control. The clock input is used to control the delta-sigma modulator that converts the analog signal from the sensor into a digital PDM signal. Typical clock frequencies for digital microphones range from about 1 MHz to 3.5 MHz. The microphone’s output is driven to the proper level on the selected clock edge and then goes into a high impedance state for the other half of the clock cycle. This allows two digital mic outputs to share a single data line. The L/R input determines which clock edge the data is valid on.
The digital microphone outputs are relatively immune to noise, but signal integrity can still be a concern due to distortion created by parasitic capacitance, resistance, and inductance between the microphone output and the SoC. Impedance mismatches can also create reflections that can distort the signals in applications with longer distances between the digital mic and the SoC.
Although codecs are not required for digital MEMS microphones, in most cases the pulse density modulated output must be converted from single-bit PDM format into multibit pulse code modulation (PCM) format. Many codecs and SoCs have PDM inputs with filters that convert the PDM data into PCM format. Microcontrollers can also use a synchronous serial interface to capture the PDM data stream from a digital mic and convert it into PCM format using filters implemented in software.