These trends call for higher-performing microphones, and some handsets also feature noise cancellation or 3D sound in video modes by using two or more microphones. In addition, the advent of intelligent digital assistants that respond to the user’s voice is changing the ways in which people interact with computers, and could drive high-performing audio subsystems into more products like wearables and IoT devices in the future.
As a result, there is rising demand for MEMS (Micro Electro-Mechanical Systems) microphones, which deliver high performance and fidelity with reliability, within compact dimensions suitable for use in portable devices. The market for MEMS microphones is expected to rise from 3.6 billion units in 2015 to over 6 billion units in 2019, according to market research company IHS Technology.
Recap on MEMS microphone architecture and operation
The MEMS microphone contains a moveable diaphragm and static backplate fabricated on a silicon-wafer substrate using familiar processes including deposition and selective etching. The backplate has perforations that allow air to pass through without causing deflection. The diaphragm is designed to flex in response to changes in air pressure caused by sound waves. This flexing causes the diaphragm to move relative to the backplate, producing a proportionate change in capacitance. A companion IC co-packaged with the MEMS transducer translates this capacitance change into an electrical signal in either analog or digital format.
There are markets for MEMS microphones with analog or digital output. Analog microphones, which essentially contain the MEMS transducer and companion analog-amplifier IC, are a popular solution for small handheld devices such as feature phones and entry to mid-level smartphones.
Digital microphones that integrate analog signal-conditioning and an Analog-to-Digital Converter (ADC) are typically preferred in equipment such as PCs or high-end smartphones. Digital technology enables greater audio performance by taking advantage of inherently higher RF and electromagnetic interference (EMI) immunity, as illustrated in figure 1. In addition, circuit design and board layout can be simplified, and design changes made easier by avoiding the need to adapt resistor and capacitor values.
Most digital microphones 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 Pulse-Density Modulated (PDM) signal. Typical clock frequencies range from about 1MHz to 3.5MHz.