MEMS麦克风应用程序及设计注意事项

　　MEMS Microphone Apps， Design Considerations

　　作者：Carolyn Mathas

　　Most modern consumer electronics use electret condenser microphones （ECMs）， also called capacitor or electrostatic microphones， which are assembled from discrete parts. Known for performance， manufacturability， and low cost， the microphones are produced at an estimated rate of more than one billion units annually.

　　A more recent solution， however， is rapidly taking hold. MEMS microphone solutions differ from the discrete-parts approach. With MEMS， a pressure-sensitive diaphragm is etched onto one silicon chip along with an integrated preamplifier. MEMS design is based on a variable capacitor with a fixed backplate and flexible membrane. As sound pressure deflects the membrane， a change in capacitance results causing voltage change. The sound is captured and converted into a digital signal on one chip， resulting in robust digital output. Available in two flavors， analog and digital， the solution is showing distinct reliability， tolerance， small footprint and height， and integrated signal conditional and A/D conversion advantages.

　　In a recent report （June 2011）， Research and Markets cite the use of multiple MEMS microphones in consumer-application designs as an impetus to rapid growth. While mobile phones use a minimum number to achieve active noise cancellation （ANC）， two， four， and six microphones are not unusual for other consumer applications in order to achieve multidirectional functions. In the report， rapid growth is expected to continue and shipments will increase to over 1 billion units by 2013.

　　IHS iSuppli agrees， forecasting an annual growth rate of 24 percent between 2009 and 2014 （see Figure 1）， and citing a breakthrough in the use of noise suppression requiring multiple microphones as well as such new applications as tablets and gaming to fuel rapid adoption.

　　Figure 1： Global MEMS microphone forecast （millions of units）。 （Source： IHS iSuppli.）

　　An example of a digital MEMS microphone is the ADMP421 by Analog Devices， a high-performance， low-power， digital-output bottom-ported omnidirectional MEMS microphone. Small and thin， it consists of a MEMS microphone element and an impedance converter amplifier followed by a modulator. The digital interface allows for the pulse density modulated （PDM） output of two microphones to be time-multiplexed on a single data line using a single clock.