How MEMS microphones work

Article By : Majeed Ahmad

MEMS microphones boast a reliable monolithic structure and far more compact form factor, which significantly lowers mechanical vibration, power consumption, and noise interference.

Like their electret condenser microphone (ECM) counterpart, MEMS microphones extract audio pressure changes as electrical signals. However, MEMS microphones boast a reliable monolithic structure and far more compact form factor, which significantly lowers mechanical vibration, power consumption, and noise interference. They also offer a better signal-to-noise ratio (SNR) and support a wide operating temperature range.

MEMS microphones, also known as silicon microphones, are now commonly used in smartphones, tablets, laptops, hearing aids, voice biometric, digital voice assistants, and more.

MEMS microphone comprises a flexibly suspended diaphragm free to move above a fixed backplate and is fabricated on a silicon wafer. When a sound pressure passes through holes in the backplate, it causes the diaphragm to move in proportion to the sound wave’s amplitude. The movement varies the distance between the diaphragm and the backplate, and that varies the capacitance. Here, a semiconductor device converts the change in capacitance into an electrical signal.

Figure 1 The MEMS membrane converts sound waves into an electrical signal while ASIC processes signals either in differential analog or digital format. Source: CUI Devices

MEMS microphones are available as both analog or digital devices. The chip inside the MEMS microphone can provide electric signals either in differential analog or digital format at the output.

Analog versus digital

Analog MEMS microphones produce an output voltage proportional to the instantaneous air pressure level, which is usually driven by an internal amplifier chip for analog processing.

Figure 2 Analog MEMS microphones usually comprise three pins: ground, power supply voltage (VDD), and output. Source: ResearchGate

Design examples include simple loudspeakers and radio communication systems. While analog MEMS microphones offer a simple and straightforward interface to the host device, analog signals mandate careful design to eliminate noise between the microphone output and the input of the IC receiving the signal.

On the other hand, digital MEMS microphones convert analog voltage signals into a digital bitstream using an analog-to-digital converter (ADC) followed by encoding with pulse density modulation (PDM) technique. Typically, digital MEMS microphones are more suitable for designs where the host is a microcontroller (MCU) or a digital signal processor (DSP). Digital MEMS microphones also offer better noise immunity, bit error tolerance, and a simple hardware interface.

Figure 3 An ADC is required to convert the output of MEMS microphones into digital format. Source: ResearchGate

The decision between choosing an analog or digital MEMS microphone often depends on the end application. Especially, how the end application is going to use the output signal.

Advanced features

MEMS microphones, now a key enabler in human-machine communication, are becoming a major building block in voice-enabled applications. For instance, STMicroelectronics has incorporated its MEMS microphone into a system-in-package (SiP) that integrates DSP Group’s ultra-low-power voice processors and Sensory’s voice recognition firmware. The design is aimed at voice-activated appliances such as smart speakers, TV remotes, wearables, and smart home systems.

MEMS microphone’s dramatic enhancement of sound quality made available in a small form factor also makes it a popular component in microphone arrays targeted at applications like concert halls, television broadcasting, and surveillance systems. The arrays of MEMS microphones are employed to significantly improve the quality of sound while creating a more directional response.

This article was originally published on Planet Analog.

Majeed Ahmad, Editor-in-Chief of EDN, has covered the electronics design industry for more than two decades.

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