MEMS microphone technology adapting to changing use cases: Page 2 of 5

April 13, 2017 //By Masahito Kanaya
Owners of devices such as smartphones and tablets continually want to be able to use their gadgets in new ways, and at the same time expect extremely high performance. On-board audio functionality is a prime example. People want to be able to record social events, music performance and expect accurate, lifelike playback, or to enjoy high voice-call quality free of background noise even when outside or travelling in a car. There is also demand for high audio quality when capturing sounds further from the microphone.

Going digital

 

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.


Figure 1: Showing noise immunity improvement with digital.

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 1 MHz to 3.5 MHz.

The microphone’s output is driven to the proper level on the selected clock edge and then enters a high-impedance state for the other half of the clock cycle. This allows two digital microphone outputs to share a single data line (figure 2). The L/R input determines the clock edge for valid data.


Figure 2: Digital microphone allows a reduction in number of transmission lines.

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