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 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.