Wide-Band MEMS Microphones Application Guide

Microphone Placement within a Product

When designing a PUI Audio MEMS microphone into your product, careful PCB placement and soundhole position need to be considered to minimize noise. The microphone should be placed to avoid antennae, amplifiers, motors, power supplies, or any other devices that may create noise interference. Additionally, microphones should not be placed on the same PCB as spring contact or surface mount speakers, as the vibration from the speaker is often transmitted to the microphone through the PCB, creating noise, echo, or feedback.

Microphone External Sound Hole Placement on a Product’s Housing

The external sound hole on a product should be limited in distance from, and larger in diameter than, the sound hole on the microphone. A direct path from the microphone’s sound hole (also referred to as the Acoustic Port) to the exterior of a product’s housing is required for optimum frequency response and to minimize unwanted noise from entering the microphone. Placing the external sound hole on a flat surface simplifies the design of a gasket between the housing and the microphone sound hole.

Microphone Sound Channel Design

When designing the sound channel that connects the external sound hole to the microphone’s soundhole, first ensure that there is no leak by gasketing the area between the external soundhole and the microphone soundhole.

Many echo and feedback problems are caused by a leaky gasket between the soundhole between a topfiring microphone and the housing or between the bottom-firing microphone’s PCB and the housing. Please ensure the gasket is fully compressed to eliminate potential problems and/or degradation in performance.

The sound channel should be kept as short as possible and larger in diameter than the microphone soundhole for top-firing microphones or be greater than or equal to the diameter of the PCB soundhole for bottom-firing microphones. The diameter of the gasket should be as close to the same size as the external sound hole to prevent creating a Helmholtz resonating chamber. A long, thin or short, wide sound channel can “tune” the microphone, creating a peak in response between 10 kHz to 15 kHz, followed by a steep roll-off in frequency response thereafter.

Analog MEMS Microphone Circuit Design

Slight variances in power supply voltage do not affect the sensitivity of PUI Audio’s analog MEMS microphones (designated by AMM in the PUI Audio part number). As such, a circuit designer only needs to ensure that the voltage output of the power supply is within the voltage range called out in the MEMS microphone specifications.

Unlike traditional ECM microphones, MEMS microphones do not require the use of a bias resistor between the power supply and the microphone. MEMS microphones have an independent output that is separate from the voltage input. It is recommended to decouple the noise of the power supply from the MEMS microphone by using a 0.1µF capacitor at C1 in the diagram below. A DC-blocking/high-pass filter capacitor should be placed between the MEMS output pin and the CODEC/ADC/preamplifier’s input pin, C2 in the diagram below. Values between 1µF and 3µF are often used, where the larger the capacitor value, the higher the frequency at which the high-pass filter’s corner frequency is placed.

In the event of electromagnetic interference, place a resistor that matches the microphone’s impedance between the amplifier’s unused differential input and the microphone’s ground.

Digital MEMS Microphones

PUI Audio offers digital output MEMS microphones (designated by DMM in the PUI Audio part number) with PDM and I2S data formats. Both feature a Left/Right channel select pin for stereo audio capture and data transfer over a single data input line on a DAC or CODEC. The I2S digital MEMS (DMM-4026-B-I2S-R) microphone also offers the ability to set up microphone arrays by changing the WCLK Hold Time.

PUI Audio’s digital MEMS microphones offer a Full Power mode, a reduced sensitivity Low Power mode, and a Sleep mode for reduced current draw and battery power savings when the microphone isn’t needed. Modes are activated by changing the input clock frequency to the microphone.