Quality Control of digital MEMS Mic Arrays

MEMS Mic

Digital MEMS (MicroElectroMechanical System) microphones are components that are used in various consumer devices as well as in automotive and industrial applications. A digital MEMS microphone combines an acoustic sensor as well as an A to D converter on one silicon chip. This requires only a very small footprint on a PCB (printed circuit board), and allows the microphones to be connected directly to a signal processor.

Due to the rapidly-growing use of voice recognition applications, digital MEMS microphones are often used in an array formation. To ensure flawless operation, the absolute specification values, and, even more important, the values of all microphones in the array relative to each other must be tested. This page explains how to interface digital MEMS microphones to an acoustic test system and how to measure the relevant key parameters for a reliable QC procedure.

At a glance

  • For measurements on single components or arrays of digital MEMS microphones
  • Seamless integration into NTi Audio microphone test solution
  • Provides different voltage supplies and operating clock frequencies

What to test

To test the acoustic parameters of a digital MEMS microphone, the digital signal must interface directly with the audio analyzer system, or be converted to a different format, e.g. analog. The typical parameters of interest for a QC test are the same as for the testing of most other microphones; Sensitivity, Frequency response, Distortion, and sometimes Signal to Noise ratio (SNR). For a complete microphone characterization typically performed in a lab environment, parameters such as EIN (Equivalent Input Noise), PSR (Power Supply Rejection), PSRR (Power Supply Rejection Rate), and Dynamic Range are measured or calculated. Optionally, the directional behavior of a microphone at different frequencies can be measured by using a turntable.

For all absolute measurements (those that are not expressed in % or dB) the units for digital MEMS microphones are different. While the sensitivity of analog microphones is expressed in mV/Pa or dBV/Pa, the unit for digital microphone is dBFs. This stands for “decibels below Fullscale” and describes the headroom of a digital microphone from 94dBSPL (1Pa) to the maximum digital output of that microphone. This point of maximum digital output is also referred to as the AOP (Acoustic Overload Point).

Acoustic vs. Digital vs. Analog Domain

Testing single MEMS microphones is very rare. In most cases, the MEMS microphones are tested on an assembled PCB containing several MEMS microphones. For characterizing the performance of that PCB, it is of interest how the assembled MEMS microphones behave relative to each other. A typical parameter is the “Sensitivity Span”; the difference between the highest and lowest sensitivity measured on the assembled MEMS microphones.


Digital MEMS microphone peculiarities

Digital MEMS microphones deliver data in the ½ cycle PDM format. The microphone requires a CLK input, and delivers its data on a DATA output. Furthermore, two microphones share one data line. Therefore, each microphone is configured to be either a “left” or “right” microphone. This is done by hardwiring the L/R input pin to either Vdd or ground. MEMS microphones are supplied mostly by 1.8V or 3.3V.

In normal operation, the “left” microphone writes a data bit on each rising edge of the clock signal, while the “right” microphone writes a data bit on each falling edge. While one microphone is writing data, the other one puts its DATA output into a high-impedance mode. On the DSP that is receiving the data, the left and right signal data are then separated and put together into two signal streams.

Normal operation of two digital MEMS microphones

But what happens when one of the two microphones is not assembled correctly or is missing?

Operation with one inoperative or missing MEMS microphone

In this example, the right microphone is missing, therefore only the left microphone is writing to the data line. At the falling edges, the left microphone puts its DATA line to high-impedance state. Therefore, the DATA line keeps its state as it was previously written by the left microphone. As a result, from the receiving DSP perspective, the right microphone seems to deliver the exact same data as the left microphone. The two data streams are identical! This problem must be addressed by the test system, as detecting a missing microphone is a fundamental feature when testing a MEMS array PCB.

The clock frequencies used to operate digital MEMS microphones range typically between a few hundred kHz, up to 3MHz. A lower clock rate means lower power consumption, but also lower audio quality.

To ensure digital signal integrity, it’s recommended to keep the distances between digital MEMS microphones and the audio test system as short as possible. These microphones are simply not designed to drive a long high-capacitance cable.

The recommended NTi Audio solution

The basic measurement system for testing digital MEMS mic arrays consists of an audio analyzer, an NTi Audio MEMS interface box, a reference loudspeaker and a reference microphone. The system is controlled by a PC software.

Test setup for measuring a 6 MEMS mic array PCB


The FX100 Audio Analyzer

The FX100 generates the test signals for the reference loudspeaker, and analyzes the signals coming from the MEMS microphones as well as from a reference microphone. Depending on the number of MEMS microphones and the time constraints, additional parallel channels or input switchers can be used.


NTi Audio MEMS Mic Interface box

MEMS Mic Test Box

MEMS Microphone Text Box

Provides an interface to connect up to 8 digital MEMS microphones in parallel. Each MEMS mic signal is converted and routed to a balanced audio output. The MEMS mic box provides 1.8V or 3.3V supply to the microphones and allows selecting between different clock frequencies. Not-connected or inoperable MEMS microphones are reliably detected and visualized by LEDs on the box. The MEMS mic box communicates to the PC via a USB interface.


The reference loudspeaker

This has to provide sufficient bandwidth and sound pressure to cover the required test conditions. It is recommended to use a coaxial design (point-source) loudspeaker to avoid non-uniform sound distribution.


The reference microphone

This is to measure the true signal coming from the loudspeaker during each measurement. With this information, any deviation or drift from the reference loudspeaker can be compensated.


The PC Software

For EOL (End Of Line) testing of digital MEMS mic array PCBs the RT-Mic software is the ideal choice. It offers an easy-to-handle configuration, guided workflows for calibration, reference data collection, and limit calculation. Each microphone is measured and judged against PASS/FAIL criteria. The results of the individual MEMS microphone tests are summarize in an overall DUT (Device Under Test) result.

RT-Mic EOL QC software


Options & Accessories

An environment sensor can measure and log temperature, relative humidity, and barometric pressure along with the measurement data.

A barcode scanner can be used to read the serial number of the measured DUT A turntable is used to determine the directional characteristic of a single MEMS microphone

Benefits

  • Connect up to 8 MEMS microphones. This covers all MEMS array PCBs used for smart devices, automotive applications, etc.
  • Fast and accurate measurement of all relevant acoustic parameters.
  • Single MEMS microphone and complete PCB evaluation.
  • Integrated detection of inoperable or missing MEMS microphones.
  • Turnkey solution for EOL testing applications.
  • Selectable MEMS microphone voltage supply and clock frequency.

Configuration

Flexus FX100 Audio Analyzer

Digital MEMS Microphone Test System

contains

  • Flexus FX100 Audio Analyzer
  • Measurement Microphone M2010
  • RT-Microphone Software for Flexus
  • MEMS Mic Interface Box

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