DIGITAL ELECTRONICS DESIGN FEATURE
STEP UP TO THE ELECTRONICS STAGE MEMS vs ECM for microphone applications
Words by Bruce Rose, principal applications engineer, CUI C
onversational interaction with smart machines, such as home digital
assistants, is a growing trend, one that is driving the development of new high- tech products that can listen to their surroundings, interpret commands, or gather contextual/environmental data. Accordingly, the market for the latest micro electro mechanical system (MEMS) microphones is growing quickly. But they are not the only technology on the block. The traditional, electret condenser microphone (ECM) remains a viable candidate for many applications. To assess the strengths of each type, let us first look at their operating principles and construction details.
MEMS MICROPHONES Their typical construction combines the MEMS transducer die with a signal- processing application specific integrated circuit (ASIC), placed side by side on a printed circuit board (PCB). A protective casing is placed over the top. A small hole is required to allow sound waves to enter the sensor, and may be fabricated either in the top cover or the PCB, depending on whether the microphone is top- or bottom-ported. The MEMS transducer is fabricated in silicon by a selective etching process that forms a mechanical diaphragm and a support structure. As the sound pressure causes the diaphragm to deflect, the resulting change in capacitance is converted to an electrical signal in the companion ASIC. The ASIC contains an audio preamplifier, whose analog output can be taken directly off chip. On the other hand, in a digital microphone, the signal from the preamplifier is connected to an analog-to-digital converter (ADC) and an encoder, also integrated in the ASIC. Pulse Density Modulation (PDM) is a popular digital encoding format that simplifies transmission and eases decoding at the receiver.
and today’s ASIC fabrication processes ensure close matching of electrical parameters, in addition to excellent temperature stability. On the other hand, ECMs have several
The typical construction of a MEMS microphone
ECM MICROPHONES Alternatively, let’s consider the ECM. The electret material has a fixed surface charge and is mounted as a diaphragm between the metal washer and plastic spacer shown. It is positioned close to the conductive pickup plate, and hence forms a capacitor in which the air gap acts as the dielectric. Sound-pressure waves make the diaphragm deflect, causing a change in capacitance that produces a varying voltage, according to ΔV = Q/ ΔC (where Q = a fixed charge).
COMPARING PERFORMANCE AND PROPERTIES
MEMS microphones hold several advantages. Firstly, consider the small package sizes. With both analog and digital signal conditioning circuitry integrated, designers can save PCB real- estate and bill of materials costs. MEMS microphones have relatively low output impedance, while digital devices are largely unaffected by external electrical noise. This makes them suitable for use in environments containing strong electromagnetic or RF fields, or where long wires are needed to connect the microphone to its respective application. This technology also offers immunity to interference from mechanical vibrations,
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features that make them an attractive alternative in certain applications. One of these is the wide variety of electrical termination options, including pins, wires, solder pads or spring contacts, in addition to SMT terminations, which can set designers free from conventional PCB mounting constraints. Additionally, their larger physical size simplifies effective sealing against dust and moisture. The ECMs versatile acoustic properties include unidirectionality and/or noise cancellation, granting designers leverage over non-uniform spatial sensitivity and thus enforcing immunity to background audio noise. ECMs tend to have a wider operating voltage range, assuring reliable operation in products where the voltage rail may be loosely regulated. In addition, an ECM may be preferred when upgrading an existing product, as legacy designs are usually tailored to this type of microphone, streamlining redesign work and accelerating time to market.
GUIDING DESIGN DECISIONS Ultimately, a combination of constraints will determine the optimum choice of microphone for any given application. A broad selection of MEMS microphones and ECMs increases flexibility for designers. MEMS technology currently dominates the statistics for market growth due to its many inherent advantages. But versatile termination options and directionality are two key strengths of ECMs that engineers will continue to value, whether updating legacy designs or developing completely new products.
CUI
www.cui.com ELECTRONICS | JUNE 2019 35
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