Designing MEMS Microphones Into Products
MEMS microphones have effectively replaced the 50-plus-year-old electret condenser microphone (ECM) in nearly all consumer electronic products. And while MEMS microphones are the de facto standard, designers still have options to consider.
The most common footprint for small surface-mounted analog microphones is 3.35 mm x 2.5 mm. This size is available from multiple vendors in pin-compatible configurations that give designers the freedom to try out products from many different vendors in a single design.
The most important performance specification for microphones is the signal-to-noise ratio (SNR). Virtually any microphone application will benefit from having better SNR, but high SNR is an absolute necessity for advanced applications.
High SNR enables high-quality background noise cancellation. High-end noise-cancelling headphones, for example, use large and expensive ECM microphones with greater-than-70 dBa SNRs to create rich, immersive audio experiences. If this level of noise cancellation could be delivered to lower-cost consumer devices, VUI would work better in environments with lots of background noise. Higher SNR also increases the effective range of microphones, enabling far-field audio and VUI at a distance. These applications should seek out the highest SNR possible.
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Power supply rejection ratio (PSRR) is another key design criteria. Having high PSRR allows the microphone to reject noise that couples into the power supply line. This noise can have many sources but is most common on systems with less clean power supplies, on large systems where the power supply is physically distant from the microphone, and on systems with lots of radio frequency (RF) components such as smartphones and tablets. Designers should look for PSRR of at least 70 dB.
Another consideration is analog versus digital. Analog microphones still make up the majority of the market, but there are applications where digital might be a better choice. Analog microphones are generally smaller than digital and great where size is the key consideration. Digital microphones contain an analog-to-digital converter (ADC), and the path from the microphone’s output to the codec is completely digital. If the path from the microphone to the codec is long or traverses areas with intensive RF noise, then a digital output will offer superior resistance to interference.
Consider Robustness of Your Microphone Selection
Quality and reliability issues have also been the Achilles heel of MEMS microphones for a long time. Most MEMS devices are sealed in hermetic packages, but microphones need to be exposed to the environment to work. Contaminants such as dust, dirt, solder flux vapors, and liquids will make traditional capacitive MEMS microphones malfunction.
Piezoelectric MEMS microphones, on the other hand, are immune to these contaminants, offering a more robust alternative to capacitive MEMS mics. They are the best choice for systems that may be exposed to water, sweat, dust, particles, or solder flux vapor and can be used outdoors and have intended lifetimes greater than two years.
Using proper microphone evaluation criteria, designers can create systems that will delight the growing number of VUI product users with rich audio experiences and avoid the frustrations and costs of reliability failures. One thing is certain, while we are still in the first year of a 10-year technology innovation cycle centered around IoT, the variety and breadth of VUI-enabled IoT devices will be big.
Matt Crowley is CEO of Vesper, a provider of piezoelectric MEMS microphones. Prior to joining Vesper, he was founder and VP of business development at Sand 9, where he pioneered the development of piezoelectric MEMS for mobile timing.
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