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Impact of cable runs on MEMS microphone tests

Posted: 02 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:micro-electro-mechanical system? MEMS? microphones? ADCs? pulse density modulation?

Virtually all smartphones, tablets, and wearables integrate multiple micro-electro-mechanical system (MEMS) microphones both for capturing sound and implementing noise-cancellation features. These small, surface-mounted components are cost effective and provide remarkably good performance. While many MEMS microphones produce only analogue signals, a new generation of digital microphones is rapidly overtaking the market. These tiny devices incorporate very simple, low-power analogue-to-digital converters (ADCs) that produce pulse density modulation (PDM) data streams.

PDM is a 1bit data technology that employs oversampling in order to match the performance of traditional pulse-code modulation (PCM) audio. By greatly oversampling the analogue signal (most commonly by a factor of 64, but with a wide range of possible values) the bandwidth of the system is increased, and the inherent noise of a 1bit system is pushed beyond audibility, where it is easily filtered.

In order to match the performance of a PCM audio system with a 48kHz sample rate, digital MEMS microphones commonly employ a sample rate of 48kHz x 64 = 3.072MHz. This high-frequency behaviour creates some issues for test and measurement.

Because the microphones are generally mounted in close proximity to destination components in the finished product, signal loss due to capacitive effects are minimal, but the situation changes for the worse when testing MEMS microphones in a lab setting. Even bench cable runs of less than 3 ft (1 m) can present problems. The low-power nature of the devices means that they can't drive much current and effectively display high-output impedance at sampling frequencies. The problem is much more severe if you're testing these acoustic devices in an anechoic chamber. All noise-generating equipment (PCs, analysers) must remain outside the chamber, typically requiring cable runs of several meters.

Of course, using high-quality cable with proper termination is important, but for long runs the most practical solution is to provide a buffer or line driver that re-transmits the PDM data stream with low output impedance, as shown below. A PDM line driver allows the analyser and associated equipment to be a comfortable distance from the microphone without introducing data loss or unwanted artifacts into the stream.

Figure 1: A PDM Line Driver boosts signals in cable runs.

Figure 2: Maximum cable length versus frequency.

Even with the line driver, losses due to cable effects are present, as shown above. With a practical limit in the neighbourhood of 45 ft (13.7 m) for the most common sample rate, this should allow for easy and accurate testing of MEMS microphones on benches and in anechoic chambers. Figure 2 shows a plot of cable length as a function of sample rate.

About the author
Brad Price contributed this article.

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