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MEMS microphone technology matches volume market performance requirements

MEMS microphone technology matches volume market performance requirements

Technology News |
By Julien Happich



These trends call for higher-performing microphones, and some handsets also feature noise cancellation or 3D sound in video modes by using two or more microphones. In addition, the advent of intelligent digital assistants that respond to the user’s voice is changing the ways in which people interact with computers, and could drive high-performing audio subsystems into more products like wearables and IoT devices in the future.

As a result, there is rising demand for MEMS (Micro Electro-Mechanical Systems) microphones, which deliver high performance and fidelity with reliability, within compact dimensions suitable for use in portable devices. The market for MEMS microphones is expected to rise from 3.6 billion units in 2015 to over 6 billion units in 2019, according to market research company IHS Technology.

 

Recap on MEMS microphone architecture and operation

The MEMS microphone contains a moveable diaphragm and static backplate fabricated on a silicon-wafer substrate using familiar processes including deposition and selective etching. The backplate has perforations that allow air to pass through without causing deflection. The diaphragm is designed to flex in response to changes in air pressure caused by sound waves. This flexing causes the diaphragm to move relative to the backplate, producing a proportionate change in capacitance. A companion IC co-packaged with the MEMS transducer translates this capacitance change into an electrical signal in either analog or digital format.

There are markets for MEMS microphones with analog or digital output. Analog microphones, which essentially contain the MEMS transducer and companion analog-amplifier IC, are a popular solution for small handheld devices such as feature phones and entry to mid-level smartphones.

 

Going digital

Digital microphones that integrate analog signal-conditioning and an Analog-to-Digital Converter (ADC) are typically preferred in equipment such as PCs or high-end smartphones. Digital technology enables greater audio performance by taking advantage of inherently higher RF and electromagnetic interference (EMI) immunity, as illustrated in figure 1. In addition, circuit design and board layout can be simplified, and design changes made easier by avoiding the need to adapt resistor and capacitor values.

Fig. 1: Showing noise immunity improvement with digital.

Most digital microphones also have inputs for a clock and a L/R control. The clock input is used to control the delta-sigma modulator that converts the analog signal from the sensor into a digital Pulse-Density Modulated (PDM) signal. Typical clock frequencies range from about 1MHz to 3.5MHz.


The microphone’s output is driven to the proper level on the selected clock edge and then enters a high-impedance state for the other half of the clock cycle. This allows two digital microphone outputs to share a single data line (figure 2). The L/R input determines the clock edge for valid data.
 

Fig. 2: Digital microphone allows a reduction in number of transmission lines.

With the advantages of high noise immunity and simplified circuit design, digital MEMS microphones lend themselves well to use in multi-microphone arrays for echo and noise cancellation, as well as beamforming to achieve directional sensitivity. To implement noise cancellation in a smartphone, a common approach is to position one or more extra microphones away from the main voice microphone, such as in the upper edge or the back of the case, to detect noise from the surrounding environment. This can be subtracted from the output of the voice microphone to help improve call quality. The noise-reduction microphones are often also used in video-recording modes.

Beamforming, also, uses an array of two or more microphones. Although most microphones have omnidirectional sensitivity, some applications can benefit from increased sensitivity in a particular direction or reduced sensitivity in others; for example to improve audio quality and intelligibility in situations such as audioconferencing or in-car calling. Beamforming makes this possible by applying digital algorithms to the outputs of microphones in the array, based on the phase differences of sounds arriving from various directions. It is also possible to identify the direction a particular sound is coming from.

 

ASIC design in detail

Microphone module makers differentiate their products by choosing a suitable MEMS microphone kit, in which an optimized pair of MEMS sensor and ASIC is already combined.

Fig. 3: Microphone specialists choose a suitable
MEMS microphone kit.

ON Semiconductor has focused on developing highly integrated ASICs for digital MEMS microphones, ready to be combined with any of a variety of MEMS transducers made by independent MEMS suppliers. An example is the LC706200 digital IC family, which integrates a feed-forward delta-sigma ADC in addition to an analog amplifier and low-pass filter, as shown in figure 4. There is also a charge pump that provides the operating voltage for the MEMS transducer.

Fig. 4: A feed-forward delta-sigma ADC enables
a small-footprint microphone with digital output.

ON Semiconductor’s digital ASICs can help overcome the challenges facing today’s MEMS microphone designers by satisfying key performance criteria. Among these, a high SNR is required to allow clear performance when microphones are used at greater distances, as well as for cleaner audio capture generally. In particular, automatic speech recognition algorithms depend on high SNR to achieve good word accuracy rates. ASICs with SNR greater than 64dB are expected today, complementing advances achieved by MEMS engineers to optimise the characteristics of the transducer.

As end users seek better results from devices like smartphones in an increasing variety of use cases, there is demand for microphones that can operate without distortion up to high Sound Pressure Levels (SPL) as encountered in loud environments. One example is to allow high-quality recording for social users to capture their experiences at music festivals.  


Digital MEMS microphone for future standalone voice command

There is high demand for voice command functions in IoT and portable device sectors thanks to speech recognition engines and powerful voice assistants like Siri®,  ”OK Google”, and Amazon Echo. Current speech recognition systems are typically executing all of the time as they consume quite a lot of power listening and recognizing speech. Future voice command function will be expected to be operated standalone and will be turn on when it is activated by voice. Low power digital MEMS microphone technology will be suitable for future standalone voice trigger solutions – it will perform very well, extremely low power, and they can be added to an existing design relatively easily.

Algorithms such as noise cancellation and beam forming, which analyse signals from multiple microphones, need to rely on close matching of the sensitivity of individual microphones in the array, ideally to within +/- 1dB. Although screening, or binning, is one potential solution, microphone designers are looking for ASICs to provide adjustable gain that enables tuning out of process-related variations in MEMS fabrication.

The LC706200 product family provides high performance solution. It has several other features that ensure enhanced linear performance over a wide operating range, including low input-referred noise of -106dBFS with the advantage of an 8kHz low-pass filter for peaking compensation, and a low noise internal bias and regulator circuit that leverages ON Semiconductor’s giga-ohm resistor process. The device also has a high Power-Supply Rejection Ratio (PSRR) that prevents unwanted noise entering the signal chain, and power management including sleep mode and a low-power mode that remains responsive to voice commands.

 

Conclusion

Changes in the way people use computers and smart devices are driving demand for reliable and high-performing MEMS microphones. Digital ASICs now in the marketplace maximise freedom for microphone developers to deliver best-in-class products to satisfy these demands.

 

About the author:

Masahito Kanaya is Product Marketer at ON Semiconductor – www.onsemi.com

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