Sound is the latest way in which consumer electronics product developers are looking to differentiate their offerings. But designers face problems, particularly when it comes to the small portable devices to which we have become accustomed. The traditional solution – the electret condenser microphone, or ECM (see fig 1) – is relatively large, doesn't suit the high temperatures of reflow soldering processes and, importantly, draws a relatively high current. The solution lies in the adoption of MEMS technology.
Todd Borkowski is MEMS marketing manager for Analog Devices. "A lot of attention has been paid to enhancing video performance and providing connectivity. Only now has audio become important. But a lot of the features that manufacturers are looking to add require high signal to noise ratios (snr), higher frequency response, sensitivity and phase matching as microphone arrays become more popular."
Analog Devices was one of the first companies to offer MEMS products, targeted primarily at automotive applications. But it has been developing its MEMS microphone portfolio over the last few years. "It's a natural move," Borkowski claimed.
Another company bringing MEMS expertise to bear in the microphone field is Wolfson Microelectronics. But Nigel Burgess, MEMS product line manager, pointed out that MEMS is only part of the problem. "MEMS microphones need to be coupled to electronics, partly for power, partly to capture the output and to amplify the signal."
Wolfson started talking with another Scottish company in 2006 regarding the latter's MEMS transducer expertise. "Although it started out as partnership discussions, the technology had a good fit with Wolfson's audio product plan and the company ended up being integrated into Wolfson. That technology has matched well with Wolfson's electronics expertise."
That combination of expertise, said Burgess, puts Wolfson in the position of being one of only a few companies with IP in MEMS microphones, the associated electronics and packaging. "We have the capability of taking microphones to the next generation because our MEMS people sit next to our electronics people."
Why is the MEMS microphone attracting attention and how does it provide better performance than the ECM? "ECMs have the advantage of low cost and multiple sources," Borkowski admitted, "but there are problems. They are larger and draw more current. They are also susceptible to noise and rf interference. They also have a low snr in small packages."
Burgess explained some application problems. "The microphone in a laptop can be situated next to the webcam. With an analogue output, this can result in a 30cm antenna. If you can convert analogue to digital at the microphone, you reduce the aerial effect and emi problems."
The advantages of MEMS microphones include better shock resistance – up to 10,000g – better temperature operating range and less current consumption. Vibration and shock resistance is important. ECMs may not resist a sound pressure of 160dB, but that's a pressure level which can be created in a car when the door is slammed.
And MEMS microphones, in general, have a better response. Borkowski explained: "Customers say that, while they like the better frequency response of ECMs, they have to filter the output and often end up losing that benefit. They find MEMS gives that back to them."
Small temperature changes can also have large effects on the ECM and these effects can be difficult to predict.
Seemingly trivial, where the sound actually enters the microphone is seen as important. Analog Devices uses a hole in the host pcb (see fig 2), while Wolfson uses a hole in the microphone packaging. Borkowski explained Analog's approach was all to do with Helmholtz resonance; an example of which is the sound created when you blow across the neck of a bottle.
"This effect is hard to correct for," he noted, "because you can't predict where the peak resonance will be. The resonance is often large and, to counter it, you need to generate a waveform with equal and opposite magnitude."
"One of the technical problems you face is trying to reduce acoustic noise in the transducer," Burgess said. "If you can reduce this noise, you get a better performing product."
Analog Devices' MEMS microphones use a piston based design. "It's much like a trampoline," Borkowski noted. Sound waves entering the microphone move a 0.9µm diaphragm, which varies the capacitance. This signal is processed by an asic and passed to the next stage.
Currently, Analog Devices uses a two chip solution: the MEMS microphone and the asic. "We could do a single chip device," Borkowski said, "but this brings two challenges. First, it would mean a thicker package and this is a critical dimension in consumer products. Second, it would affect time to market. With the two chip approach, we can select either an analogue or a digital asic."
Analog Devices is rolling out two additions to its MEMS microphone range. The ADMP404 and 405 are both analogue devices with a sensitivity of -38dBV and an A-weighted snr of 62dB. The only difference between them is frequency response. The 404 has a flat response from 100Hz to 15kHz, while the 405's response starts at 200Hz. The reason? "It helps to filter out wind and fan noise," Borkowski pointed out.
Unlike Analog Devices, Wolfson has integration in mind. "We can make the microphone and the asic on the same piece of silicon," said Burgess, "and we want to get to that point. Step one is building microphones, step two will be integrating them to create a smaller, better performing product."
Wolfson has four analogue variants at the moment; the WM7110 and 7120, each available in an enhanced format. The WM7110 has a typical consumption of 140µA and an snr of 59dB. It offers low total harmonic distortion, good linearity and a flat phase response.
As Burgess hinted, Wolfson will be expanding its MEMS microphone range into the digital domain in the near future.
SNR is an important figure of merit. "Every 3dB of snr allows you to double the distance you can be away from the microphone. A good quality mike can eliminate the need for a headset," Borkowski claimed.
And why analogue? "People think analogue is gone and the future is digital," he continued. "I don't believe that's so and we'll probably see a 50:50 split in a couple of years."
Burgess outlined where demand for MEMS microphones is coming from. "We're starting to see the need for multiple microphones in mobile phones. For example, companies are beginning to put a microphone on the back of the phone with the camera. The microphone therefore faces the people you're trying to video and results in better quality. And there's a move to use MEMS mikes for noise cancellation; that can mean another two devices. Other applications are looking to improve hands free performance and there are moves to include mikes in digital still cameras and handheld gaming devices."
If an application uses multiple mikes, then sensitivity becomes important. "Our devices are matched closely and only have a ±1dB variation," Burgess claimed.
"One of the key things customers want is higher performance and that means better sound quality. Wolfson is looking to offer a range of mikes with the highest performance in the smallest packages," Burgess concluded.
