Contributed by Jefferson Chua, Circuit Engineer
What Thomas Edison did for today's semiconductors
I recently watched a program on the Discovery Channel describing how telephones work. Now normally, I'd rather be doing circuit extraction, or perhaps watching the NBA, but having studied electronics for many years, how could I not know how telephones work? There was a part of the show where the three basic elements of a telephone were described. These are the switch, the speaker, and the all important microphone.
Back in school we all learned - thanks to Thomas Edison's ingenious invention of the carbon microphone - that we can capture sound waves and convert them to electrical signal. Although the carbon microphone is one of the oldest types of microphones invented, it is still in use today. For all those who forgot (or never knew) how this works - including me - the show told us that as sound waves hit the diaphragm, they compress the carbon dust, and that changes its resistance. The variable resistance in turn, changes the amount of current that flows.
Although that satisfied most of my curiosity, I was still curious as to why a microphone is called "micro" phone? I mean, it's true that the carbon microphone or other types of microphones are small, but isn’t the word "micro" supposed to be 'extremely small' or simply 'small-small?'
Surprisingly, my question was answered in the 21st century with the AKU2000, which according to Akustica, is the first CMOS-implemented single chip microphone using MEMS technology.
Die Photo - AKU2000 (Click to see larger image)
This device is a surface-mountable digital output microphone which integrates the MEMS acoustic transducer (with a movable diaphragm), pre-amplifiers, internal voltage regulators, a 4th order 14-bit sigma-delta modulator plus many more, all in a single CMOS-process chip. Furthermore, the output of the microphone is Pulse Density Modulated (PDM). That is, it’s a single-bit digital output stream that is insensitive to radio frequency (RF) and electromagnetic(EM) interference. In a way, it is specifically designed for use in microphone array applications which require a high level of noise immunity.
So how does it work? Just like the carbon or condenser microphone, as sound waves hit the MEMS transducer 'diaphragm' which is etched directly on the silicon chip, a corresponding analog signal is created and fed to the pre-amplifier stage. The amplified signal is then integrated into the 4th order sigma-delta modulator creating a robust digital output. Since its application caters to the portable industry, the digital signal coming from the ADC eliminates the need for additional circuitry, such as analog filters, and thereby simplifying the overall circuit design.
The chip measures to be 1.7 mm x 1.7 mm and believe me, it is very small…
Now that’s what I call “micro”-phone.
In related MEMS news, our presentations during Transducers07 in Lyon were met with great reception. Although we didn't specifically present MEMS microphone devices, we did take the attendees inside some of the latest in MEMS inertial sensors and printhead technologies. If you would like to receive a copy of these presentations please contact us.
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