Monday, February 27, 2006

"CHIPS: How one-chip Akustica mic does it"

The standard way to add a microphone to a digital appliance such as a notebook PC, Bluetooth headset or PDA is through a combina- tion of four components: a miniature electret condenser microphone, a discrete field-effect transistor (FET), a separate operational preamplifier chip and an analog-to-digital converter chip. Akustica's digital-output CMOS microphone chip concentrates all four functions in a single device. An electret microphone uses a metallic diaphragm with an air gap between it and a fixed metallic backplate, which acts as the plates of a capacitor. The gate of the FET is attached to this capacitor, and as sound waves move the diaphragm, they change the capacitance between the diaphragm and the backplate. This invokes a change in voltage output from the FET that is fed to the external preamplifier, which in turn feeds an analog-to-digital converter. Akustica's design, too, is based on a metallic diaphragm inducing a change in capacitance, but eliminates the high-impedance input to the FET by directly connecting the diaphragm to an impedance-matched preamplifier that is on the same chip as the diaphragm. Likewise, the A/D converter is also integrated on-chip. Because of the short distance between the diaphragm and the preamplifier, the single-chip MEMS microphone has better isolation from the power supply and output stage as well as being virtually immune to stray RF, electromagnetic and power supply fluctuations. Moreover, its output is digital, making it a perfect fit for digital appliances. The short trace lengths eliminate antenna effects and make reliable, repeatable impedance matching possible for the on-chip preamp. Also, the diaphragm can be less than 0.5 mm in diameter, compared with 4 to 6 mm for the diaphragm of an electret mic.