MEMS microphone has digital output
By Graham Prophet, EDN Europe -- EDN, May 11, 2006
Akustica's claim that its AKU2000 is the first CMOS MEMS (microelectromechanical-system), direct-digital-output, single-chip microphone. The company licensed the technology, which it believes is comprehensively patent-protected, from Carnegie-Mellon University (Pittsburgh). The technology allows Akustica to build MEMS using the metallization layers of a standard-CMOS process to form the membrane that makes the sensing element of the microphone. Because the technology is standard-CMOS, the company can build an output amplifier and fourth-order sigma-delta modulator onto the same chip.
Unlike the electret-capacitor-microphone technology the Akustica product aspires to replace, the chip is surface-mountable in a standard flow-soldering process. It requires a clock input of 1 to 4 MHz and operates from 2.8 to 3.6V, using less than 750 µA. Akustica designed the 4×4-mm package for reflow soldering on the reverse of a pc board. The board must have a through-hole aligned with the membrane opening on the chip, but this arrangement simplifies sealing the assembly to an external surface. The output is a pulse-density-modulated, serial bit stream. Akustica defines the chip's frequency response by a mask that specifies it to be ±3 dB over 200 Hz to 6 kHz.
Akustica initially targets the laptop/notebook-computer market, although the cellular phone is a potentially bigger market. However, the company believes that the standard electret-capacitor microphone is more deeply entrenched in cell phones and that Akustica will be better able to attack that sector when it has driven its product further down the price curve.
The fact that the device has a digital output makes the AKU2000 of great interest to laptop/notebook designers and also those working in a similar physical format. The high impedance and low signal level of an electret make it vulnerable to electrical noise: It is difficult to route screened wiring through a laptop's hinge, so the microphone often resides in the chassis of the machine, close to sources of mechanical noise, such as hard-disk drives and fans. The device's digital output not only overcomes that problem, but also makes it feasible to mount two or four of them on a screen bezel. The designer can use them as a small array to shape the overall acoustic response into a high-sensitivity zone where the user is and cancel out noise from elsewhere. The AKU2000 sells for $3.87 (1000).
