AN INTEGRATED SILICON CAPACITIVE MICROPHONE WITH FREQUENCY-MODULATED DIGITAL OUTPUT

In this paper a new integrated capacitive silicon microphone with freq uency-modulated digital output is described. The FM output can be proc essed directly by a digital system without the need of analog-to-digit al conversion, which is normally required with capacitive microphones in digital systems. The integrated microphone comprises a polyimide st ructure fabricated directly on a silicon substrate, on which the oscil lator has already been completed using a standard CMOS process. This i s possible since the polyimide microphone process is fully IC-compatib le. The capacitance of the microphone modulates the output frequency o f the oscillator, and from experiments a sensitivity of 234 Hz/Pa is m easured at a carrier frequency of 263 kHz. The frequency response is f lat (+/- 1 dB) within the measured range of 100 Hz to 15 kHz. The A-we ighted noise is 4.5 Hz for a bandwidth of 15 kHz, yielding an equivale nt noise level of the microphone of 60 dB(A) re. 20 mu Pa. The measure ments have shown to be in good agreement with the theoretical models. (C) 1998 Elsevier Science S.A. All rights reserved.