Capacitive sensing circuit for closed-loop control of wide tuning range microspectrometers

We examine the problem of simultaneous drive and capacitance sensing, on a microelectromechanical systems (MEMS) device, where the drive is a bipolar AC waveform. The attention of this paper is particularly focused on wavelength calibration of the microspectrometer, a MEMS micromachined Fabry Perot filter monolithically integrated with a photodetector. However, this work is also very pertinent to other bipolar AC driven MEMS devices, which presently use separate measurement MEMS structures. To avoid charging effects, the microspectrometer must be driven by an AC waveform and, the only option for capacitance measurement is to do so simultaneously, on the same terminals, as the drive waveform is applied. We propose a novel differential capacitive sensing circuit to determine the centre wavelength of the MEMS-based micro-spectrometer, allowing closed-loop control of the microspectrometer's centre wavelength. Automatic calibration can be realized with the addition of a known light source.