An oversampled capacitance-to-voltage converter IC with application to time-domain characterization of MEMS resonators

This paper reports the first electronic circuit used to measure the motion of a microelectromechanical systems (MEMS) resonator in the time domain. The measurement of the shuttle position is made using a capacitance-to-voltage converter IC that has been developed by combining correlated double sampling with delta modulation in a fully differential circuit topology. This oversampling circuit may be adjusted to trade bandwidth (sample rate) for resolution, while reference levels may be adjusted to set the desired sensitivity to accommodate a large range of capacitive sensor interface applications. The IC was fabricated using an inexpensive, 1.5-mu m, double-metal, double-polysilicon CMOS technology, and test results demonstrate a resolution of 170 aF for a signal bandwidth of 3 kHz, a 68-dB dynamic range, and nonlinearity less than 0.16%. The converter IC was used to characterize a comb-drive, SiC lateral MEMS resonator by time-domain measurement of its shuttle-comb capacitance. Resonant frequency was found to be 16.6 kHz, independent of operating pressure, but quality factor varied from 51 at 760-Torr pressure to 6000 at 175 mTorr. The ability to accurately characterize the SiC resonator shows that the packaging approach used in this study is sufficient to interface capacitive-based MEMS devices with Si ICs in cases where on-chip integration is not feasible or possible.