A Readout circuit for resonant MEMS temperature sensors

Asadi, Hamed
High precision is the dominant advantage that resonant sensors have over other types of analog sensors (sensors with a subsequent analog-to-digital converter). The resolution of these precise sensors is determined by both frequency resolution and sensitivity of the sensor. The sensitivity is highly related to the sensitivity of the MEMS resonator. However, the frequency resolution is dominantly defined by the closed-loop circuitry if noise contribution of the resonator is assumed to be smaller than that of the electronic circuitry. In this thesis, resolution reduction of a closed-loop resonant sensor is intended to be accomplished. The DETF (double-ended tuning-fork) MEMS resonator used in this study is already designed and fabricated in METU MEMS. The TCF (temperature coefficient of frequency) of this resonator is 53 ppm/K which results in a sensitivity of 9.27 Hz/K with 174.818 kHz resonant frequency. This implies that the sensitivity is fixed and the only remaining parameter to achieve a better resolution for the sensor is the frequency resolution of the closed-loop circuitry. Since closed-loop with an AGC (automatic gain control) block has superior far-from-carrier phase noise performance in comparison to a closed-loop circuit with a limiter (e.g., comparator), this scheme is chosen to be implemented in XFAB 0.35um CMOS process. For the implemented closed-loop circuit in CMOS technology, the minimum achievable noise floor at the steady state oscillation is obtained to be -107 dBc/Hz. Based on the simulation results, when the "white phase" noise is filtered out, the sensor resolution is obtained to be around 0.021 degrees Celsius. To verify the functionality of the design in the CMOS technology, the closed-loop circuit is implemented again by discrete components. The stability and resolution evaluations of the implemented circuit are achieved by Allan deviation. The bias instability point is measured to be about 0.0045 degrees Celsius with 5.28 second averaging time. In this thesis, the main goal of the design, especially in XFAB 0.35um CMOS Process, is to study and characterize the noise and phase noise performances of the closed loop circuit. Thus, the resolution improvement can be accomplished by minimizing the noise in the loop.