A Switch-Bridge-Based Readout Circuit for Differential Capacitance Measurement in MEMS Resonators

This paper demonstrates a study of a switch-bridge-based readout circuit for differential capacitance measurement in MEMS resonators. The proposed switch-bridge circuit is improved upon a diode-bridge circuit, which is simple, highly sensitive, and environmentally robust. However, the diode-bridge circuit suffers an output limitation due to the requirement of reverse-bias for diodes. The switch-bridge circuit keeps the advantages of the diode-bridge circuit but eliminates its limitation by replacing diodes with switches. As a result, the sensitivity of the switch-bridge circuit can be further improved by increasing the reference voltage. The presented behaviors and characteristics of the switch-bridge circuit are also suitable for the diode-bridge circuit, whose detailed theoretical analyses have not been reported yet. Then, we introduce an error model to investigate the impacts of nonideal switches in practical applications. In addition, the noise feature is discussed by using a power spectral density model. Experimental results show that the switch-bridge-based readout circuit, when applied to a capacitive MEMS gyroscope, achieves a gain of 7.1 V/pF, or equivalently 1.96 V/ \mu {\mathrm{ m}} , and a noise density of 0.077 aF/sqrt(Hz) between 20 Hz to 50 kHz.