SPICE Analysis of a Micro Gyro and Mixed Analog-Digital Sensor Interface Circuit

MEMS (Micro Electro Mechanical Systems) is a fast growing field both in the commercial companies and in the universities. Different design tools such as VEM and MAGIC are available for the design of both surface and bulk micromachining devices. A tool to predict the behavior of the design is not yet available. The designs of the mechanical part of the sensor and the electronics are done separately, and only the electronics can be simulated in order to predict its behavior after fabrication. The performance of the mechanical structure can be estimated using Finite Element Analysis but cannot be combined together with the electronics. This work tries to address a possible solution for the simulation of a mechanical structure in a method that will enable one to predict the behavior of the complete sensor, together with the electronics. Since many companies and universities are currently searching for a low cost gyro to be used in automobile applications, the author decided to conduct an analysis of a comb drive structure that acts as a gyro, and to simulate its electronic equivalent circuit using SPICE.

The second part of this work introduces the design of a circuit which reads differential pick-off sensor signals. The circuit eliminates the need for a transformer, unlike the Blurnlein Capacitance Bridge, and relies only on a capacitance bridge. The circuit uses an approach to reduce the effect of the 1/f noise by using a chopping method in order to enhance the resolution of the circuit. Since MOS transistors are used as switches, the operation of the circuit helps to reduce the charge injection into the circuit. In addition, a double sampling approach is taken: During one part of the period the circuit samples the sensor signal due to a mechanical effect on the sensor and at the other part of the period it samples the null position so that both can be compared in order to eliminate parasitic effects.