Research On Interface Circuits Of MEMS Capacitive Sensors

A capacitive sensor is disclosed that includes a variable capacitor transducer which varies its capacitance with changes in an environmental parameter. It can find wide applications in parameter measurement such as pressure, humidity, temperature and acceleration, et al. With the fast development of the design and manufacturing level of MEMS, several sorts of MEMS capacitive sensors, such as pressure sensor, humidity sensor, accelerometer and biochemical sensors have been put into use. Because of the very small capacitance, an interface IC is necessary for the MEMS capacitive sensor.In this paper, a novel integrated interface circuit for the MEMS capacitive sensors has been developed. It works as a capacitance-to-frequency converter and contains a variable relaxation oscillator, whose output frequency is related to the capacitive sensor. A differential frequency circuit is designed following the oscillator. It develops the difference frequency between the sensor oscillator output and the reference oscillator output. It gets great improvement in the temperature property and PSRR of the circuit, and moves the output frequency into the band which can be easily dealt by MCU. The influence of VDD fluctuation is reduced by 97 percent, and the influence of Vth fluctuation and Temperature is also reduced by 80 percent and 77 percent. The sensitivity of the circuit can reach 25Hz/fF by simulation. The layout of the circuit is developed by TannerTM L-edit layout edit software. To realize the compatibility of MEMS sensors and the circuit, a CMOS process is chosen. The layout is designed strictly following the design rules. The layout parameters extract files which are essential to get the post simulation netlist are also given out. The LVS check is completed by post simulation.The interface circuit is processed in 2μm P-well standard CMOS process. An overall testing on the circuit is completed, and the methods and results of testing are presented. The output frequency of the circuit depends on the capacitance of the sensor, and the resistance of current source. The sensitivity of the interface circuit under testing is about 400Hz/pF. Analysis about the testing result is also given. The circuit is both suitable for our humidity sensor and pressure sensor. The integration of the sensor and IC is considered. It is also considered how to protect the circuit during the post process. A plan is given out to describe how to build a detect system based on the circuit.