Design Of Open Loop Detection Circuit For MEMS Accelerometer

MEMS accelerometer is a kind of important inertial sensor,it is more and more popular in various fields. According the basic requirements of basic indictors and application based on the sensor, a kind of detection circuit is designed for capacitive mems accelerometer.Open loop project is used for the detection circuit, whose theoretical basis is modulation and demodulation of signal, the open loop detection system comprises the voltage buffer, which is used as C-V convertor, the sample and hold demodulator,which is used to demodulate the signal, the instrumentation amplifier, which is used to amplify the signal, the switched capacitor low pass filter, which is used to attenuated the high frequency signal.Follow a top-down design method, the basic principle of accelerometer and signal detection is analyzed at first, and the Simulink model of the open loop detection is introduced,which can provide the theoretical basis for circuit designing. Secondly, the electrical model of accelerometer is built, which is used to simulate the behavior of the sensor, the idea electrical model of the other models is built, too, combined with the sensor model and the electrical model, that can realize the machine electricity integration simulation, which can provide specification for circuit designing, the underlying circuit is designed according to the specification.With the L50 W fabrication of MXIC,the design of the open loop detecting circuit is completed. As the modulated carrier, the square wave, whose frequency is 100 kHz, is loaded on the top and bottom electrodes of the capacitive sensor. The amplifier of the buffer is used two folded cascode structure, whose bandwidth and slew rate can be 4 MHz and 5V/us. The switched-demodulator, with two sampling channels, whose sampling frequency is 100 kHz,can sample the positive and negative cycle of the signal. The instrumentation amplifier can realize double input single output function and amplify the signal. Low pass filter is 2 order switched capacitor structure, whose filter function is butterworth, whose circuit structure is MFB, it can realize the cutoff frequency of 3 kHz,with signal attenuation is lower than 0.001%. The supply voltage is 5 V, according to the simulation result, the max acceleration of the circuit is 2.7 g, the minimum detectable acceleration is 100 ug,which is meet the design requirements.