MEMS INTERFACE CIRCUITS

Abstract

In this thesis, MEMS electronic interface for an optical micro-accelerometer is designed. At first, specification for the interface circuit design is developed by studying the mechanical properties under MEMS application conditions. According to this specification, two schemes of interface circuits have been implemented. In the first design, a fully differential transimpedance amplifier (TIA) converts the photo-diode current to voltage. The fully differential TIA has two current inputs. One is from PD1which reflects the input acceleration, while the other is from PD2, which is shielded from illumination and thus reflects the dark cur­ rent and shot noise of photocliodes. The differential output from TIA stage goes through a buffer stage which is also used as a low-pass filter. In the second design, only one photodiode is used to convert the displacement into corresponding cur­ rent. This current is then amplified by the transimpedance stage and sent to the buffer amplifier. Tho background and low frequency flicker noises arc removed by integrating the output signal from buffer amplifier and feeding it back to the input with a switched-capacitor integrator. Simulation results show that they can meet the specification. As a special topic of this thesis, buffer amplifier design is critically examined. Previous designs in this field have been classified and reviewed. One novel buffer amplifier circuits has been designed and implemented by fabrication using Alpha 0.8µ.m analog service. In the new design, a class-AB low voltage and large output swing CMOS buffer amplifier using improved quasi-complementary output stage and improved adaptive loads is developed. Improved quasi-complementary out­ put stage enables it more suitable for the low voltage applications, while adaptive load is used to reduce the sensitivity of the quiescent current to process variation. The performance of the adaptive load in this buffer has been improved compared with the previous one introduced in [I].