Biography
Enrollment Date: 2009
Graduation Date：2012
Degree：M.S.
Defense Date：2012.05.28
Advisors：Zhihua Wang
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：Research on the Multi-Range Readout Circuits for Capacitive Sensors
Abstract:
Over the past a few decades, sensor system has grown very fast. In recent years, with the rapid development of the fabrication of micro-electronics and the increasing motivation of consumer electronics, micro-sensors made by micro-electronics process has its own benefits of small size, low power, light weight, good reliability, easy to integrate and mass production. It has become very popular in health care, navigation, industrial control and automotive electronics. Due to the advantages of simple structure, low temperature coefficients and low power, capacitive sensors are wildly used now. For capacitive sensor system, the performance of readout circuits is the key influence factor. Since one single consumer electronic products has integrated more and more capacitive sensors, universal capacitive sensor readout circuits can satisfy these demands. Firstly, this paper carried out research on the single range capacitance readout circuit. Based on the single range capacitance readout circuit, research and design of multi-range capacitance readout circuit was carried out. Firstly, a capacitance-to-voltage converter based on transimpedance amplifier was designed, with its measuring range from 0 to 5 pF. This paper proposed a novel fully differential amplifier with its balancing branch. Chopper stabilization technique was used to remove the op-amp effects of offset voltage and low frequency noise. This chip was fabricated by UMC 0.18 μm CMOS process. The supply voltage is 3.3 V; clock frequency is 2 MHz; and the chip area is 0.74 mm x 0.42 mm. Measurement results show that the sensitivity is 370 mV/pF; linearity error below 1 % and power consumption as low as 2.5 mW. On-chip under-test capacitance arrays controlled by FPGA demonstrated the good dynamic performance of capacitance voltage converter, showing the readout circuit can respond frequency of capacitance variation of 1 kHz and below. Secondly, with the existing capacitance-to-voltage converter, in order to digitize the under-test capacitance, a voltage-to-frequency converter was designed. This circuit was composed of voltage-current converter, bidirectional current integrator, comparator and its related flip-flops. This chip was fabricated by CSMC 0.5 μm CMOS. The chip has its supply voltage of 5V, chip area 1 mm x 0.8 mm, linear input voltage range 2 V – 4 V. Measurement results show that sensitivity is 1.6 MHz/V, linearity error below 3 %. The max power consumption is 2.35 mW at 4 V input voltage. On the basis of the two previous circuit designs, a multi-range readout circuit for capacitive sensors was designed. This circuit was composed of three stages: the first stage was a charge transfer integrator; the second stage was an offset-compensated voltage amplifier; the third stage was output demodulator. Configuring the on-chip capacitance by SPI controlling words, it can realize the measuring range of 10 fF, 100 fF, 1 pF and 10 pF. This chip was fabricated by UMC 0.18 μm CMOS process. The supply voltage is 3.3 V. Chip area is 1.5 mm x 0.9 mm. Chip area is 2.1 mW. Simulation results show that: for 10 pF measuring range, its sensitivity is 0.4 V/pF and linearity error below 0.05 %; for 1 pF measuring range, its sensitivity is 4 V/pF and linearity error below 0.3 %; for 100 fF measuring range, its sensitivity is 40 mV/fF and linearity error below 0.7 %; for 10 fF measuring range, its sensitivity is 0.4 V/fF and linearity error below 1 %.