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Design of the Interface Circuit for C/V Integrated Capacitive Humidity Sensor

Author: ChenJianChun
Tutor: CaoYiJiang
School: Harbin University of Science and Technology
Course: Microelectronics and Solid State Electronics
Keywords: Switched capacitor CMOS integrated Operational amplifier Capacitive sensor
CLC: TP212
Type: Master's thesis
Year: 2009
Downloads: 172
Quote: 3
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Abstract


The capacitance type sensor is an important class of sensors in the sensor family. It is the use of the principle of charge-discharge, the change with the analyte variable changes in the electrical capacity converted to facilitate the measurement and transmission of the parameters of the voltage, current or frequency, etc. The device, in the humidity, temperature, displacement, pressure, etc. measurements obtained very broad Application However, due to the small capacitance change amount is very small, generally in pF magnitude, so using a dedicated integrated interface circuit to become an important method for detecting and processing the small capacitance. According to the charge-discharge detection principle, based C / V (capacitance / voltage) conversion CMOS integrated capacitive humidity sensor interface circuit design to achieve high-precision measurement of micro-capacitance. The interface circuit is mainly constituted by an operational amplifier and the switched capacitor circuit. The core part of the circuit design for the design of the operational amplifier, op amp performance requirements, interface circuit using two op-amp structure, first stage folded cascode structure to provide high gain; first two common source-level structure, to provide higher output swing; reference current source for the op amp bias current. The switched capacitor circuit using the principle of charge and discharge to complete the detection of micro-capacitance, due to the use of CMOS analog switches, to improve the accuracy of the circuit. CSMC 0.6μm CMOS process, the interface circuit transistor parameters were set, and use of the HSpice simulation software circuit simulation and parameter adjustment. The simulation results show that: 91.842dB, the op amp's open-loop gain and phase margin 58 ° 3dB bandwidth of 1KHz, unity-gain bandwidth of 14.042MHz, to meet the work requirements of the interface circuit; overall interface circuit output voltage of the input voltage The symmetrical square wave signal of the same frequency, and its amplitude varies with the sensing the change in capacitance when the change in capacitor 10pF ~ 15pF, the output voltage changes from 0.5V to 2.3V, and whenever sensing capacitance change 0.01pF, The output voltage will be generated 3.6mV of change to achieve the detection of the small capacitance. Use Cadence software interface circuit layout, fully consider the match between devices reduce parasitics and improve the circuit performance and design rule checking and layout and schematic consistency, verify that the territory The correctness of the design. Compared with the conventional interface circuit, the circuit having a high gain, precision, and a simple structure and can be compatible with the CMOS process, and suitable for most of the capacitance type sensor, in order to better achieve the detection of small capacitance.

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