Design Of Capacitive Gas Sensor And Peripheral Reference Current Source Circuit

Over the past decade, technical advantages of Micro Systems Engineering in development speed and new application numbers give us a very deep impression. Micro Systems Engineering includes Micro Electro-Mechanical Systems (MEMS) and related equipment design, manufacture and package. Micro Systems Engineering is applied in the aerospace, automotive, biotechnology, consumer products, defense, environmental protection and safety, health care, pharmaceutical, telecommunications industry, and other aspects of data measurement. Based on MEMS technology, a novel capacitive SO2 gas sensor and its peripheral current reference circuit was proposed in this paper.The response mechanism of traditional gas sensor is based on surface effect by one-dimensional diffusion. A new design of capacitive gas sensor for sulfur dioxide is discussed in this paper. The structure is made up of cylinder model with three-dimensional diffusion, which upper and side surfaces can meet the gas at the same time to enlarge the area with gas. According to the theory of film diffusion, the response time of gas sensor is analyzed and compared with traditional sensor, and the new gas sensor has less response time is made out in the end.As the external signal processing circuit core modules of MEMS gas sensors, low temperature coefficient and high power supply rejection ratio (PSRR) of reference current source are the core parameters of the circuit design.A new low temperature coefficient and high-PSRR current reference design was presented. Positive temperature coefficient current was generated by Widlar reference current source; negative temperature coefficient current was generated by base-emitter votage of BJT. The two current were compensated, resulting in current reference. Based on the traditional Widlar-type structure reference current source, the BJT small-signal model was analyzed, the traditional Widlar type current reference feedback gain is less than 1 positive feedback. Operational amplifier was added in Widlar type current reference feedback loop to increase the circuit's PSRR, and the feedback properties was changed to stabilize the circuit operating point. As the temperature characteristic curve of transistor is highly volatile in the high and low temperature phase, particularly in the high temperature phase, the base-emitter voltage temperature characteristics appears obvious non-linear. Self-compensation diode loop was designed to increase current reference curvature compensation, that is, positive temperature coefficient and negative temperature coefficient current direct coupling at the low temperature phase, the compensation effect is obvious; due to base-emitter current rapid decline at the high temperature phase, compensation current curve was decreased rapidly, the compensation current was generated from the compensation diode loop compensation, the high-temperature phase current reference curvature compensation was realized. The circuit was simulated with SMIC 0.35um BICMOS technology process, the reference current has a PSRR of 144.07dB and temperature coefficient of 12.4ppm/℃.