Research On Controlling And Measuring Circuits Of CMOS Integrated Thermal Wind Sensor

With CMOS integrated circuits processes and the newly developed MEMS processes, many mechanical structures can be made in silicon substrates. This produced new fields for the design of the sensors. It is possible to realize the so-called SOC (System On a Chip) by integrating sensors with the processing circuits in one substrate.CMOS silicon thermal flow sensor with CMOS processes is very cheap. Integrating the controlling and signal processing circuits with the sensor structure in one substrate can realize smart wind sensor. The sensors based on silicon have very low sensitivity and the output signals are ultra weak. All these weaknesses should be make up by the circuits on the same chip. In this thesis, according to the operational principle of the sensor, the controlling and signal processing circuits were designed to meat the requirement of the sensor.The operational principle in this thesis is based on temperature difference. Under some kind of controlling, the surface of the sensor is heated above the environment. The information of the wind velocity and direction can be obtained by measuring the temperature difference between two points symmetrically located on the surface of the sensor. In the chapter of the controlling circuits, three kinds of modes were discussed. They are constant power, constant temperature difference and temperature balance modes. Constant temperature difference mode was selected to meat the sensor structure and the requirements. In the scope of measuring wind velocity, under a closed controlling loop, the temperature of the surface of the sensor is kept a constant difference above the surroundings. The circuits designed can meet the requirements after simulation.The sensor uses thermopiles to measure the temperature difference. So the output voltage signal is ultra low (micro-Volt) and in very low frequency. Those amplifiers fabricated with CMOS are restricted with their serious offset and noise, especially the low frequency 1/f noise. It is necessary to design the on chip low offset low noise amplifiers. In this thesis, two kinds of mainly used low offset low noise design techniques are discussed. One is auto-zero and the other is chopping. The chopping amplifier with its many parts of modulation switches, pre-amplifier, band-pass selective amplifier, and voltage follower and demodulation switches is designed. The circuit is simulated with HSPICE to optimize. The residual offset of microvolt and the equivalent input noise PSD of 70 nV /Hz were obtained. The designed circuits were taped out using the 5μm P well processes. The circuits fabricated have been tested, and the shortcomings were analyzed. The design was optimized again for fabrication.In this thesis, the PCB controlling and signal measuring circuits were also fabricated to test the CMOS sensor. The sensor was tested under wind tunnel. It can measure the velocity of the scope of 0 to 23m/s and with the sensitivity of all directions. These fatherly confirmed the validity and feasibility of the controlling and signal measuring circuits. The fabrication of the circuits still produced much experience for the next design works.