Analysis And Design Of Temperature And Humidity Sensor

Temperature and humidity are two basic physical quantities,which are widely used in numerous industries and are strongly related to our daily life.Temperature and humidity of environment cannot be detect without sensors.Integrated temperature and humidity sensor has replaced the traditional discrete sensors in many areas because of its characteristics of small size,portable,low cost and can be easily implanted.System on a Chip has developmented a lot in recent years,the demands for temperature and humidity sensors made of standard CMOS technology is growing rapidly.One of the most significant specifications of a smart temperature and humidity sensor is high accuracy,while other performance such as low-power dissipation,low-cost,high resolution are also highly required.In this paper,UMC 0.28?m standard CMOS technology is used to manufature a temperature sensor with digital output and a humidity sensor with analog differential output.Bipolar transistor is proven to have the best temperature characteristics of all semiconductor devices.The temperature coefficience of Bipolar transistor's base emitter voltage V_BE is negative.There is a difference of?V_BE between two bipolar transistor's base emitter voltage when working under different current density,and?V_BE has good linearity with the change of temperature.Temperature sensor take advantages of substrate PNP bipolar transistor in standard CMOS process.V_BEE and?V_BE are used to produce bandgap reference,and?V_BE is used to detect temperature as it is insensitive to the varations of power supply voltage,processs and package.The used of a two-stage?-?ADC in this chip makes its digital interference.A programmable quantitative reference voltage of the ADC is included to compensate the nonlinearity of V_BE.To reduce offset errors due to charge injection of the switches and mismatch of devices,auto-zero,chopper and dynamic element match techniques are used.The post simulation shows the inaccuracy of±0.326?can be achieved cooperating with tuning circuit over the full military temperature range of-55?to 125?.It takes about 8ms to convert anbient temperature to digital output.The overall current of this sensor is 964?A when supply power is 3.3V,and the area of the chip is 1.68mm×1.3mm.The dielectric constant of Polyimide film changes after absorbing water vapor in the air.The advantages of a polyimide capacitance are fast response speed,low power consumption and good linearity with the variation of temperature relative humidity.It is compatible with stardard CMOS process,only surface process is enough to manufacture it.A sandwiched-type capacitor is fabricated by using polyimide film and metal.The change of relative humidity can be converted to the varation of capacitance,then convert to differential output in a switch-capacitor amplifier.Post simulation illustrates the humidity sensor can convert 1fF varation of polyimide capacitance to 10.5mV differential output,and the error between post-simulation and theoretical calculating value is less than 3.6mV.The current comsuption of the humidity sensor is 688?A,and the total area is 1.3mm×0.75mm.