Design Of Capacitive Liquid Level Sensor ASIC

The aerospace vehicle propulsion technology is developing rapidly,low temperature liquid propellant which has characteristics of non-toxic and pollution-free is widely used in the propelling system of aerospace vehicles.In the process of the launch of aerospace vehicles,we need to measure the liquid level of low temperature liquid propellant precisely,capacitive liquid level measuring method is one of the most common methods.Capacitive liquid level sensor is often used in aerospace field because of its high sensitivity,strong stability and good dynamic response.We can obtain the information of liquid level by measuring the output capacitance of capacitive liquid level sensor.Nowadays,in the domestic aerospace carrier rockets,most of the weak-capacitance detection circuits of capacitive liquid level sensor adopt traditional topological structures of circuits of capacitance measurement.Meanwhile,these weak-capacitance circuits consist of traditional discrete components such as resistance,capacitance,etc.Therefore,the detection precision of these circuits is relatively low,which is only 10-14 pF.The output weak-capacitance signals of sensor are submerged by noise signals easily,which limits the resolution of measurement of liquid level in carrier rockets significantly and cannot meet the demand of high precision measurement of liquid level in new carrier rockets.However,the ASIC that are manufactured by microelectronic technology possess the characteristics of small in size,high accuracy,high integration and simple to be applied,which are taking place of the traditional discrete components measurement circuits gradually.This paper designed a weak-capacitance detection interface ASIC which is appropriate for the capacitive liquid level sensor in aerospace carrier rockets in a standard 0.5?m CMOS process.This interface ASIC has realized linear detection of the output capacitance of capacitive liquid level sensor,which convert the output capacitance of sensor to voltage.This paper analyzed the high linearity detection and the high resolution of capacitance detection.Besides,it also analyzed the noise characteristic of the whole circuit and adopted correlated double sampling(CDS)technology to eliminate the flicker noise of the circuit in low frequency.In this paper,all of the modules within the ASIC have been designed,which are driving circuit,front-end charge amplifier,front-stage amplifying circuit,correlated double sampling and holding circuit,PI controller,low-pass filter,three-stage operational amplifier,CMOS switches and digital clock circuits.After designing and simulating the whole circuit by Cadence Spectre,this ASIC realized the function that converting the output capacitance of the sensor to the output voltage of the ASIC successfully.The simulation results of the ASIC show: when the output capacitance of the sensor range from 40 pF to 60 pF,the nonlinearity of capacitance detection of the ASIC is 0.01%,output noise is 6.8aF/?Hz;when the output capacitance of the sensor range from 0.236 p F to 0.344 pF,the nonlinearity of capacitance detection of the ASIC is 0.04%,output noise is 0.13aF/?Hz.This interface ASIC achieved the requirement of high accuracy measurement of capacitance,which can be applied in many other kinds of capacitive detection sensors.