Study And Design Of A Portable And High-precision Analog Front End For Asthma Monitor System

In recent years,with the increasing aggravated environmental problems,the incidence of asthma is increasing year by year.Due to the lung function parameters of asthmatic patients will emerge before asthmatic symptoms are detected several hours ago,even a few days ago.Therefore,if the asthma monitoring system can use as an early warning system,the patients can fully understand their illness and take timely measures to avoid the unpredictable consequences causing by a sudden attack of asthma before asthma is detected.In the process of data acquisition,the pressure sensor used in front end of the asthma monitoring system exist a problem of low resolution within the scope of low pressure.The measured data discover that the output voltage only change about 3 mv when expiratory flow vary from 10 to 30 LPM.Such small voltage resolution against acquisition of the subsequent ADC and the processing of MCU.Aiming to solve the problem of low resolution within the scope of low pressure,this thesis adopt the method of program-controlled and increase the resolution of the asthma monitoring system within the scope of low pressure.Furthermore,it designed a conditioning circuit with high resolution to acquire data from the front end of the asthma monitoring system within the scope of low pressure.Besides,this thesis has compared and analysed the experimental data of the program controlled circuit with the original circuit.Experiment results show flow value of the program-controlled curve closer to the actual value of 0LPM in the absence of exhalation(expiratory flow value is 0LPM).Compared to the circuit without program-controlled,program-controlled flow monitoring error increased by 2.35 LPM at most and 0.14 LPM at least.As a result,the flow monitoring value is more accurate than the circuit without program-controlled.This method has further improved the accuracy of the real-time monitoring of exhalation flow and has a higher resolution of flow.Special for this system,this thesis also designed a Programmable Gain Amplifier(PGA)chip,which includes 24 dB two stage operational amplifier,6dB attenuator,serial-parallel conversion interface and channel selector.In this thesis,a 0.18?m standard CMOS process is used to realize the schematic design of PGA which is powered by 5V.The simulation results show that the output swing of PGA is 4.63 V and the program-controlled gain are 24.07 dB,12.03 dB,6.011 dB and 8.617 md B respectively.The maximum gain error is 0.14%,as well as the minimum is 0.1%.Due to PGA output swing,program-controlled gain and gain error has meet the parameter requirements of the asthma monitoring system,it can be applied to the asthma monitoring system.