基于电流频率变换法的微弱电流测量电路

A current to frequency conversion based circuit for low-level current measurement

电离辐射探测中经常需要开展微弱电离电流测量。微弱电离电流的测量因待测信号极其微弱需要克服各类型干扰源的影响,同时需要达到较宽的测量范围,故具有较大挑战。为了完成辐射防护用电离室输出微弱电流信号的测量,设计了一款基于电流频率变换法的宽量程微弱电离电流测量电路,其有效量程为10^(−13)~10^(−8) A。通过Keithley 6430标准电流源测试,电流测量相对误差小于3%,配合场所监测高气压电离室探测器在137Cs参考辐射场下,剂量率测量相对误差优于5.9%。

[Background]Weak ionization current measurement is often needed in ionizing radiation detection,it is a great challenge due to extreme weak signal to be measured.Various types of interference sources need to be overcome for a wide measurement range.[Purpose]This study aims to complete the measurement of weak current signal output from ionization chamber for radiation protection.[Methods]First of all,a wide-range weak ionization current measurement circuit based on current-frequency conversion method was designed to convert the weak current signal into a voltage signal by a current integrator.Then the trigger logic signal was produced by the threshold discrimination circuit and the reset circuit composed of two high-speed comparators.Finally,the pulse frequency signal was output by the Monostable trigger circuit to calculate the current,and the Keithley 6430 standard current source was employed in experimental test.[Results]Experimental results of designed weak current measuring circuit show that the effective measurement range is 10^(−13)~10^(−8) A.The relative error of current measurement is less than 3%.Under the 137Cs reference radiation field,the relative error of the dose rate measurement for the high pressure ionization chamber detector used for on-site monitoring is better than 5.9%.[Conclusions]Weak current measuring circuit proposed in this study is applicable to most of ionization chamber for radiation protection.