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

Abstract

BackgroundWeak 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.PurposeThis study aims to complete the measurement of weak current signal output from ionization chamber for radiation protection.MethodsFirst 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.ResultsExperimental 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%.ConclusionsWeak current measuring circuit proposed in this study is applicable to most of ionization chamber for radiation protection.

Keywords

• current frequency conversion
• weak current measurement
• wide range
• high pressure ionization chamber