Metrological Evaluation of a Programmable Gain Current-Sensing Digitizer Suitable for Retarding Potential Analyzers

Retarding potential analyzers (RPAs) are plasma diagnostic sensors that are widely preferred in the aerospace field for the study of space plasmas like the ionosphere and solar winds. This article describes a low-cost programmable-gain current-digitizing front-end for RPAs to be mounted on spacecraft. The requirement of a controlled high-speed and accurate measurement, considering the fast transit speed of the spacecraft, can be met using the proposed scheme. The circuit features a transimpedance amplifier as the primary stage followed by a programmable-gain integrator. The gain of the integrator is intelligently chosen by adjusting the integration time based on the magnitude of the incoming current. This enables the measurement of wide-range low-current from RPAs with minimal effects of analog-to-digital converter (ADC) quantization error with reduced response time. The design and analysis of the scheme, simulation, and experimental results for a current range of +/-(100 pA, 100 nA) is explained. The effects of offset errors of the scheme along with noise analysis were also reported. The maximum observed nonlinearity from the experimental model was 2.77%. From the results and inferences, the proposed scheme has been proven to work with RPA sensors. Recommendations for optimizing RPA operation, in conjunction with the proposed digitizing scheme are also discussed.