Design and Analysis of a Wide-Range Low-Current Digitizing Platform for Spacecraft-Mounted Plasma Diagnostic Probes

This paper presents a digitizing platform specifically designed and developed for interfacing spacecraft-mounted plasma diagnostic probes. These probes are typically current-output sensors with a wide dynamic range. The proposed circuit consists of a probe-specific converter as the primary stage, followed by a programmable-gain analog backend. This backend circuit intelligently selects the appropriate gain based on the input current magnitude, thereby minimizing digitization errors. Key features of the scheme include precise and controlled measurement accuracy with bipolar current sensing, minimal error influence, and fast response. The architecture is designed to mitigate the effects of ON resistance in gain-stage switches. Additionally, an error compensation mechanism is introduced to ensure the sensor output remains independent of its operating conditions. The paper details the design, error, and noise analysis of the proposed platform, along with simulation and experimental results. The system is evaluated for a current range of ±500 pA to ±5 μA. Hardware tests demonstrate a maximum nonlinearity of 1.15%, a conversion time of approximately 5 ms, and an SNR exceeding 46 dB, across the entire current span when used with a 10-bit, 5V ADC. The effectiveness of the platform was further tested with Langmuir Probes and Retarding Potential Analyzers using their respective current profiles. The output profile was observed to trace the input current profile. Test results confirm that the proposed scheme serves as a cost-effective, universal platform for plasma diagnostic probes. Additionally, the paper discusses the design considerations and guidelines for implementing this scheme in space and other relevant applications.