An Efficient Digitizer for Measurement of Low-Magnitude Currents With Wide Span

This article proposes an efficient digitizer for the measurement of low currents over a wide span. The proposed digitizer for low-current measurement (D-LCM) technique provides a digital output proportional to the sensed current, with reduced quantization error of the analog-to-digital converter (ADC). In addition, the proposed D-LCM technique ensures the following features: 1) simple and cost-effective architecture for current-output sensors; 2) bidirectional current sensing over a large span for a unipolar ADC; and 3) reduced effects of various error sources. The novel D-LCM design, comprising a transimpedance amplifier and a multiregime integrator, helps to realize the aforementioned features. The analog circuitry is intelligently controlled by a simple digital unit such that the input span is split using a geometric-mean approach and operated in multiple regimes. The entire methodology and its circuit realization are mathematically derived, and its capability to reduce the effects of the ADC quantization errors is discussed. Detailed evaluation of other error sources and necessary compensation/design guidelines are also elaborated. The performance of the D-LCM technique is evaluated using simulation studies and experimental tests on a developed hardware prototype. Experimental tests demonstrate that the dual-regime-based D-LCM technique acts as a digitizer for the current range tested (i.e., −100 nA to 100 nA) with a nonlinearity error of 1.3%. It is also shown that the error could be reduced to around 0.47% using a triple-regime-based D-LCM technique. The precision-related performance of the D-LCM is also experimentally obtained and shown to be adequate. The developed D-LCM technique is useful for many instruments, such as retarding potential analyzers (RPAs) and photodiode-based systems. The proposed technique was implemented and verified in an RPA flight prototype used for the study of Earth's ionosphere.