An Improved Digitizing Interface Circuit for Wide-Range Current-Output Sensors

Abstract

A current digitizer, suitable for interfacing with a wide-range current-output sensor (COS) is proposed. The circuit features a transimpedance amplifier with a programmable-gain attenuating amplifier in its feedback which mimics the high-valued resistance required for a wide-range low-current to voltage conversion. The proposed scheme offers a fast response and is immune to switch resistances in the feedback circuit. This low-cost circuit uses a unipolar analog to digital converter (ADC) to sense bidirectional currents with reduced ADC quantization noise and offset error. A mathematical model of the scheme with error analysis is presented. The performance of the digitizing interface was established using simulations as well as experiments on a hardware prototype. The results show that the developed interface works well over the current range of 500 pA to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 ~\mu \text{A}$ </tex-math></inline-formula> (dynamic range of 104) with nonlinearity (N. L.) less than 1.2%.