A Current-to-Digital ∆Σ ADC for Low-Noise High-Precision Applications

This paper presents the system design of a new high precision potentiostat. Our direct current-to-digital conversion is capable of sensing picoampere (pA) currents without a need for transimpedance amplifiers(TIAs). Our idea utilizes a ∆Σ modulator with one very important difference, current feedback via slope scaling. This feature allows us to utilize noiseless elements such as capacitors in the feedback loop to achieve high performance. We have validated the system using both theory and experiment. Performance improvement over current systems is first demonstrated by performing a theoretical analysis of the expected noise of the system. A 10×10cm2 PCB prototype was fabricated as a proof of concept. The slope scaling idea is applied to both first and second order ∆Σ Analog to Digital Converters (ADCs) with signal input bandwidth of 1.57 KHz with an oversampling ratio of 64. Signal-to-Noise-and-Distortion Ratio (SNDR) of 40 dB and 60 dB is achieved with 1st and 2nd order ∆Σ ADCs, respectively. The noise floor of the implemented circuit is 569 fA over a 1KHz bandwidth and was tested for input currents ranging from 100 pA to 1 µA.