A subthreshold current-sensing ΣΔ modulator for low-voltage and low-power sensor interfaces

A continuous-time (CT) Sigma Delta modulator for sensing and direct analog-to-digital conversion of nA-range (subthreshold) currents is presented in this work. The presented modulator uses a subthreshold technique based on subthreshold source-coupled logic cells to efficiently convert subthreshold current to digital code without performing current-to-voltage conversion. As a benefit of this technique, the current-sensing CT Sigma Delta modulator operates at low voltage and consumes very low power, which makes it convenient for low-power and low-voltage current-mode sensor interfaces. The prototype design is implemented in a 0.18 mu m standard complementary metal-oxide semiconductor technology. The modulator operates with a supply voltage of 0.8V and consumes 5.43 mu W of power at the maximum bandwidth of 20kHz. The obtainable current-sensing resolution ranges from effective number of bits (ENOB)=7.1bits at a 5kHz bandwidth to ENOB=6.5bits at a 20kHz bandwidth (ENOB). The obtained power efficiency (peak FoM=1.5pJ/conv) outperforms existing current-mode analog-to-digital converter designs and is comparable with the voltage-mode CT Sigma Delta modulators. The modulator generates very low levels of switching noise thanks to CT operation and subthreshold current-mode circuits that draw a constant subthreshold current from the voltage supply. The presented modulator is used as a readout interface for sensors with current-mode output in ultra low-power conditions and is also suitable to perform on-chip current measurements in power management circuits. Copyright (c) 2014 John Wiley & Sons, Ltd.