A 2.5-µW Beyond-the-Rails Current Sensor With a Tunable Voltage Reference and ±0.6% Gain Error From −40 °C to +85 °C

Abstract

This letter presents a low-power, fully integrated current sensor for Coulomb-counting. It employs a hybrid delta-sigma modulator ( M) with an FIR-DAC to digitize the voltage drop across a shunt. The modulator's first stage consists of a capacitively coupled chopper amplifier, which enables a beyond-the-rails (−0.3 to 5 V) input commonmode voltage range from a 1.8-V supply. A tunable voltage reference is used to accurately compensate for the large temperature coefficient (∼3500 ppm/ • C) of low-cost metal shunts. With a 20-m on-chip shunt, ±2 A currents can be digitized with 0.35% gain error from −40 • C to 85 • C, after a 1-point trim. With a 3-m PCB trace, currents up to ±15 A can be digitized with 0.6% gain error over the same temperature range. Fabricated in a standard 0.18-μm CMOS process, the sensor occupies 1.6 mm 2 and consumes 2.5 μW, which is 3× less than the state of the art. It also achieves competitive energy efficiency, with a figure of merit (FoM) of 149 dB.