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

This letter presents a low-power, fully-integrated current sensor for Coulomb-counting. It employs a hybrid delta-sigma modulator (ΔΣM) with a FIR-DAC to digitize the voltage drop across a shunt. The modulator's 1st stage consists of a capacitively-coupled chopper amplifier, which enables a beyond-the-rails (-0.3 to 5 V) input common-mode voltage range from a 1.8 V supply. A tunable voltage reference is used to accurately compensate for the large temperature coefficient (∼ 3500 ppm/oC) of lowcost metal shunts. With a 20 mΩ on-chip shunt, ±2 A currents can be digitized with 0.35% gain error from -40 to 85 ∘C, after a 1-point trim. With a 3 mΩ 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 lm CMOS process, the sensor occupies 1.6 mm2 and consumes 2.5 μW, which is 3x less than the state-of-the-art. It also achieves competitive energy efficiency, with a Figure-of Merit (FoM) of 149 dB.