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

This letter presents a low-power, fully integrated current sensor for Coulomb-counting. It employs a hybrid delta-sigma modulator (AIM) 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 common-mode voltage range from a 1.8-V supply. A tunable voltage reference is used to accurately compensate for the large temperature coefficient (similar to 3500 ppm/degrees C) of low-cost metal shunts. With a 20-m Omega on-chip shunt, +/- 2 A currents can be digitized with 0.35% gain error from -40 degrees C to 85 degrees C, after a 1-point trim. With a 3-m Omega 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-mu m CMOS process, the sensor occupies 1.6 mm(2) and consumes 2.5 mu W, 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.