Current feedback uses a sense resistor to accurately measure the current of the battery. An instrumentation amplifier is used to measure the voltage across the sense resistor and scale it to ADC input level. Offset and gain errors are not a major concern because all test equipment gets calibrated. But gain and temperature drift parameters of the circuit are important to achieve current control accuracy below ±0.01% or ± 100 ppm over ±5°C temperature variation. For an example, selecting a 10 ppm/°C current-sense resistor gives only about 50 PPM margin. The INA821 device has 0.1-μV/°C input offset drift and 5-ppm/°C gain drift. For a 3-mΩ sense resistor and 10-A maximum current, the total drift will be 8 ppm/°C. ADC drift comes from its reference and with a reference of 3 ppm/°C device, the error is just slightly over 100 ppm.

Flicker or 1/f noise of the instrumentation amplifier might be concerning if the current sense resistor is too small. For an example, with a 3-mΩ sense resistor and 10-A output current, 1 fine current step corresponds to 3 μV change at the input of the instrumentation amplifier. Therefore, the 1/f noise of the amplifier should be smaller than 3 μV.

Voltage feedback path gives better error margin over temperature change. A 1-μV/C drift operation amplifier is less than 0.2 ppm/C error. Including error due to ADC, it gives a margin of more than 80 ppm.