 High-side current sensing

High-side current sensing measures an electric current with a sensor between the supply and the load, rather than between the load and its return reference as in low-side current sensing. With a shunt resistor, the useful signal is the difference between two input potentials that may both lie far above the amplifier's local reference. The amplifier input common-mode range therefore matters independently of the differential signal size.^[ref]

For an illustrative12volt supply, a50milliohm shunt resistor carrying2amperes developsThe sense terminals are at12.0 and11.9volts relative to the supply return, so their common-mode voltage is11.95volts. An ideal differential amplifier of gain20 would convert the100millivolt difference to2volts. A3.3volt output or digitizer range does not by itself establish that the amplifier inputs can accept11.95volts: that requires a suitable input stage and operating specifications.

The LTC6115 demonstrates an actual high-side arrangement: an external shunt resistor generates a differential electric potential difference, and external resistors set its conversion to an output signal. Its separate current-sense and voltage-sense supply conditions must not be conflated.^[ref]

Keeping the load return connected directly to the reference avoids the reference shift introduced by a return-path shunt resistor. The supply-side drop still reduces the available load electric potential difference. Separate sense connections implement four-terminal sensing; measuring through shared high-current tracks instead adds their drops to the inferred electric current.^[ref]