 Low-side current sensing

Low-side current sensing places a sensor between a load and its return reference. For a shunt resistor, the electric potential difference across the shunt resistor is proportional to the returning electric current. The readout inputs can operate near the return potential, but the load's local return is raised by the burden voltage. High-side current sensing instead places the sensor toward the supply.^[ref]

For an illustrative12volt supply, a6ohm load and a50milliohm return-path shunt resistor, Ohm's law givesThe load return is about99millivolts above the source return, and the load itself receives about11.901volts. These potentials matter if a second instrument or communication cable connects the load return to another reference. A parallel return path can bypass the shunt resistor, so the measured branch electric current need not equal the total load electric current.

Pete Semig's reference design provides a concrete apparatus: a25ohm shunt resistor, an instrumentation amplifier and a single5volt supply measure a specified range from10microamperes to10milliamperes. Its circuit analysis and simulated responses separate the ideal current-to-output relation from practical offset and low-current errors.^[ref]

For a new setup, recording a zero-current output and several known electric currents tests offset and gain. Four-terminal sensing helps keep high-current connection drops out of the sensed electric potential difference; an amplifier input common-mode range extending to the required near-return potentials is still necessary. Neither a near-zero common-mode voltage nor a successful gain check proves that the return-path insertion leaves the load undisturbed.