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Current sensing determines an electric current from a measurable response to that current. A shunt resistor converts it into an electric potential difference; magnetic methods can instead respond to the field associated with the current. The sensor, amplifier, reference, digitizer and conversion model jointly determine the inferred current.

A basic resistive signal chain iswhere $G$ is amplifier gain and $b$ is an output offset. Pete Semig's single-supply low-side reference design demonstrates current measurement with a sensing resistor and an instrumentation-amplifier readout.^[ref]

For an original example, choose $R_s=20\mathrm{m\Omega }$, $G=50$, $b=0.10\mathrm{V}$ and measured output1.10volts. The estimate is1ampere. Ignoring the offset would instead give1.10amperes. A1percent error in the product $G{R}_s$ produces a corresponding scale error, even if the displayed voltage is perfectly repeatable.

In low-side current sensing, the shunt sits between the load and its return reference; its burden can raise the load's local reference potential. In high-side current sensing, it sits toward the supply, so the readout must handle the relevant common-mode voltage. A small differential signal does not imply a small voltage of either input relative to the amplifier's own supply. The LTC6115 is an example of an amplifier designed for high-side current sensing.^[ref]

An analog-to-digital converter can record the amplified signal, but digitization does not remove sensor loading, gain error or aliasing. A useful measurement procedure records zero-current output, several reference currents, connection points and temperature, with enough bandwidth for the waveform of interest. Four-terminal sensing can reduce connection errors for a shunt; separate gain/offset calibration addresses different error sources. The desired time resolution, electrical loading and uncertainty must all be included in the current estimate.

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.