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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.

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]

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.

A shunt resistor is a resistor used to establish a known relation between current and a measured electric potential difference. For current sensing, it is placed in the measured current path, and Ohm's law givesThe voltage-sensing circuit should draw negligible extra current for this simple relation to describe the desired branch current. Its small series voltage drop is often called the burden voltage.

An original example uses $R_s=10\mathrm{m\Omega }$ and $I=5\mathrm{A}$. The sensed drop is50millivolts and Joule heating dissipatesReducing resistance lowers both the burden and dissipation, but also reduces signal relative to a fixed amplifier offset or readout noise. The nominal resistor value alone therefore does not determine measurement quality.

Lead and contact resistance can dominate a low-value shunt. If a10milliohm shunt is inadvertently measured together with2milliohms of current-carrying interconnect, interpreting the total voltage using only the nominal shunt value overestimates current by20percent. Four-terminal sensing separates the high-current connections from the voltage-sense connections. Analog Devices' LTC6115 data sheet illustrates these Kelvin sense paths and explains how interconnect resistance introduces error.^[ref]

A practical characterization drives known currents through the shunt while measuring the voltage at its sense terminals, then repeats after thermal settling. Temperature coefficient of resistance, self-heating and connection geometry affect the realized conversion. The amplifier's operating range and measurement calibration must be assessed alongside the resistor.

A shunt converts current into a local voltage signal; it does not inherently provide electrical isolation between the measured circuit and its readout. High-side current sensing and low-side current sensing describe alternative placements of that measurement relative to the load and reference conductor.