\b[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 is
$$
V_s=IR_s,\qquad V_{\rm out}=G V_s+b,\qquad
\widehat I=\frac{V_{\rm out}-b}{G R_s},
$$
where $G$ 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.<10 µA-10 mA, Single-Supply, Low-Side, Current Sensing Solution by Pete Semig>[Circuit Description and Theory of Operation]{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.<LTC6115 data sheet by Analog Devices>[Description and Applications Information]{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.


 Current sensing (source code)