 Current sensing

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.