= 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.<10 µA-10 mA, Single-Supply, Low-Side, Current Sensing Solution by Pete Semig>[Page1 circuit and Circuit Description; Theory of Operation]{ref}
For an illustrative12volt supply, a6ohm load and a50milliohm return-path <shunt resistor>, <Ohm's law> gives
$$
I=\frac{12}{6+0.05}\,\mathrm A\approx1.9835\,\mathrm A,\qquad V_s=0.05I\approx0.09917\,\mathrm V.
$$
The 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.<10 µA-10 mA, Single-Supply, Low-Side, Current Sensing Solution by Pete Semig>[Page1 schematic; §2 Theory of Operation; §§4.1–4.4, printed pp9–12, simulated response and error analysis]{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.
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