= Burden voltage
<Burden voltage> is the <electric potential difference> introduced between the terminals of an <ammeter> when an <electric current> flows through it. A <shunt resistor> makes a measurable signal by deliberately developing this drop, but the drop can also change the <electric current> being measured. In a real <ammeter>, leads, connections and a protection fuse can contribute as well as the <shunt resistor>.<Minimizing Errors for Low-Current Measurements Using NI Hardware by National Instruments>[Shunt Ammeters versus Feedback Ammeters, Figures1–3]{ref}
Consider an ideal1volt source driving a100ohm <resistor>. Before inserting an <ammeter>, <Ohm's law> gives10milliamperes. If the inserted <ammeter> has10ohms of total series <electrical resistance>,
$$
I=\frac{1\,\mathrm V}{(100+10)\,\Omega}=9.091\,\mathrm{mA},\qquad V_b=I(10\,\Omega)=90.91\,\mathrm{mV}.
$$
The <ammeter> could report its own current perfectly and still measure a current9.09percent below the undisturbed value. This is a change in the measured <electrical circuit>, rather than a display-rounding error.
Reducing the <shunt resistor>'s <electrical resistance> reduces <burden voltage> but also reduces the signal presented to an amplifier. A feedback <ammeter> instead uses a <transimpedance amplifier> to keep its input near a reference potential. National Instruments illustrates the distinction with a low-voltage leakage-current measurement, where the <burden voltage> consumes a substantial fraction of the applied test <electric potential difference>.<Minimizing Errors for Low-Current Measurements Using NI Hardware by National Instruments>[Figures2–3 and associated leakage-current comparison]{ref}
An experimental check uses several sensing <resistors> and records both the inferred <electric current> and the load's terminal <electric potential difference>. Dependence on the inserted <electrical resistance> reveals loading. <Four-terminal sensing> reduces errors from connection drops in the sensed signal; it does not remove the physical <burden voltage> across the <shunt resistor>.
Back to article page
