Direct current voltage measurement

In the field of metrology, the measurement of DC voltage is of fundamental importance, as a significant proportion of measurement processes result in this quantity following a conversion operation. Two main approaches coexist: so-called opposition or zero methods, based on a reference voltage and ratio balancing techniques, and direct reading methods, which rely on the use of a voltmeter. This article outlines the principles, devices, and limitations of these techniques, as well as the disruptive phenomena that can degrade the quality of the results.

Comparison methods involve balancing the voltage under test with a reference source. They require a reference device (Josephson junctions, stabilized Zener diodes, voltage calibrators), a balancing element (resistive dividers or transformers), and a zero detector (galvanometer or nanovoltmeter). Josephson junction arrays, based on quantum effects, have become the international standard for the definition of the volt. By assembling thousands of junctions, they provide stable and fully traceable voltages up to 10 V. Semiconductor references, based on temperature-compensated Zener diodes or reference amplifiers, offer robust solutions that are widely used in laboratories and commercial instruments. Programmable calibrators, which combine these references with high-precision digital-to-analog converters, ensure coverage of a wide voltage range. Finally, fixed or variable potentiometric bridges, as well as current comparator bridges, enable precise comparisons through voltage division or current transfer, achieving levels of accuracy unattainable by direct methods.

Voltmetric methods rely on the use of an analog or digital voltmeter. Although their uncertainty is generally higher, they are simpler and faster to use and are well-suited for everyday applications. Analog voltmeters operate on various principles (magnetoelectric, electrodynamic, and electrostatic galvanometers) and are still used in specific contexts. Digital voltmeters, which are much more widespread today, rely on various analog-to-digital converters: double-slope or multiple-slope integration, successive approximation (SAR), sigma-delta, or charge redistribution. These techniques determine the resolution, speed, and linearity of the devices.

In addition to the instruments’ intrinsic performance, measurement reliability is significantly affected by disruptive phenomena. The source’s output impedance, cable leakage currents, amplifier input currents, and settling times associated with parasitic capacitance can introduce significant biases. Thermoelectric forces, generated by metal junctions subjected to temperature gradients, constitute a particularly critical source of error at low voltages. Finally, external electromagnetic disturbances...