Zero detectors
Null method in AC

In so-called "zero" measurement methods, bridge equilibrium corresponds to the cancellation of a potential difference between two well-defined points of the circuit when it is energized. The voltmeter, which measures this potential difference, plays a vital role. It must enable this balance to be achieved as accurately as possible. To this end, it must possess certain specific characteristics, essentially linked to the fact that we're dealing here with AC measurements, and be used with certain precautions. In particular, it must be able to detect the useful signal, which becomes very weak in the vicinity of equilibrium, while remaining as insensitive as possible to any parasitic signals and noise, which can then become predominant. This is why we recommend always working at frequencies slightly offset from those of the power supply or its harmonics. There are currently two main approaches...