Measurement of low AC currents by the standard current probe equipped with lock-in amplifier

Most of standard current probes are intended for the measurement of medium or higher currents. Usually, due to limited dynamic range, characteristic sensitivity and inner noise of the probe, it is hardly possible to measure currents lower than 100 mA. The paper deals with the possibility to extend the dynamic range of the current probe by an additional external lock-in amplifier. The paper describes the principle of the lock-in amplifier in general and design of the laboratory fixture of lock-in amplifier based on AD630 circuit as well. The designed amplifier was used to condition the output of the Agilentll46A current probe. The experiment is described in detail. The results show that thanks to the lockin amplifier the currents even lower than 1 mA can be easily detected.