Low-cost rectifier for measuring of AC voltage or current frequency compensation proposal

Abstract

Usually the rectifiers are the most problematic devices of the low-cost measurement instruments. The features of a simple full wave rectifier construction with an operational amplifier are usually determined by the rectifying diodes and the slew rate of the operational amplifier and unfortunately, they suffer from the dependence on the amplitude of the measured signal. Of course, special topologies as well as current-mode operating devices can be used, but this paper considers the possibility of how the frequency range of a cheap conventional rectifier can be broadened. It is supposed that the rectifier operates on a single signal level at which it can be well frequency compensated. The measurement is processed indirectly by a microcontroller that changes the amplification factor of a preamplifier in order the target level was achieved. Subsequently the measured value is computed from the amplification factor required to achieve the target level. However, this approach, although it looks simple, can result in very time consuming processing if the frequency of the measured signal is too low. Therefore several approaches and proposals are discussed. The authors believe that the proposed solution, although quite complex, can bring satisfactory results when used in multimeters equipped with a microcontroller that employ a three digit display and are capable of operation up to 100 kHz or 1 MHz in dependence on the construction of its internal circuits.