For example, some appliances, such as hair dryers and fans, feature a control that switches in half-wave rectification of the mains waveform to reduce the power and/ or speed. The resulting waveform usually resembles the signal shown in Figure 3b. To measure the rms value of current signals that aren’t true sinewaves, it will be necessary to couple IC2’s output into an rmsto- dc converter, such as Analog Devices’ AD737 or Linear Technology’s LTC1966.

Note that the scheme isn’t suitable for measuring very low currents where the inherent offsets and nonlinearities in the circuit become significant relative to the very small signal produced by the Hall Effect sensor. Consequently, at currents below about half an ampere, the circuit’s accuracy and linearity start to deteriorate. Also, the circuit isn’t intended for precision current measurement—you may need to consider other techniques if you require accuracy better than ±5%.

Repeatability (in terms of differences in measured sensitivity from one unit to another) is influenced mainly by variations in overall circuit gain and by part-to-part shifts in the Hall Effect sensor’s magnetic sensitivity. (The A1321’s sensitivity is nominally 5 mV/Gauss, but can vary from 4.75 to 5.25 mV/Gauss.) Still, measurements on two prototype boards produced strong results, where the difference in sensitivity was less than 1% of nominal.

REFERENCE:
1. Dobrev, Dobromir, “Two op amps provide averaged absolute value,” p. 98, EDN, Oct. 30, 2003.

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