Evaluation of a Digitizer Designed to Interface a Non-Intrusive AC Voltage Measurement Probe

A low-cost yet efficient digitizer, suitable for a capacitive type non-contact ac voltage probe is presented. The existing measurement scheme for such a probe requires an interfacing circuit, an analog-to-digital converter followed by a processor to compute fast Fourier transform (FFT) to obtain an output. It will be beneficial and less complex if an analog to digital converter can be developed that can directly interface with the probe and get the final output without FFT computation. Such a scheme that operates based on the dual-slope analog-to-digital conversion technique is presented in this paper. Its output is directly proportional to the fundamental component of the unknown ac voltage. Output has negligible sensitivity to the variations in the probe-to-wire capacitance. The proposed scheme has a new and simple compensation approach to accurately obtain the fundamental of the unknown voltage, even if it has significant 3rd harmonics. A detailed analysis of various sources of errors affecting the performance of the scheme is conducted and the details are presented in the paper. A prototype of the proposed scheme has been designed, developed and interfaced with a laboratory-made non-intrusive ac voltage probe and tested for 0-600 V, at two frequencies, 50 Hz and 60 Hz. The output of the digitizer was linear, with worst-case linearity of 0.26%. The effective number of bits observed for the prototype were 11.16 and 11.85 for 50 Hz and 60Hz measurement respectively.