An efficient digitizer for non-intrusive ac voltage measurement

This paper presents a new low-cost but reliable digitizer, which in combination with a capacitive non-contact ac voltage probe provides a digital output that is directly proportional to the unknown voltage being measured by the probe. The digitizer is designed such that it not only digitizes the signal but also implements the necessary computations required to get the final output directly, with negligible sensitivity to the probe-to-wire capacitance. It was otherwise implemented in earlier approaches using typical Analog to Digital Converter and computation of FFT of the ADC data. The new digitizer utilizes the principle of dual-slope A/D conversion, hence processes high accuracy over the full-scale range. A prototype of the proposed digitizer has been designed, developed and interfaced with a laboratory made non-intrusive ac voltage probe and tested for 0-120 V range at 60 Hz. The measurement was compared with the conventional direct method and the worst-case error was less than 0.85%.