A Digital Impedance Comparator with High Resolution

Impedance comparisons with precision below one part-per million (ppm) are made by the combination of a two-stage-transformer bridge circuit with electrically switched ratios and a modified commercial microprocessor-based impedance meter. This system has much better immunity to both series connection impedance and shunt admittance to ground than does the conventional comparator circuit. It also has greatly improved deviation accuracy so that high precision is possible over a practical difference range. Its integrating multiple-sampling detector has a narrow noise bandwidth giving a standard deviation of less than 1 part in 107 for measurements made in less than 1 s with a 1-kHz 1-V signal applied. It should be possible to extend the method used to get a wide-range automatic impedance measuring system.