Four-Terminal Pair Digital Sampling Impedance Bridge up to 1MHz

This paper describes a new four-terminal pair (4TP) digital sampling impedance bridge designed for the frequency range up to 1MHz and small impedances, such as shunts. The bridge is capable of comparing impedance standards of arbitrary ratios in a full complex plane from approximately 100kΩ down to 50mΩ, limited by the maximum achievable current 3A. To keep low uncertainties, a new multiplexer was designed and a very simple and fully automated linearity correction method based on the pair of calculable resistors was developed and validated. This paper describes the design and details of the bridge topology, basic uncertainty budget, and first results of the validation. The expanded uncertainty of impedance module is about 50μΩ/Ω at 1MHz for impedance ratios up to 1:16 and voltage drops above 10 mV and the expanded uncertainty of a phase angle was about 360μrad/MHz. Expanded uncertainty for frequency 100kHz about 10 μΩ/Ω was reached. Typical expanded uncertainty for low-impedance ratios below 1:1.1 is only 35μΩ/Ω. Set of measurement of impedance standards of known values and a small international comparison of ac-dc and phase angle errors of current shunts were carried out to validate the bridge capabilities in a wide range of impedances. The validation measurements showed the deviations of the bridge are below 35μΩ/Ω and below 350μrad at 1MHz.