A Capacitance Measurement System Using an IVD Bridge

A new system to characterize the magnitude and phase characteristics of 1 pF to 1 F capacitance standards over the 50 Hz to 100 kHz frequency range is described. The system has been developed to provide measurement support for LCR meters and general impedance measurement services. It consists of an inductive voltage divider (IVD) bridge capable of accurately comparing two capacitance standards and a set of 1 pF to 1 nF capacitance standards with known magnitude and phase characteristics. Details of the measurement system will be given, along with preliminary measurement results and major sources of error. Introduction The Electricity Division at the National Institute of Standards and Technology (NIST) is developing expanded impedance measurement services for its customers. There is a growing demand for measurements supporting commercial impedance meters over a broader frequency range than previously offered by NIST. The effort to meet this demand has lead Electricity Division staff to apply the NIST Binary Inductive Voltage Divider (BIVD) Bridge (1-3) to the measurement of capacitance standards. The bridge is intended to measure both four-terminalpair and high-accuracy three-terminal standards. More correctly, the bridge compares an unknown standard against a known standard using an IVD set to an appropriate ratio, enabling an accurate scaling of capacitance measurement. The bridge has been successfully operated at frequencies from 50 Hz to 100 kHz for IVD ratio measurements (3) , although preliminary capacitance measurements have been performed only at 1 kHz and 10 kHz. Key attributes of the BIVD Bridge include fully automatic operation, in-phase and quadrature (magnitude and phase) characterization (thus loss can be determined using an appropriate loss standard), and sensitivity (resolution) for both the in-phase and quadrature components of well below a part in 10 6 . For indepth ac bridge theory, see Coaxial AC Bridges, by Kibble and Rayner (4) . † Contribution of the U.S. Government. Not subject to copyright in the U.S. ‡ Electronics and Electrical Engineering Laboratory; Technology Administration; U.S. Department of Commerce. IVD Bridge System The bridge system consists of a dual-channel voltage source, SOURCE A and SOURCE B (one channel to drive the bridge and the second to provide the quadrature injection signals); a reference IVD, RIVD, and associated drive IVD, DIVD; a standard and a test capacitor, CS and CT; several injection-control IVDs, IVD1-IVD4, and associated injection transformers, IT1-IT4; tuned null detectors, DET1-DET2; resistive voltage dividers, R1/R2 and R3/R4, and a detection transformer, DT, as shown in Fig. 1. The bridge is configured with all arms ground-referenced. The first two arms of the bridge consist of the in-phase SOURCE A signal driving the standard capacitor, CS, and the other two arms are made up of an inverted-phase signal (at the output of inverting amplifier A1) driving the reference IVD, RIVD, the drive IVD, DIVD, and the test capacitor, CT. The output tap of the RIVD is isolated from CT using unloading circuitry, consisting of DIVD, IT3, IT4, and DT. A binary inductive voltage divider (BIVD) with very high resolution (30 bits) and accuracy is used as the reference IVD (3) . The bridge requires two separate balances to determine the value of CT, relative to CS. The first balance involves adjusting the RIVD and DIVD settings until the center point of CS and CT is R 3