Dean G. Jarrett

Automated guarded bridge for calibration of multimegohm standard resistors from 10 M/spl Omega/ to 1 T/spl Omega/

The implementation of an automated guarded bridge for calibrating multimegohm standard resistors is described. A guarded multimegohm bridge has been assembled with programmable dc calibrators in two of the arms allowing multiple ratios and test voltages to be remotely selected. A programmable electrometer with a resolution of /spl plusmn/3 fA in the current mode is used to measure the difference in currents flowing through the remaining two arms of the bridge consisting of unknown and standard resistors. The balancing algorithm used to estimate the calibrator setting required to obtain a null is described along with a graphical user interface (GUI) that has been written to provide flexibility to the measurement system and improve control of the instrumentation. Evaluation of the multimegohm bridge from 10 M/spl Omega/ to 1 T/spl Omega/ is reported along with a comparison of the multimegohm bridge performance to that of the existing manual and semi-automated systems that the multimegohm bridge will replace.

Evaluation of guarded high-resistance Hamon transfer standards

An improved design for a guarded transfer standard in the resistance range 1 M/spl Omega/ to 100 G/spl Omega/ is described. Existing transfer standards and their limitations are reviewed along with a description of guard circuit theory. Measurements made to evaluate the effectiveness of guard networks are described and results are reported that verify the guard circuit behaviour. Guarded transfer standards have been tested with internal and external guard networks showing the benefits of guarding transfer standards for high-resistance measurements. Interchangeable guard networks are used in the improved transfer standards to ensure complete guarding during all phases of the measurement process thus reducing errors caused by leakages to ground. These improved transfer standards have been developed to support National Institute of Standards and Technology (NIST) calibration services over the range 10 M/spl Omega/ to 1 T/spl Omega/ and to support extension of the calibration service to 10 T/spl Omega/ and 100 T/spl Omega/.

NIST Measurement Service for DC Standard Resistors

At the National Institute of Standards and Technology (NIST), the U.S. representation of the ohm is based on the quantum Hall effect, and it is maintained and disseminated at various resistance levels by working reference groups of standards. This document describes the measurement systems and procedures used to calibrate standard resistors and current shunts of nominal decade values in the resistance range from 10 Ω to 10 Ω. Resistance scaling techniques used to assign values to the working standards are discussed. Also included is an assessment of the calibration uncertainties at each resistance level.

Fabrication of high-value standard resistors

The National Institute of Standards and Technology (NIST) has fabricated stable, transportable 10 M/spl Omega/ and 1 G/spl Omega/ standard resistors for use in an international comparison of high resistances. This fabrication process is being applied to the construction of standard resistors of values up to 10 T/spl Omega/, with initial results indicating significant improvements in stability and fewer adverse effects induced by mechanical shock and vibration.

Automatic inductive voltage divider bridge for operation from 10 Hz to 100 kHz

A bridge to calibrate programmable and manual inductive voltage dividers is described. The bridge is based on a programmable 30-b binary inductive voltage divider with terminal linearity of +or-0.1 ppm in phase and +or-2 ppm quadrature at 400 Hz. Measurements of programmable test dividers can be automated using software developed to align the bridge components and perform an automatic balance. >

Final report on key comparison CCEM-K2 of resistance standards at 10 M? and 1 G?

An international comparison of dc resistance at 10 M? and 1 G? was organized under the auspices of the Consultative Committee for Electricity and Magnetism (CCEM) and piloted by the National Institute of Standards and Technology (NIST, USA). This CCEM comparison began in August 1996 and was completed in March 2000 with the participation of fourteen other national metrology institutes (NMIs). The travelling package included three wirewound 10 M? standards and three film-type 1 G? standards designed by the NIST. Results indicate that the differences at 10 M? and 1 G? between each laboratorys values and the respective reference value, are all within each laboratorys expanded relative uncertainty at a coverage factor k = 2.

Direct resistance comparisons from the QHR to100 M/spl Omega/ using a cryogenic current comparator

Measurements of room-temperature 100 M/spl Omega/ standard resistors and cryogenic thin-film resistors based directly on a quantized Hall resistance standard have been made with a cryogenic current comparator (CCC) bridge. This 15 496:2 ratio CCC attains a current sensitivity of 10.7 fA/Hz/sup 1/2/ in measurements of cryogenic thin-film resistors, without extensive shielding or filtering. A resistive primary winding helps the CCC maintain stability in the presence of external noise. The resistive-winding technique may be useful for the absolute measurement of small currents delivered by single-electron tunneling devices.

Settling Times of High-Value Standard Resistors

An investigation of the response of high-value resistance standards and elements to applied potential has shown that the processes used to prepare and construct high-value resistance standards can affect the settling times of these standards. Improvements made to the processes used at NIST to fabricate high-value resistance standards will be discussed

Characterization of four-terminal-pair resistance standards: a comparison of measurements and theory

Coaxial straight-wire resistance standards with calculable frequency dependence have been used to tie alternating-current (AC) measurements to direct-current (DC) measurements of resistance. Coaxial standards of values 100 /spl Omega/ and 1000 /spl Omega/ are compared with each other and with other resistance standards at several frequencies up to 15920 Hz using AC bridges.

Design and evaluation of a low thermal electromotive force guarded scanner for resistance measurements

The design and testing of a low thermal electromotive force guarded scanner, developed to provide completely guarded switching when used with actively guarded resistance bridge networks, is described. The design provides a continuous guard circuit trace on the scanner circuit boards that surrounds the relay contacts and protects the measurement circuit from leakages to ground. Modification to the circuit boards and relays of the guarded scanner are explained. Several tests were developed to evaluate the guarding effectiveness, including isolating sections of the guard circuit to create a potential drop between the main and guard circuits. Calibration of standard resistors using the guarded scanner has shown relative differences less than 1×10 −6 , 30×10 −6 , and 150×10 −6 for measurements made with and without the guarded scanner at nominal resistances of 1, 10, and 100 GΩ, respectively. The substitution method was used to significantly reduce the relative differences between channels to less than 0.5×10 −6 , 3×10 −6 , and 30×10 −6 for nominal resistances of 1, 10, and 100 GΩ, respectively. Applications for the guarded scanner in automated direct currentmeasurement systems are presented.