Section 4
4-1
4. THEORY OF OPERATION
4.1. THEORY I INTRODUCTION
The Guildline Model 6675A is a microprocessor-based, direct-current-comparator bridge designed
for the automatic measurement and display of the ratio of two resistances to accuracies approaching
0.1 ppm. The ratio of the two resistances is determined from a direct measurement of the voltage of
the bridge imbalance while an ampere-turn balance is maintained. A microprocessor-based, 16 bit
technology, is used to control the direct-current-comparator, set the bridge measurement currents
and measurement configuration, initiate the voltage measurement cycle and maintain the ampere-
turn balance. This fully automated measurement process achieves optimum resistance ratio
resolution and accuracy at the sub-ppm level.
The bridge is designed for flexibility and ease of use and is based on the manually operated direct-
current-comparator bridge manufactured by Guildline Instruments (Model 9975) and also on the
original work of the National Research Council of Canada
1
. Remote access to the embedded
microprocessor-controller, through a simple control language command set and GPIB
communication interface allows the 6675A operator to fully exploit the capabilities of the direct-
current-comparator to suit each user requirement while still maintaining optimum resistance ratio
measurement accuracy at the sub-ppm level.
The design approach taken for the 6675A results in an effective bridge measurement accuracy of 27
bits (to a maximum of 32 bits depending on configuration), allowing the resistance ratio of
resistances in the range 0.001 to 1 G to be automatically measured.
1
MacMartin, M.P. and Kusters, N.L., "A direct -current-comparator for four terminal
resistance measurements", IEEE Trans. Instrumentation and Measurement, vol IM-
15, pp.212-220, December 1966.
To Next Page
Loading...