Section 4
4-4
At balance, the ratio of the two currents (Ix and Is) flowing in the two windings is the same
as the ratio of the turns set for each winding (Nx and Ns).
Complete electrical isolation between circuit currents is maintained throughout the 6675A by
making full use of fibre optic technology. A fibre optic serial data communication link is
used to transfer data between each functional component and the embedded microcontroller.
A link joining the two potential leads of the unknown resistance (Rx) and the reference
resistance (Rs) and the nanovoltmeter detector, provides the only connection between the
two halves of the bridge. Power supply isolation techniques as recommended by Kibble
2
are used throughout to further isolate the bridge halves. The nanovolt detector is a high input
impedance (>1 G) amplifier arrangement capable of resolving voltages at the nanovolt
level with a typical linearity accuracy of 14 bits (to a maximum of 18 bits). In the automatic
direct-current-comparator bridge arrangement this amplifier is used to measure the voltage
difference between the two bridge circuits (bridge imbalance).
Knowledge of the reference resistance value (Rs), the effective bridge voltage imbalance and
confirmation that ampere-turn balance is maintained, with the turns (Nx, Ns) and current (Ix,
Is) settings, allows the unknown resistance value (Rx) to be calculated.
2
Kibble, B.P. and Robinson, I.A., "An Isolated Mains Driven Power Supply", NPL
Memorandum DES 54, September 1985.
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