Section 4
4-8
Rx
Rs
=
Tn-t
N
+
2.t
N
.NVexpr
(16)
Where:
NVexpr =
1
1+
nV(+t)(+)- nV(+t)(-)
nV(-t)(-)-nV(-t)(+)
(17)
Expression (16) allows us to include the offsets due to:
Vos = the combined offsets due to thermals and nanovoltmeter offsets
Ios = the offset in the primary current
and:
Ic = the core offset current.
The complete operation of the 6675A, including coarse control of the slave current (Is) to
track both the turns setting (Nx), and the current setting (Ix), is achieved through the
microprocessor / fibre optic control function. A graphical display unit and keypad
arrangement controlled by the microprocessor are used for display of the calculated ratio
result and operator interaction respectively.
Bridge currents to ±150 mA with up to 30 volt compliance can be generated by the 6675A
when measuring four terminal resistances. When measuring very low resistance values to
0.001 , a high current range extender option is available for use with the 6675A. A stable,
microprocessor controlled, adjustable power supply capable of sourcing up to 990 V DC is
provided by the 6675A to allow higher resistance values (100 k to 1 G) to be measured
in a two terminal configuration.
4.2. THEORY II RESISTANCE MEASUREMENT - 9975 BACKGROUND.
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