Fluke 8508A - Specific Measurement Calibration; DC Voltage and Current Calibration

8508A
Users Manual
INPUT HI
INPUT LO
adj092f.eps
Figure 3-6. 4-Wire Measurements
4-wire High Resistance Measurements
When making very high resistance measurements above about 1 MΩ, a metal screen can
be wrapped around the resistor to reduce noise, usually caused by charge injection.
Connecting the
GUARD terminal to the screen will intercept leakage via the screen (in
parallel with the unknown resistor). The resistor under test should not be grounded, as
this will make the measurement noisier.
INPUT HI
SENSE HI
GUARD
SENSE LO
INPUT LO
SENSE HI
SENSE LO
Rx
SENSE HI
SENSE HI
SENSE LO
SENSE LO
GUARD
INPUT HI
INPUT HI
INPUT LO
INPUT LO
Rx
Metal
Screen
GUARD
adj093f.eps
Figure 3-7. 4-Wire High Resistance Measurements
4-wire Resistance Zero
For accurate measurements of resistance it is essential that a correctly connected zero
source be used when performing an Input-Zero operation before making a series of
measurements. The preferred arrangement shown in Figure 3-8 ensures that thermal and
induced EMF effects, and bias current effects, associated with the multimeter and the
measurement cables are eliminated.
Two precision 4wire short accessories are supplied as standard within the 8508A –
LEAD leadkit. Fitted over the Input Hi, Input Lo, Sense Hi and Sense Lo terminals these
provide a convenient means of zeroing the 8508A inputs at the terminals. Use of the
4-wire short device at the multimeter terminals does not address potential sources of error
within measurement cables.
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