4-Wire Resistance Zero; Guard - Fluke 8508A User Manual

8508A
Users Manual

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.

Guard

In the resistance function with Ext Grd selected the Guard terminal functions as
Guard. Using the Guard
'in-circuit' resistance measurements by guarding out parallel resistance paths so that only
the value of Rx will be displayed.
Similarly,
capacitance and a suitable tapping point is available. The connections for making

Guard

Providing that Ra and Rb are no less than 1 k (10 k on 2M range and above), and
the
displayed value Rd by:
Deviation fraction 'E' can be found within 1 % by the simplified formula:
(Where Rg is the
Example:
If Rd = 100 , Rg = 1 , Ra = Rb = 10 k , then the value of E is given by:
The value of Rx is thus given by:
3-18
INPUT HI
SENSE HI
GUARD
SENSE LO
INPUT LO
Figure 3-8. 4-Wire Resistance Zero Measurements
terminal as
can be used to reduce the settling time if Rx is shunted by any
Guard
measurements are shown in Figure 3-9. Selection of Ext Grd is required.
resistance (Rg) is less than 1 ; the actual value can be calculated from the
Guard
Rx = Rd x (1 + E)
E = (Rd x Rg) / (Ra x Rb)
lead-resistance from the junction of Ra and Rb)
Guard
E = (100 x 1) / (10 k x 10 k) = 10
-6
Rx = 100 x (1 + 10 ) Ohms,
= 100.0001 Ohms
Metal
Screen
INPUT HI
SENSE HI
INPUT LO
Rx
GUARD
SENSE LO
Guard, the Guard feature can be used to make
-6
(1 ppm of readings)
adj094f.eps