Fluke 8508A - Ω Guard

3
Making Measurements
Using the Measurement Functions
SENSE HI
SENSE HI
SENSE LO
SENSE LO
GUARD
GUARD
INPUT HI
INPUT HI
INPUT LO
INPUT LO
Rx
Metal
Screen
adj094f.eps
Figure 3-8. 4-Wire Resistance Zero Measurements
Ω Guard
In the resistance function with Ext Grd selected the Guard terminal functions as
Ω Guard. Using the Guard
terminal as Ω Guard, the Ω Guard feature can be used to make
‘in-circuit’ resistance measurements by guarding out parallel resistance paths so that only
the value of Rx will be displayed.
Similarly,
Ω Guard can be used to reduce the settling time if Rx is shunted by any
capacitance and a suitable tapping point is available. The connections for making
Ω Guard measurements are shown in Figure 3-9. Selection of External Guard is required.
Table 3-3. Minimum Guard Resistances
Range Minimum value for Ra and Rb
2 , 20  200 
200  2 k
2 k 20 k, 200 k, 2 M 20 k
20 M, 200 M, 2 G, 20 G 200 k
Providing that Ra and Rb are greater than the values shown in Table 3-3. Minimum
Guard Resistances, and the
Ω Guard resistance (Rg) is less than 1 Ω; the actual value can
be calculated from the displayed value Rd by:
Rx = Rd x (1 + E)
Deviation fraction ‘E’ can be found within 1 % by the simplified formula:
E = (Rd x Rg) / (Ra x Rb)
(Where Rg is the Ω Guard lead-resistance from the junction of Ra and Rb)
Example:
If Rd = 100 Ω, Rg = 1 Ω, Ra = Rb = 10 kΩ, then the value of E is given by:
E = (100 x 1) / (10 k x 10 k) = 10
-6
(1 ppm of readings)
The value of Rx is thus given by:
Rx = 100 x (1 + 10
-6
) Ohms,
= 100.0001 Ohms
3-19