Ω Guard - Fluke 8588A Operator's Manual

8588A/8558A
Operators Manual
Ω Guard
In the Resistance function, with Ext. Guard selected (see also Input Terminal
Selection), the GUARD terminal functions as an Ω Guard. Use the GUARD
terminal as Ω Guard and the Ω Guard feature can make 'in-circuit' resistance
measurements by guarding out parallel resistance paths. This results in only the
value of Rx being shown.
Similarly, use Ω Guard 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 9. Push  then select Ext.
Guard to toggle external guard between ON and OFF. See Table 10.
1 Ω, 10 Ω
100 Ω
1 kΩ, 10 kΩ, 100 kΩ, 1 MΩ
10 MΩ, 100 MΩ, 1 GΩ, 10 GΩ
Providing that Ra and Rb are greater than the values shown in Table 10, and the
Ω Guard resistance (Rg) is <1 Ω, the actual value can be calculated from the
displayed value Rd by:
Deviation fraction 'E' can be found within 1 % by the simplified formula:
(Where Rg is the Ω Guard lead-resistance from the junction of Ra and Rb)
38
Figure 9. Ohms Guard Measurements
Table 10. Minimum Guard Resistances
Range
Rx = Rd x (1 + E)
E = (Rd x Rg) / (Ra x Rb)
Minimum value for Ra and Rb
100 Ω
1 kΩ
10 kΩ
100 kΩ
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