8588A/8558A
Operators Manual
38
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.
adj095f.emf
Figure 9. Ohms Guard Measurements
Table 10. Minimum Guard Resistances
Range Minimum value for Ra and Rb
1 Ω, 10 Ω 100 Ω
100 Ω 1 kΩ
1 kΩ, 10 kΩ, 100 kΩ, 1 MΩ 10 kΩ
10 MΩ, 100 MΩ, 1 GΩ, 10 GΩ 100 kΩ
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:
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)
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