Volts and Ohms Measurements 3-13
Guarding the circuit minimizes these effects by driving the inner shield of the triax cable at
signal potential, as shown in Figure 3-8. Here, a unity gain amplifier with a high input imped-
ance and low output impedance is used. Since the center conductor (HI) and the inner shield
(Guard) of the cable are at virtually the same potential, the potential across R
L
is zero, so no cur-
rent flows. Also, with a zero potential across C
L
, there is no capacitor charging process to slow
down the measurement response.
For the volts and ohms functions, the input of Model 6514 places the driven guard on the
inner shield of the triax cable when GRD is enabled.
Inner Shield
Triax Cable
R
S
HI
6514 Input
LO
E
S
R
L
C
L
Source
Center
Conductor
A = I
Guard
150kΩ
Figure 3-8
Guarded input cable
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