Figure 3-8 Guarded Input Cable - Keithley 6514 Instruction Manual

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
rent flows. Also, with a zero potential across C
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.
Figure 3-8
Guarded input cable
Triax Cable
Source
R
L
R
S
E
S
Volts and Ohms Measurements
, there is no capacitor charging process to slow
L
Center
Conductor
HI
A = I
LO
C
L
6514 Input
Inner Shield
150kΩ
3-13
is zero, so no cur-
L
Guard