Keithley 6514

3-12 Volts and Ohms Measurements
Guarding input cable
Guarding should be used for high-impedance measurements and for low voltage measure-
ments that use long input cables.
To better understand the concept of guarding, review the unguarded circuit shown in Figure
3-7. E
S
and R
S
represents the resistance and voltage components of the source, and R
L
and C
L
represents the leakage resistance and cable capacitance of the triax input cable. The equivalent
circuit shows the divider that is formed. If R
S
is large enough, the divider will significantly atten-
uate the voltage seen at the input of Model 6514 (see “Cable Leakage Resistance”). Also, R
S
and
the cable capacitance (C
L
) could create a long RC time constant resulting in a slow measurement
response (see “Input Capacitance”).
RC
0 1.0 2.0 3.0 4.0 5.0
Time
0
63
100
Percent
of Final
Value
Figure 3-6
Settling time
Inner Shield
Triax Cable
R
S
HI
To 6514
Input
LO
E
S
R
L
C
L
Source
Center
Conductor
Equivalent Circuit
R
S
HI
To 6514
Input
LO
E
S
R
L
C
L
Figure 3-7
Unguarded input cable