Guarded Operation; Figure 2-6 Unguarded Circuit; Figure 2-7 Guarded Circuit - Keithley 6512 Instruction Manual

panel V, Ω GUARD switch allows guarding to be easily ap-
plied to the input circuit; see paragraph 2.7.4 for details.
At low signal levels, noise may affect accuracy. Shielding of
the unknown voltage can reduce noise effects substantially.
When using shielding, the shield should normally be con-
nected to input low, although connecting the shield to chassis
ground may yield better results in some cases.

2.7.4 Guarded operation

Guarding consists of using a conductor driven by a low-im-
pedance source to totally surround the leads carrying a high-
impedance signal. The output of this low-impedance source
is kept at the same potential as the signal itself, resulting in
drastically reduced leakage currents.
To approach the concept of guarding, let us first review the
unguarded circuit shown in Figure 2-6. The measured signal
is represented by the voltage source E
tance R . Cable leakage impedance is represented by Z
S
The source resistance and leakage impedance form a voltage
divider that attenuates the source voltage as follows:
E
O
Z L
R S
E O
E S
A. BASIC CONFIGURATION
Figure 2-6
Unguarded circuit
Thus, to keep the error due to leakage resistance under 0.1%,
the leakage resistance must be at least 1,000 times the source
resistance value.
Guarding the circuit minimizes these effects by driving the
shield at signal potential, as shown in Figure 2-7. Here, a uni-
and the source resis-
S
Z E
L S
= -------------------- -
Z R
+
L S
E S Z L
E O =
R S + Z L
R S
E S
Z L
B. EQUIVALENT CIRCUIT
ty-gain amplifier with a high input impedance and low output
impedance is used. The input of the amplifier is connected to
the signal, while the output is used to drive the shield. Since
the amplifier has unity gain, the potential across Z
cally <1mV, so no leakage current flows. Leakage between
the cable shield and ground may be considerable, but it is of
no consequence since that current is supplied by the low-im-
pedance source, rather than by the signal itself.
OV
R S
E S
Figure 2-7
.
Guarded circuit
L
When the rear panel V, Ω/ GUARD switch is placed in the
ON position, guard potential is placed on the inner shield of
the triaxial cable. The outer shield remains at chassis ground.
Thus, it is necessary to use the COM terminal for low signal
connections, as shown in Figure 2-8. For very critical mea-
surements, a shielded, guarded enclosure should be used.
Hazardous voltage (up to 300V) may be
present on the inner shield when V, Ω/
GUARD is on, depending on the input
E O
signal. A safety shield, connected to chas-
sis ground is recommended when mak-
ing voltage measurements over 30V or
for guarded resistance measurements.
The use of guarding is not recommended
for the amps or coulombs functions.
The PREAMP OUT terminal may be used for guarding in
the volts and ohms modes in a similar manner. In this mode,
Z L
A=1
6512
PREAMP
Z S
( SHIELD)
WARNING
NOTE
Operation
is typi-
L
E O
2-11