Keithley 6521, Инструкция по эксплуатации, Страница: 38 / 65

Operation
3-10
isolated LO terminals. However, all instruments in the test
setup may not be designed in this manner. When in doubt,
consult the manual for all instrumentation in the test setup.
3.6.4 Keeping connectors clean
As is the case with any high-resistance device, the integrity
of the triax and BNC connectors used on the Model 6521/22
can be compromised if they are not handled properly. If con-
nector insulators become contaminated, the insulation resis-
tance will be substantially reduced, affecting high-
impedance measurement paths. Oils and salts from the skin
can contaminate connector insulators, reducing their resis-
tance. Also, contaminants present in the air can be deposited
on the insulator surface. To avoid these problems, never
touch BNC or triax connector insulating materials. In addi-
tion, the scanner card should be used only in clean, dry envi-
ronments to avoid contamination.
If the connector insulators should become contaminated, ei-
ther by inadvertent touching, or from air-borne deposits, they
can be cleaned with a cotton swab dipped in clean methanol.
After thorough cleaning, they should be allowed to dry for
several hours in a low-humidity environment before use, or
they can be dried more quickly using dry nitrogen.
See Section 4 for additional information on cleaning scanner
card connectors.
3.6.5 Guarded operation
Guarding is useful when making measurements from voltage
sources that have high internal source resistances. As shown
in Figure 3-5, guarding consists of surrounding a signal-car-
rying lead with a guard signal that is driven by the low-im-
pedance electrometer preamp output. Since the guard shield
is at the same potential as the HI signal lead, no leakage cur-
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
Figure 3-4
Eliminating ground loops
rent is supplied by the high-impedance source being mea-
sured, and no loading occurs. Leakage between the guard
shield and the outer (chassis ground shield) may be consid-
erable, but that leakage current is of no consequence since it
is supplied by the low-impedance preamp rather than by the
high-impedance source itself.
In order to use guarding with the Model 6522 Scanner Card,
be sure to make appropriate signal connections, and place the
Model 6517 in the guarded mode (see the Model 6517 User’s
Manual for details).
NOTE
The Model 6521 Scanner Card is designed
for current measurements and cannot be
used for guarded voltage measurements.
The Model 6517 Electrometer cannot be
placed in the guarded mode when the
Model 6521 is installed.
3.6.6 Voltage/current scan mode
The Model 6517 Electrometer can be programmed for the
voltage or current scan mode. When in the voltage scan
mode, the relays are operated under a break-before-make ba-
sis, which means that the relay for the present channel opens
before the relay for the next channel closes. Thus, to avoid
possible damage to the Model 6521/6522 or external cir-
cuits, you should always program the Model 6517 for the
voltage scan mode when switching voltages. The voltage
scan mode is also recommended for resistance or charge
function switching.
Voltage
Source
6517
Preamp output
+
-
HI
HI
LO LO
R
G
R
S
V
S
R
L
R
S
= Source resistance
R
L
= Leakage resistance
R
G
= Guard-to-ground resistance
Figure 3-5
Guarded operation