Operation
4-16
Figure 4-14 shows how to connect several instruments to-
gether to eliminate this type of ground loop problem. Here,
only one instrument is connected to power line ground.
Ground loops are not normally a problem with instruments
having 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.
4.5.7 Keeping connectors clean
As is the case with any high-resistance device, the integrity
of connectors can be compromised if they are not handled
properly. If connector insulation becomes contaminated, the
insulation resistance will be substantially reduced, affecting
high-impedance measurement paths.
Oils and salts from the skin can contaminate connector insu-
lators, reducing their resistance. Also, contaminants present
Figure 4-14
Eliminating ground loops
Instrument 1
Instrument 2
Instrument 3
Power Line Ground
in the air can be deposited on the insulator surface. To avoid
these problems, never touch the connector insulating materi-
al. In addition, the multiplexer card should be used only in
clean, dry environments 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.
4.5.8 Scanning speed considerations
One key advantage of the Model 7015 is the faster switching
times affordable by the solid-state relays. The following
paragraphs discuss considerations for taking full advantage
of this faster switching speed while scanning.
Break-before-make (Model 7002 only)
Normally, break-before-make is enabled to ensure that the
presently closed channel fully opens before the subsequent
channel in the scan list closes. While this feature does protect
the mux card and external circuits, it also slows down the
scanning speed somewhat. To maximize scanning speed, dis-
able break-before-make operation. Note, however, that cau-
tion should be used, as simultaneous connection of two or
more active sources may damage the mux card or external
circuits. Break-before-make can be enabled or disabled by
using the appropriate selection in the GENERAL MENU
(Model 7002 only).
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