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Operation
5-14
Ground loops
When two or more instruments are connected together, care
must be taken to avoid unwanted signals caused by ground
loops. Ground loops usually occur when sensitive instru-
mentation is connected to other instrumentation with more
than one signal return path such as power line ground. As
shown in Figure 5-13, the resulting ground loop causes cur-
rent to flow through the instrument LO signal leads and then
back through power line ground. This circulating current
develops a small but undesirable voltage between the LO ter-
minals of the two instruments. This voltage will be added to
the source voltage, affecting the accuracy of the measure-
ment.
Figure 5-14 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.
Figure 5-13
Power line ground loops
Figure 5-14
Eliminating ground loops
Keeping connectors clean
As is the case with any high-resistance device, the integrity
of connectors can be damaged if they are not handled prop-
erly. If connector insulation becomes contaminated, the insu-
lation 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
in the air can be deposited on the insulator surface. To avoid
these problems, never touch the connector insulating mate-
rial. In addition, the card should be used only in clean, dry
environments to avoid contamination.
If the connector insulators should become contaminated,
either by inadvertent touching or from airborne deposits,
they can be cleaned with a cotton swab dipped in clean meth-
anol. After thoroughly 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.
AC frequency response
The AC frequency response of the Model 7022 is important
in test systems that switch AC signals. Refer to the specifica-
tions at the front of this manual.
Instrument 1
Instrument 2
Instrument 3
Power Line Ground
Signal Leads
Ground Loop
Current
Instrument 1
Instrument 2
Instrument 3
Power Line Ground
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