Keithley 7012-S, Instruction Manual

Page: 61 / 93

Operation
4-15
Figure 4-13 shows how to connect several instruments
together 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 instru-
ments having isolated LO terminals. However, all in-
struments in the test setup may not be designed in this
Figure 4-12
Power line ground loops
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
Signal Leads
Ground Loop
Current
Figure 4-13
Eliminating ground loops
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
manner. When in doubt, consult the manual for all in-
strumentation in the test setup.
4.5.5 Keeping connectors clean
As is the case with any high-resistance device, the in-
tegrity of connectors can be damaged if they are not
handled properly. If connector insulation becomes con-
taminated, the insulation resistance will be substantial-
ly reduced, affecting high-impedance measurement
paths.
Oils and salts from the skin can contaminate connector
insulators, reducing their resistance. Also, contami-
nants present in the air can be deposited on the insula-
tor surface. To avoid these problems, never touch the
connector insulating material. In addition, the multi-
plexer card should be used only in clean, dry environ-
ments to avoid contamination.
If the connector insulators should become contaminat-
ed, either by inadvertent touching, or from air-borne
deposits, they can be cleaned with a cotton swab
dipped in clean methanol. 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.
4.5.6 AC frequency response
The AC frequency response of the Model 7012 is im-
portant in test systems that switch AC signals. Refer to
the speci
Þ cations at the front of this manual.
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