Keeping Connectors Clean; Noise Currents Caused By Cable Flexing; Shielding - Keithley 7072 Instruction Manual

OPERATION
2.7.4 Keeping Connectors Clean
As is the case with any high-resistance device, the integrity
of biaxial and other connectors can be damaged if they
are not handled properly. If the connector insulation
becomes contaminated, the insulation resistance will be
substantially reduced, affecting high-impedance measure-
ment paths.
Oils and salts from the skin can contaminate connector in-
sulators, reducing their resistance. Also, contaminants pre-
sent in the air can be deposited on the insulator surface.
To avoid these problems, never touch the connector in-
sulating material. In addition, the matrix 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 ah-borne deposits,
they can be cleaned with a cotton swab dipped in clean
methanol. After thorough cleaning, they should be allow-
ed to dry for several hours in a low-humidity environment
before use, or they can be dried more quickly using dry
nitrogen.
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2.7.5 Noise Currents Caused by Cable Flexing
Noise currents can be generated by bending or flexing
coaxial or triaxial cables. Such currents, which are known
as triboelectric currents, are generated by charges created
between a conductor and insulator caused by friction.
Low-noise cable can be used to minimize these effects.
Such cable has a special graphite coating under the shield
to provide lubrication and to provide a conduction path
to equalize charges.
Even low-noise cable generates some noise currents when
flexed or subjected to vibration. To minimize these effects,
keep the cables as short as possible, and do not subject
them to temperature variations that could cause expansion
or contraction. Tie down offending cables securely to avoid
movement, and isolate or remove vibration sources such
as motors or pumps.
2.7.6 Shielding
Proper shielding of all unguarded signal paths and devices
under test is important to minimize noise pickup in vir-
tually any switching matrix system. Otherwise, inter-
ference from such noise sources as line frequency and RF
fields can seriously corrupt a measurement.
In order for shielding to be effective, the shield surround-
ing the HI signal path should be connected to signal LO
(or chassis ground for instruments without isolated Ix) ter-
minals). Since most Model 7072 matrix applications call
for separately switching LO, a separate connection from
LO to the cable shield at the source or measurement end
must be provided, as in the example of Figure 2-21. Here,
we are using the GUARD path of the Model 7072 to carry
the shield out to the device under test. Needless to say,
this arrangement should not be used with guarding, as
GUARD and LO should not be connected together.
WARNING
Hazardous voltage may be present if LO on any
instrument is floated above ground potential.
If the device under test is to be shielded, the shield should
be should be connected to the Lo terminal. If you are us-
ing the GUARD connection as shield, care should be taken
to insulate the outer ring of the triaxial connector mounted
on the test fixture from the test fixture itself. Otherwise,
Lo will be connected to chassis ground, possibly resulting
in a ground loop. An alternative is to use two shields, one
mounted within (and insulated from) the other. In this
case, the GUARD path would be connected to the inner
shield, while the outer shield would be chassis ground-
ed. This arrangement is shown in Figure 2-22. Incidental-
ly, this configuration is also recommended for guarded ap-
plications, with the inner shield as guard, and the outer
shield acting as a safety shield.
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