Keithley 7172, Instruction Manual

Page: 43 / 98

SECTION 2
Operation
2-25
2.7.2 Electromagnetic Interference (EMI)
The electromagnetic interference characteristics of the
Model 7172 Low Current 8
×
12 Matrix Card comply with
the electromagnetic compatibility (EMC) requirements
of the European Union as denoted by the CE mark.
However, it is still possible for sensitive measurements
to be affected by external sources. In these instances,
special precautions may be required in the measure-
ment setup.
Sources of EMI include:
•
radio and television broadcast transmitters
•
communications transmitters, including cellular
phones and handheld radios
•
devices incorporating microprocessors and high
speed digital circuits
•
impulse sources as in the case of arcing in high-
voltage environments
The effect on instrument performance can be consider-
able if enough of the unwanted signal is present. A com-
mon problem is the recti
Þ cation by semiconductor
junctions of RF picked up by the leads.
The equipment and signal leads should be kept as far
away as possible from any EMI sources. Additional
shielding of the measuring instrument, signal leads, and
sources will often reduce EMI to an acceptable level. In
extreme cases, a specially constructed screen room may
be required to suf
Þ ciently attenuate the troublesome
signal.
Many instruments incorporate internal Þ ltering that
may help to reduce RFI effects in some situations. In
some cases, external
Þ ltering may also be required. Such
Þ ltering, however, may have detrimental effects on the
desired signal.
2.7.3 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 instrumentation is connected to other
instrumentation with more than one signal return path
such as power line ground. As shown in Figure 2-20, the
resulting ground loop causes current to
ß ow 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 terminals
of the two instruments. This voltage will be added to
the source voltage, affecting the accuracy of the
measurement.
Figure 2-21 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
instruments in the test setup may not be designed in
this manner. When in doubt, consult the manual for
each instrument in the test setup.
2.7.4 Keeping Connectors Clean
As is the case with any high-resistance device, the integ-
rity of triaxial and other connectors can be damaged if
they are not handled properly. If the connector insula-
tion becomes contaminated, the insulation resistance
will be substantially reduced, affecting high-impedance
measurement paths.
Oils and salts from the skin can contaminate connector
insulators, reducing their resistance. Also, contaminants
present in the air can be deposited on the insulator sur-
INSTRUMENT 1 INSTRUMENT 2 INSTRUMENT 3
SIGNAL LEADS
POWER LINE GROUND
GROUND
LOOP
CURRENT
Figure 2-20. Power Line Ground Loops
INSTRUMENT 1 INSTRUMENT 2 INSTRUMENT 3
POWER LINE GROUND
Figure 2-21. Eliminating Ground Loops
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