Magnetic Fields; Radio Frequency Interference; Ground Loops - Keithley 7021 Instruction Manual

Operation
Magnetic fields
When a conductor cuts through magnetic lines of force, a
very small current is generated. This phenomenon will
frequently cause unwanted signals to occur in the test
leads of a switching multiplexer system. If the conductor
has sufficient length, even weak magnetic fields like those
of the earth can create sufficient signals to affect low-level
measurements.
Two ways to reduce these effects are: (1) reduce the lengths
of the test leads, and (2) minimize the exposed circuit area.
In extreme cases, magnetic shielding may be required. Spe-
cial metal with high permeability at low flux densities (such
as mu metal) is effective at reducing these effects.
Even when the conductor is stationary, magnetically
induced signals may still be a problem. Fields can be pro-
duced by various signals such as the AC power line voltage.
Large inductors such as power transformers can generate
substantial magnetic fields, so care must be taken to keep
the switching and measuring circuits a good distance away
from these potential noise sources.
At high current levels, even a single conductor can generate
significant fields. These effects can be minimized by using
twisted pairs, which will cancel out most of the resulting
fields.

Radio frequency interference

Radio Frequency Interference (RFI) is a general term used
to describe electromagnetic interference over a wide range
of frequencies across the spectrum. Such RFI can be partic-
ularly troublesome at low signal levels, but it can also affect
measurements at high levels if the problem is of sufficient
severity.
RFI can be caused by steady-state sources such as radio or
TV signals or some types of electronic equipment (micro-
processors, high speed digital circuits, etc.), or it can result
from impulse sources, as in the case of arcing in high-volt-
age environments. In either case, the effect on the measure-
ment can be considerable if enough of the unwanted signal
is present.
RFI can be minimized in several ways. The most obvious
method is to keep the equipment and signal leads as far
away from the RFI source as possible. Shielding the switch-
ing card, signal leads, sources, and measuring instruments
will often reduce RFI to an acceptable level. In extreme
cases, a specially constructed screen room may be required
to sufficiently attenuate the troublesome signal.
5-14
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Many instruments incorporate internal filtering that may
help to reduce RFI effects in some situations. In some cases,
additional external filtering may also be required. Keep in
mind, however, that filtering may have detrimental effects on
the desired signal.

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 instrumen-
tation 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 current 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 terminals of
the two instruments. This voltage will be added to the source
voltage, affecting the accuracy of the measurement.
Signal Leads
Instrument 1 Instrument 2
Ground Loop
Current
Power Line Ground
Figure 5-13
Power line ground loops
Figure 5-14 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.
Instrument 1 Instrument 2
Figure 5-14
Eliminating ground loops
Instrument 3
Instrument 3
Power Line Ground