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C-6 Measurement Considerations
Magnetic fields
When a conductor loop 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 test system. If the conductor has sufficient length or cross-sectional area, even weak magnetic
fields such as those of the earth can create sufficient signals to affect low-level measurements.
Three ways to reduce these effects are: (1) reduce the lengths of the connecting cables, (2)
minimize the exposed circuit area, and (3) change the orientation of the leads or cables. In
extreme cases, magnetic shielding may be required. Special metal with high permeability at low
flux densities (such as mu metal) are effective at reducing these effects.
Even when the conductor is stationary, magnetically-induced signals may still be a problem.
Fields can be produced by various sources 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 Model 2182 voltage source and connecting cables a good distance away from these
potential noise sources.
Radio frequency interference
RFI (Radio Frequency Interference) is a general term used to describe electromagnetic
interference over a wide range of frequencies across the spectrum. Such RFI can be particularly
troublesome at low signal levels, but it can also affect measurements at high levels if the fields
are of sufficient magnitude.
RFI can be caused by steady-state sources such as radio or TV signals, or some types of
electronic equipment (microprocessors, high speed digital circuits, etc.), or it can result from
impulse sources, as in the case of arcing in high-voltage environments. In either case, the effect
on the measurement 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 Model 2182
voltage source and signal leads as far away from the RFI source as possible. Additional shielding
of the instrument, signal leads, sources, and other 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.
The Model 2182 digital filter 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 such as increased settling time 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 instrumentation is
connected to other instrumentation with more than one signal return path such as power line
ground. As shown in
Figure C-2 , the resulting ground loop causes current to flow through the