Measurement Considerations; Magnetic Fields - Keithley 7072 Instruction Manual

OPERATION
Trim Tee
Adapters
Figure 2-16. Using Triax Tee Adapters to Daisy Chain Cards
2.7 MEASUREMENT CONSIDERATIONS
Many measurements made with the Model 7072 concern
low-level signals. Such measurements are subject to
various types of noise that can seriously affect low-level
measurement accuracy. The following paragraphs discuss
possible noise sources that might affect these
measurements.
2.7.1 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 matrix system. If the conductor has suffi-
cient length, even weak magnetic fields lie 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.
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 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.
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