Section 3: Operation
Model 2000-SCAN Scanner Card User's Manual
3-18
2000-SCAN-900-01 Rev. A / April 2018
Magnetic fields
When a conductor cuts through magnetic lines of force, a small current is generated. This may cause
unwanted signals to occur in the test leads of a scanning system. If the conductor has sufficient
length, even weak magnetic fields can create sufficient signals that affect low-level measurements.
Two ways to reduce these effects are to:
•
Reduce the lengths of the test leads
•
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, is 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 signals such as AC power line voltage. Large inductors, such as power
transformers, can generate substantial magnetic fields. Care must be taken to give the switching and
measuring circuits enough distance 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.
Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the DMM6500 comply with the electromagnetic
compatibility (EMC) requirements of the European Union (EU) directives 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 test setup.
Sources of EMI include:
•
Radio and television broadcast transmitters.
•
Communications transmitters, including cellular phones and hand-held radios.
•
Devices using microprocessors and high-speed digital circuits.
•
Impulse sources like arcing in high-voltage environments.
The instrument, measurement leads, and other cables should be kept as far away as possible from
any EMI sources. Shielding measurement leads and the multimeter may reduce EMI to acceptable
levels. In extreme cases, a specially-constructed screen room may be required to sufficiently
attenuate troublesome signals.
Many cases use internal filtering that may help reduce EMI effects. In some cases, additional external
filtering may be required. However, keep in mind that filtering may have negative effects on the
measurement.
