Keithley 2000-Scan, Instruction Manual, Page: 55 / 86

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 scanning
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. 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 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.
Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the Model 2000 Multimeter 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 incorporating microprocessors and high-speed digital circuits.
• Impulse sources as in the case of 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.
R
E
DUT
DUT
R
PATH
E
OUT
R
PATH
R
DUT
R
PATH
+
=
E
DUT
E
OUT
Figure 3-6
Voltage attenuation by
path isolation
resistance
Operation 3-21