Ground Loops - Keithley 2000-SCAN-20 Instruction Manual

3-14 Operation
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 instruments incorporate internal filtering that may help reduce EMI effects. In some
cases, additional external filtering may be required. Keep in mind, however, that filtering may
have detrimental effects on the measurement.

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
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.
Figure 3-6
Power line ground loops
Instrument 1
inFigure 3-6, the resulting ground loop causes current to flow through the
Signal Leads
Instrument 2
Ground Loop
Current
Power Line Ground
Instrument 3