Ground Loops; Figure 3-7 Power Line Ground Loops; Figure 3-8 Eliminating Ground Loops - Keithley 2000-Scan Instruction Manual

3-22 Operation
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 in
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 3-7
Power line ground
loops
Figure 3-8
loop problem. Here, only one instrument is connected to power line ground. Ground loops are
not normally a problem with instruments having isolated LO terminals. However, all
instruments in the test setup may not be de-signed in this manner. When in doubt, consult the
manual for all instrumentation in the test setup.
Figure 3-8
Eliminating ground
loops
Figure 3-7, the resulting ground loop causes current to flow through the
Signal Leads
Instrument 1 Instrument 2
Ground Loop
Current
shows how to connect several instruments together to eliminate this type of ground
Instrument 1 Instrument 2
Power Line Ground
Instrument 3
Power Line Ground
Instrument 3