Keithley 7999-2, Инструкция по эксплуатации

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Operating Considerations 3-5
Ground loops
When two or more devices are connected together, care must be taken to avoid unwanted
signals caused by ground loops. Ground loops usually occur when devices are connected with
more than one signal return path such as power line ground. As shown in Figure 3-3, the result-
ing ground loop causes current to flow through 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 devices. This voltage will be induced into the desired signal affecting its
integrity.
Figure 3-4 shows how to connect several devices together to eliminate this type of ground
loop problem. Here, only one device is connected to power line ground.
Ground loops are not normally a problem with isolated LO terminals. However, all devices
in the test setup may not be designed in this manner. When in doubt, consult the manual for all
instrumentation in the test setup.
Device 1 Device 2 Device 3
Signal Leads
Ground
Loop
Current
Power Line Ground
Figure 3-3
Ground loops
Device 1 Device 2 Device 3
Power Line Ground
Figure 3-4
Eliminating
ground loops