Ground Loops And Other Smu Grounding Considerations - Keithley 4200A-SCS Reference Manual

Section 12: Maintenance Model 4200A-SCS Parameter Analyzer Reference Manual

Ground loops and other SMU grounding considerations

Ground loops, which occur when more than one point in a test system is connected to earth ground,
can create error signals that cause erratic or erroneous performance. The configuration shown in the
next figure shows a ground loop that is created by connecting both 4200A-SCS signal COMMON and
DUT LO to earth ground. A large ground current flowing in the loop will encounter small resistances,
either in the conductors or at the connecting points. This small resistance results in voltage drops that
can affect performance.
To prevent ground loops, the test system should be connected to ground at only a single point. If it is
not possible to remove the DUT ground, the ground link between the GNDU COMMON terminal and
chassis ground should be removed, as shown in the following figure. Note, however, that removing
the COMMON-to-chassis link may result in oscillations (refer to Making stable measurements (on
page 12-8)).
Figure 498: Ground loops
Figure 499: Eliminating ground loops
12-18 4200A-901-01 Rev. C / February 2017