Keithley 4200A-SCS Reference Manual

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Keithley 4200A-SCS Reference Manual Manual pdf 845 page image

Section 12: Maintenance Model 4200A-SCS Parameter Analyzer Reference Manual12-18 4200A-901-01 Rev. C / February 2017Ground loops and other SMU grounding considerationsGround 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 thenext figure shows a ground loop that is created by connecting both 4200A-SCS signal COMMON andDUT 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 thatcan affect performance.To prevent ground loops, the test system should be connected to ground at only a single point. If it isnot possible to remove the DUT ground, the ground link between the GNDU COMMON terminal andchassis ground should be removed, as shown in the following figure. Note, however, that removingthe COMMON-to-chassis link may result in oscillations (refer to Making stable measurements (onpage 12-8)).Figure 498: Ground loopsFigure 499: Eliminating ground loops