Tektronix Keithley 2651A Reference Manual page 242

Section 4: Theory of operation Model 2651A High Power System SourceMeter® Instrument Reference Manual
Guard
GUARD is at the same potential as output HI. If hazardous voltages are present at output HI,
they are also present at the GUARD terminal.
The rear-panel GUARD terminals are always enabled and provide a buffered voltage that is at the
same level as the HI (or SENSE HI for remote sense) voltage. The purpose of guarding is to eliminate
the effects of leakage current (and capacitance) that can exist between HI and LO. In the absence of
a driven guard, leakage in the external test circuit could be high enough to adversely affect the
performance of the SMU.
Leakage current can occur through parasitic or nonparasitic leakage paths. An example of parasitic
resistance is the leakage path across the insulator in a coaxial or triaxial cable. An example of
nonparasitic resistance is the leakage path through a resistor that is connected in parallel to the
device-under-test (DUT).
Guard connections
Guard is typically used to drive the guard shields of cables and test fixtures. Guard is extended to a
test fixture from the cable guard shield. Inside the test fixture, the guard can be connected to a guard
plate or shield that surrounds the device under test (DUT).
To prevent injury or death, a safety shield must be used to prevent physical contact with a
guard plate or guard shield that is at a hazardous potential (>30 V RMS or 42.4 V ). This
PEAK
safety shield must completely enclose the guard plate or shield and must be connected to
safety earth ground. The figure in this topic shows the metal case of a test fixture being used
as a safety shield.
See Guarding and shielding (on page 2-57) for details about guarded test connections.
Inside the test fixture, a triaxial cable can be used to extend guard to the device under test (DUT).
The center conductor of the cable is used for HI, and the inner shield is used for guard.
The figures below show how cable guard can eliminate leakage current through the insulators in a
test fixture. In this figure, leakage current (I ) flows through the insulators (R and R ) to LO,
L L1 L2
adversely affecting the low-current (or high-resistance) measurement of the DUT.
4-24 2651A-901-01 Rev. C October 2021