Keithley 2657A, Reference Manual, Page: 224 / 805

Model 2657A High Power System SourceMeter® Instrument Reference Manual Section 4: Theory of operation
2657A-901-01 Rev. B/December 2012 4-23
Guard
GUARD is at the same potential as output HI. Thus, if hazardous voltages are present at output HI,
they are also present at the GUARD terminal.
Guard overview
The driven guard (available at the rear panel GUARD terminals) is always enabled and provides 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 High Power System SourceMeter
®
instrument.
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 peak). This 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 2-55) for details about guarded test connections.
WARNING
WARNING
NOTE