Keithley SourceMeter 2600 Series Reference Manual page 235

Series 2600 System SourceMeters Reference Manual
Guard
WARNING
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 Input/Out-
put HI (or Sense HI for remote sense) voltage. The purpose of guarding is to elim-
inate the effects of leakage current (and capacitance) that can exist between
input/output high and low. In the absence of a driven guard, leakage in the exter-
nal test circuit could be high enough to adversely affect the performance of the
SourceMeter.
Leakage current can occur through parasitic or non-parasitic leakage paths. An
example of parasitic resistance is the leakage path across the insulator in a coax
or triax cable. An example of non-parasitic resistance is the leakage path through
a resistor that is connected in parallel to the 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
DUT.
WARNING
Inside the test fixture, a triaxial cable can be used to extend guard to the DUT.
The center conductor of the cable is used for In/Out HI, the inner shield is used for
guard, and the outer shield is used for In/Out LO and is connected to the safety
shield (which is connected to safety earth ground).
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.
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 (>30Vrms or 42.4V peak). This
safety shield must completely enclose the guard plate or shield
and must be connected to safety earth ground.
shows the metal case of a test fixture being used as a safety
shield.
NOTE See Section 3
Return to
Source-Measure Concepts 8-29
for details on guarded test connections.
Section 8 topics 2600S-901-01 Rev. A / May 2006
Figure 8-15B