Keithley 6220 Reference Manual page 65

Model 6220/6221 Reference Manual
Figure 2-7
Guarded triax cable – inner shield connected to Cable Guard (see
Guarded DUT mounting plate
There may be significant leakage and capacitance in the test fixture. A DUT is typ-
ically mounted on a metal plate using insulated terminal posts. In an unguarded
configuration, current leakage may occur from output high to output low through
the insulators of the terminal posts. Also, capacitance may exist across the DUT.
Figure 2-8
Model 622x. The unguarded configuration is shown in
and R
L2
and C
2
The test fixture shunt capacitance across the DUT acts the same as capacitance
in an unguarded triax cable and slows down response (increases settling time).
For a high impedance DUT (>10MΩ), significant leakage current may occur
through the terminal post insulators. The current through the DUT will be the
output current (I
Figure
The driven guard is connected to the metal mounting plate (now called "Metal
Guard Plate"). Since output high and guard are at nearly the same potential, the
voltage drop across resistor R
With such a minimal voltage drop, leakage current through R
and the charging current through C
Resistor R
ally all the current flows through the DUT.
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176 - TestEquipmentDepot.com
shows how a DUT mounted on a metal plate can be connected to the
represent the leakage resistance across the DUT, and the capacitors C
represent the shunt capacitance across the DUT.
) from the Model 622x minus the leakage current (I
S
2-8B shows how to use guarding to eliminate leakage current at the DUT.
and capacitor C
L2 2
Return to
Ω
Ω
and capacitor C will be almost 0V.
L1 1
is minimized.
1
are effectively removed from the test circuit. Virtu-
Section 2 topics
Output Connections 2-11
Figure 2-5A)
Ω
Figure 2-8A. Resistors R
).
L
is greatly reduced,
L1
L1
1