Keithley 6220, Reference Manual

Page: 65 / 454

Model 6220/6221 Reference Manual Output Connections 2-11
Return to Section 2 topics
Figure 2-7
Guarded triax cable – inner shield connected to Cable Guard (see Figure 2-5 A)
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 shows how a DUT mounted on a metal plate can be connected to the
Model 622x. The unguarded configuration is shown in Figure 2-8 A. Resistors R
L1
and R
L2
represent the leakage resistance across the DUT, and the capacitors C
1
and C
2
represent the shunt capacitance across the DUT.
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 (>10M
Ω
), significant leakage current may occur
through the terminal post insulators. The current through the DUT will be the
output current (I
S
) from the Model 622x minus the leakage current (I
L
).
Figure 2-8 B shows how to use guarding to eliminate leakage current at the DUT.
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
L1
and capacitor C
1
will be almost 0V.
With such a minimal voltage drop, leakage current through R
L1
is greatly reduced,
and the charging current through C
1
is minimized.
Resistor R
L2
and capacitor C
2
are effectively removed from the test circuit. Virtu-
ally all the current flows through the DUT.
100G
Ω
R
L1
100G
Ω
R
L2
100nA
1G
Ω
R
DUT
0V
100nA
100V
100pF
C
L1
100pF
C
L1
100V
High
Guard
Low
Output
(10nA)
622x
Triax Cable
DUT
0V
(Earth Ground)
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