Keithley 6220, Справочное руководство

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2-12 Output Connections Model 6220/6221 Reference Manual
Return to Section 2 topics
Figure 2-8
DUT mounting plate (unguarded and guarded)
Banana Jack Guard
A typical test for the Model 622x is to source a current to a DUT and then use a
voltmeter to measure the voltage across the DUT. When using a voltmeter that
has low input impedance, a more accurate voltage reading may be obtained by
taking the voltage measurement at the banana jack Guard terminal.
Figure 2-9 A shows how the conventional voltage measurement method can result
in loading error when using a low-impedance voltmeter. As shown, a 10M
Ω
impedance voltmeter results in 50% loading error when trying to measure a 10M
Ω
DUT. Half the sourced current (5µA) flows through the DUT, and the other half
flows through the voltmeter. The result is a voltage reading of 50V (instead of the
expected 100V). A higher impedance DUT results in even more error, while a
lower impedance DUT results in less error.
A technique to eliminate the loading factor is to make the measurement at the
banana jack Guard terminal. As shown in
Figure 2-9 B, there is no loading on the
DUT. All of the sourced current (10µA) is applied to the DUT. The 100V across the
DUT is also seen at the Guard terminal. Accuracy for Guard is typically ±1mV.
Metal Mounting Plate
I
L
Output
( I
S
)
622x
High
Low
R
L1
Earth
I
S
I
S
= Sourced current
I
D
= DUT current
I
L
= Leakage current
R
L2
DUT
Metal Guard Plate
Output
( I
S
)
622x
High
Low
R
L1
I
D
= I
S
I
S
I
S
= Sourced current
I
D
= DUT current
DUT
Guard
0V
I
D
= I
S
I
L
Insulated terminal post
(1 of 2)
Insulated terminal post
(1 of 2)
Ground
A) Unguarded circuit inner shield connected to Output Low
B) Guarded circuit inner shield connected to Cable Guard
Cable
(Earth Ground)
C
1
C
2
C
1
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