Keithley 2657A, Reference Manual, Page: 225 / 805

Section 4: Theory of operation Model 2657A High Power System SourceMeter® Instrument Reference Manual
4-24 2657A-901-01 Rev. B/December 2012
Inside the test fixture, a triaxial cable can be used to extend guard to the device under test (DUT).
The center conductor of the cable is used for HI, and the inner shield is used for guard.
The figures below show how cable guard can eliminate leakage current through the insulators in a
test fixture. In this figure, leakage current (I
L
) flows through the insulators (R
L1
and R
L2
) to LO,
adversely affecting the low-current (or high-resistance) measurement of the DUT.
Also in the figures below, the driven guard is connected to the cable shield and extended to the metal
guard plate for the insulators. Since the voltage on either end of R
L1
is the same (0 V drop), no
current can flow through the leakage resistance path. Thus, the SourceMeter instrument only
measures the current through the DUT.
Figure 89: Unguarded measurements
Figure 90: Guarded measurements
Insulator
I
DUT
DUT
Metal mounting plate
I
L
R
L1
R
L2
Insulator
I
M
= Measured current
I
DUT
= DUT current
I
L
= Leakage current
HI
LO
SourceMeter
I-meter
V-source
+
–
I
M
= I
DUT
+ I
L
Safety shield
Connect to protective earth
(safety ground) using
#16 AWG wire or larger
I
DUT
DUT
Metal mounting plate
R
L1
I
L
I
L
Insulator
HI
LO
x1
GUARD
0 V
SourceMeter
Connect to protective earth
(safety ground) using
#16 AWG wire or larger
Cable shield
Safety shield
I-meter
V-source
+
–
I
M
= I
DUT
R
L2
I
M
= Measured current
I
DUT
= DUT current
I
L
= Leakage current