Figure 5-12 High-Impedance Measurements - Keithley 6430 Instruction Manual

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).
A coaxial cable can be used if the guard potential does not exceed 30Vrms (42.4V peak).
The center conductor is used for In/Out HI, and the outer shield is used for guard. For higher
guard potentials, use a triaxial cable as previously explained.
Figure 5-12 shows how cable guard can eliminate leakage current through the insulators in a
test fixture. In Figure 5-12A, leakage current (I
In/Out LO, adversely affecting the low-current (or high-resistance) measurement of the DUT.
In Figure 5-12B, the driven guard is connected to the metal guard plate for the insulators.
Since the voltage on either end of R
leakage resistance path. Thus, the SourceMeter only measures the current through the DUT.
Cable guard should be used when sourcing or measuring low current (<1µA).
NOTE
Figure 5-12
High-impedance
6430
measurements
V-Source
A. Unguarded
6430
V-Source
B. Guarded
When using shielded, triaxial, or coaxial cabling with guard, cable guard (not ohms
guard) must be used to prevent oscillations. CABLE guard is the factory default
setting.
In/Out HI
I-Meter
Guard
REMOTE
x1
PreAmp
In/Out LO
In/Out HI
I-Meter
Guard
REMOTE
x1
PreAmp
In/Out LO
Source-Measure Concepts
) flows through the insulators (R
L
is the same (0V drop), no current can flow through the
L1
IN/OUT HIGH
HI (In/Out)
Guard (Cable)
LO (In/Out)
IN/OUT HIGH
HI (In/Out)
Guard (Cable)
LO (In/Out)
Connect to earth safety
ground using #18 AWG
wire or larger
and R
L1
Insulator
Insulator
I
I = + I
D
M
DUT
R R
L1 I L2
L
Metal Mounting Plate
I
= Measured current
D
I
= DUT current
D
I
= Leakage current
L
Metal Case
I
D
DUT
R
0V
L1
Metal Mounting Plate
5-23
) to
L2
I
L
I = I
M D