Source-Measure Concepts
5-23
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
L
) flows through the insulators (R
L1
and R
L2
) to
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
L1
is the same (0V drop), no current can flow through the
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
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.
6430
I-Meter
x1
REMOTE
PreAmp
In/Out HI
In/Out LO
V-Source
Guard
6430
I-Meter
x1
REMOTE
PreAmp
In/Out HI
In/Out LO
V-Source
Guard
HI (In/Out)
Guard (Cable)
Guard (Cable)
LO (In/Out)
LO (In/Out)
I
D
I
L
R
L2
I
M
I
D
I
L
DUT
Metal Mounting Plate
= +
I
D
I
D
I
L
= Measured current
= DUT current
= Leakage current
Insulator
Insulator
A. Unguarded
B. Guarded
Metal Case
I
D
R
L1
I
M
I
D
=
DUT
0V
Metal Mounting Plate
Connect to earth safety
ground using #18 AWG
wire or larger
IN/OUT HIGH
IN/OUT HIGH
R
L1
HI (In/Out)
Figure 5-12
High-impedance
measurements
Summary of Contents for 6430
Page 26: ......
Page 32: ......
Page 78: ...2 14 Connections ...
Page 98: ...3 20 Basic Source Measure Operation ...
Page 138: ...5 30 Source Measure Concepts ...
Page 156: ...6 18 Range Digits Speed and Filters ...
Page 168: ...7 12 Relative and Math ...
Page 176: ...8 8 Data Store ...
Page 202: ...9 26 Sweep Operation ...
Page 248: ...11 22 Limit Testing ...
Page 310: ...16 6 SCPI Signal Oriented Measurement Commands ...
Page 418: ...17 108 SCPI Command Reference ...
Page 450: ...18 32 Performance Verification ...
Page 477: ...A Specifications ...
Page 489: ...B StatusandErrorMessages ...
Page 498: ...B 10 Status and Error Messages ...
Page 499: ...C DataFlow ...
Page 503: ...D IEEE 488BusOverview ...
Page 518: ...D 16 IEEE 488 Bus Overview ...
Page 519: ...E IEEE 488andSCPI ConformanceInformation ...
Page 523: ...F MeasurementConsiderations ...
Page 539: ...G GPIB488 1Protocol ...
Page 557: ......