Section 2: General operation
Model 2651A High Power System SourceMeter® Instrument Reference Manual
2-58
2651A-901-01 Rev. A / March 2011
Floating a SMU
Using an external source in the test system may require that a Model 2651A source-measure unit
(SMU) float off chassis earth ground. An example of such a test system is shown below, which
includes an external voltage source. Notice that output low of the voltage source is connected to
chassis earth ground.
For the test circuit shown below, the Model 2651A must float off chassis earth ground. As shown,
SMU LO of the Model 2651A is fl10 V above chassis earth ground. If SMU LO of the Model
2651A was instead connected to chassis ground, the external voltage source would be shorted
through chassis ground.
Figure 23: Floating the Model 2651A schematic
V-source
HI
LO
10V
+10V
Chassis earth ground
Low
Output low connected
to chassis
High
+ –
Chassis
Chassis
Source chassis connected to chassis
earth ground through the power cord
External Source
+
–
Model 2651A LO
NOT connected to
chassis ground (floating)
Model
2651A
SMU
Model 2651A chassis
connected to chassis
earth ground through the
power cord.
DUT
The Model 2651A connections for the floating configuration are shown below. In order to float the
SMU, input/output LO must be isolated from chassis ground. This is accomplished by not connecting
input/output LO to chassis ground.
Figure 24: Floating the 2651A SMU connections
G
G G G
S
LO
SENSE/GUARD
S
HI
LO
HI
OUTPUT
40V, 50A MAX.
DUT
Low
LO
HI
10 V
+ –
Chassis
External Source
Output low connected
to chassis
Source chassis connected to chassis
earth ground through the power cord
High
Model 2651A chassis
connected to chassis
earth ground through the
power cord
Model 2651A LO
NOT connected to
chassis ground (floating)
The external voltage source can be a SMU of a second Model 2651A instrument or other instrument.
Keep in mind that if the combined outputs of the sources exceeds 42 V, then a safety shield will be
required for the DUT (see the following WARNINGS).
