National Instruments 1128, SCXI-1127 - page 42

Chapter 2 Using the SCXI-1127/1128
© National Instruments Corporation 2-7 SCXI-1127/1128 User Manual
Figure 2-6. 4-Wire Block Diagram
When used with a DMM for 4-wire ohms measurement, the upper
differential multiplexer provides the excitation source for the 4-wire
channel being scanned.
The EXCITATION input to the upper multiplexer is available through the
OUT0+ and OUT0– screw terminals on the SCXI-1331. The SENSE
output from the lower 16-to-1 multiplexer is available through the OUT2+
and OUT2– screw terminals on the SCXI-1331. You can also connect the
EXCITATION and SENSE commons of the SCXI-1127/1128 to the HVAB
backplane. Use switches AB0 and AB2 to make this connection. You can
do this through NI-SWITCH. This is the recommended usage with
the DMM.
Figure 2-7 shows an example of a 4-wire channel configuration of the
SCXI-1331 terminal block. Channel 6 consists of channel 6A+ and 6A–,
and 6B+ and 6B–. Channel 6A of 4-wire channel 6 is the excitation channel
and channel 6B is the voltage sense channel. Therefore, to make a 4-wire
resistance measurement, a DMM should excite the current through
high-voltage AB0 or OUT0 using channel 6A and sense the voltage across
the resistor through AB2 or OUT2 using channel 6B.
HVAB
Backplane
16-to-1
Multiplexer AB0+
AB0
Switch
AB0–
Out0–
EXCITATION Multiplexer
SENSE Multiplexer
0A+
0A–
Out0+
16-to-1
Multiplexer
AB2+
AB2
Switch
AB2–
Out2–
Out2+
15A+
15A–
0B+
0B–
15B+
15B–