Chapter 4
Connecting and Using Accessories and Transducers
©
National Instruments Corporation
4-31
When you use an external supply, the order of supply and load does not
matter as long as you do not create a second loop through which current
flows. If both your supply and load are grounded, you must connect your
signals as shown in Figure 4-19a. The circuit in Figure 4-19b does not
work.
Figure 4-19.
Current Loop Connection, Load and External Supply Both Grounded
Chassis Ground Connections
The front signal connector also has a chassis ground pin. If you have any
floating channels (channels that are not already referenced to ground from
your signal connections) that you want to reference to earth ground,
connect the isolated ground of the channel to the chassis ground pin. Do not
use this pin as a safety earth ground because the ground trace cannot handle
the potentially large currents that result when the channel is short-circuited
to high voltage. Use the strain-relief tab on the terminal block for the safety
earth ground.
LVDTs, RVDTs, and Resolvers
LVDTs, RVDTs, and resolvers require signal conditioning with AC
excitation and synchronous demodulation. The SCXI-1540 provides these
specialized functions.
Uncontrolled
Loop
Current
+
Load
SCXI-1124
V
EXT
0-20 mA
+
Load
I
I
ISINK
GND
ISINK
GND
SCXI-1124
a. Correct
b. Incorrect
V
EXT
–
–
