Signal Connections
Chapter 3
SCXI-1122 User Manual
3-8
© National Instruments Corporation
Scanned
100
times
CH0 (one sample)
CH1 (one sample)
CH2 (one sample)
CH3 (one sample)
No
100
Scans
Done?
Yes
average the samples for each channel
CH0 x 100 samples
CH1 x 100 samples
CH2 x 100 samples
CH3 x 100 samples
average the samples for each channel
a. Bad technique—hardware-driven
b. Good technique—software-
scanning wears out relays 100 times
driven scanning saves relay life.
faster than the software-driven
scanning.
Figure 3-6. Avoiding Relay Wear by Sampling and Averaging Rather Than
Single-Sample Channel Scanning
Excitation Channel Signal Connections
Your SCXI-1122 has a voltage (VEX) and a current (IEX) excitation channel, which are
available at the front connector. In addition, VEX/2 is available for half-bridge and quarter-
bridge transducers. Both channels are isolated from earth ground up to 480 Vrms working
common-mode voltage. Notice that the voltage and current excitations are electrically isolated
from each other but do not provide a safety isolation between them.
Warning:
Exceeding the overvoltage protection or isolation rating on the excitation output
can damage the SCXI-1122, the SCXIbus, and the DAQ board. National
Instruments is
NOT
liable for any damages or injuries resulting from such signal
connections.
Excitation Level
Each excitation channel of your SCXI-1122 has one level:
•
Current excitation–1 mA
•
Voltage excitation–3.333 V
It is important that you do not exceed the maximum permissible load of each channel, listed in
Table 3-1.
