![background image](http://html1.mh-extra.com/html/national-instruments/scxi-1112/scxi-1112_user-manual_3544744022.webp)
Chapter 3
Signal Connections
©
National Instruments Corporation
3-3
Caution
Exceeding the input damage level (±42 VAC peak or VDC between input
channels and chassis ground) can damage the SCXI-1112 module, the SCXIbus, and the
DAQ device. Furthermore, applying a voltage greater than
±
42 VAC peak or VDC to the
SCXI-1112 is an electrical shock hazard. National Instruments is not liable for any
damages or injuries resulting from exceeding these voltage limits.
Note
Exceeding the differential or common-mode input channel ranges results in a
distorted signal measurement.
Cold-Junction Sensors
Each channel of the SCXI-1112 has a built in cold-junction sensor. You can
access these sensors through software to perform thermocouple
cold-junction compensation.
In LabVIEW, read the cold-junction sensor (thermistor) for channel x using
the string
cjtemp
x
. Using analog input functions in NI-DAQ, refer to the
cold-junction sensor for the channel x using the integer –(1 + x).
Auto-Zero Calibration Switches
Each channel of the SCXI-1112 has an auto-zero calibration switch. When
this switch is closed, the input signal is disconnected and the differential
input terminals of the channel are shorted together and grounded. Any
voltage reading taken with this switch closed is a measure of the combined
voltage offset of the module and DAQ device. You can subtract this reading
from subsequent readings, when the auto-zero calibration switch is open, to
correct these readings for the system offset error corresponding to that
particular SCXI-1112 channel.
In LabVIEW, close the auto-zero calibration switch and take a reading on
channel x by providing the channel string
calgnd
x
to an analog input
DAQ VI. During multichannel scanning, you cannot combine
calgnd
x
and normal or virtual channel names such as
ob0 ! sc1 ! md2 ! 0:3
or
temperature1
in the same channel string
array. Auto-zero calibration readings must be taken by themselves.
For information on how to do auto-zero calibration using NI-DAQ function
calls, see the
SCXI_Calibrate_Setup
entry in the NI-DAQ Function
Reference Manual for PC Compatibles.