Download LabVIEW Data Acquisition Basics Manual

Chapter 21
Common SCXI Applications
For more information, look at the SCXI Thermocouple example VIs, found
in the appropriate module DLL in labview\examples\daq\scxi\.
These VIs use the cjtemp or mtemp string to read the temperature sensor
and use the reading for thermocouple cold-junction compensation.
Amplifier Offset
The SCXI-1100, SCXI-1101, SCXI-1112, SCXI-1122, SCXI-1125,
SCXI-1141, SCXI-1142, and SCXI-1143 have a special calibration feature
that enables LabVIEW to ground the module amplifier inputs so that you
can read the amplifier offset. For the other SCXI analog input modules, you
must physically wire your terminals to ground. The measured amplifier
offset is for the entire signal path including the SCXI module and the DAQ
device.
To read the grounded amplifier on the SCXI-1100, SCXI-1101, or
SCXI-1122, use the standard SCXI string syntax in the channels array with
calgnd substituted for the channel number, as shown in the following
table.
Channel List Element
Channel Specified
ob0!scx!mdy!calgnd
(SCXI-1100 and SCXI-1122 only)
The grounded amplifier of the module
in slot y of the chassis with ID x.
ob0!scx!mdy!calgndz
Where z is the appropriate SCXI
channel needing shunting for the
SCXI-1112, SCXI-1125, SCXI-1141,
SCXI-1142, or SCXI-1143.
For example, you can run the Getting Started Analog Input VI, found
in labview\examples\daq\run_me.llb, with the channel string
ob0!sc1!md1!calgnd to read the grounded amplifier of the module in
slot 1 of SCXI chassis 1. The voltage reading should be very close to 0 V.
The AI Start VI grounds the amplifier before starting the acquisition, and
the AI Clear VI removes the grounds from the amplifier after the
acquisition completes.
The SCXI-1112, SCXI-1125, SCXI-1141, SCXI-1142, and SCXI-1143
have separate amplifiers for each channel, so you must specify the channel
number when you ground the amplifier. To specify the channel number,
attach the channel number to the end of the string calgnd. For example,
calgnd2 grounds the amplifier inputs for channel 2 and reads the offset.
© National Instruments Corporation
21-5
LabVIEW Data Acquisition Basics Manual