Programming instructions

Chapter 21 Common SCXI Applications
© National Instruments Corporation 21-5 LabVIEW Data Acquisition Basics Manual
Amplifier Offset
The SCXI-1100, SCXI-1122, and SCXI-1141 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 or SCXI-1122, use the
standard SCXI string syntax in the
channels
array with
calgnd
substituted
for the channel number. Refer to the following table for an example of this.
For example, you can run the Getting Started Analog Input VI, found
in the
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-1141 has a separate amplifier 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. You can also specify a range of channels. The string
calgnd0:7
grounds the amplifier inputs for channels 0 through 7 and reads the offset
for each amplifier.
Use the Scaling Constant Tuner VI from
Functions»Data Acquisition»
Signal Conditioning
to modify the scaling constants so that LabVIEW
automatically compensates for the amplifier offset when scaling
binary data to voltage. The SCXI-1100 Voltage example, found in
labview\examples\daq\scxi\scxi_ai.llb
, shows you a way to use
the Scaling Constant Tuner VI.
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
.