Programming instructions

Chapter 20 Special Programming Considerations for SCXI
© National Instruments Corporation 20-3 LabVIEW Data Acquisition Basics Manual
SCXI Gains
SCXI modules provide higher analog input gains than those available on
most DAQ plug-in devices.
Note
Before reading this section, you should have already read the Limit Settings
section in Chapter 3, Basic LabVIEW Data Acquisition Concepts.
Enter the gain jumper settings in the NI-DAQ Configuration utility for each
channel on each module with jumpered gains. LabVIEW stores these gain
settings and uses them to scale the input data. When you use the
input
limits
control of the analog input VIs, LabVIEW chooses onboard gains
that complement the jumpered SCXI gains to achieve the given input limits
as closely as possible.
For analog input modules with programmable gains, LabVIEW uses the
gain setting you enter in the NI-DAQ Configuration utility for each module
as the
default gain
for that module. LabVIEW uses the default gain for the
module whenever you leave the
input limits
terminal to the analog input
VIs unwired, or if you enter
0
for your upper and lower input limits.
You can experiment with the default gain setting by using the original
Getting Started Analog Input VI found in
labview\examples\daq\
run_me.llb
. This VI does not use input limits. After you execute the VI,
you can open the NI-DAQ Configuration utility while LabVIEW is open
and change the default gain setting there. Be sure to save your changes by
choosing
File»Save
(for the NI-DAQ 4.8.
x
, save changes by closing the
utility) before switching back to LabVIEW to run the VI again. Remember
that the larger the gain setting, the more precise your measurements will be
as long as the signal is within the resulting range of the channel.
When you use the
input limits
to specify non-zero limits for a module with
programmable gains, LabVIEW chooses the most appropriate SCXI gain
for the given limits. LabVIEW selects the highest SCXI gain possible for
the given limits, and then selects additional DAQ device gain if necessary.
If your module has programmable gains and only one gain for all channels
and you are using an MIO/AI DAQ device, you can specify different input
limits for channels on the same module by splitting up your channel range
over multiple elements of the channel array, and using a different set
of input limits for each element. LabVIEW selects one module gain
suitable for all of the input limits for that module, then chooses different
MIO/AI gains to achieve the different input limits. The last three examples