Rockwell Automation 1746-XXXX SLC 500 4-Channel Analog I/O Modules User Manual - Manual (Page 101)

1 Publication 1746-UM005B-EN-P - June 2004
Appendix
C
Optional Analog Input Software Calibration
This appendix helps you calibrate an analog input channel using
software offsets to increase the expected accuracy of an analog input
circuit. Examples of equations and a ladder diagram are provided for
your reference. A software calibration reduces the offset error and
gain error at a given temperature by scaling the values read at
calibration time.
Calibrating an Analog Input
Channel The following procedure can be adapted to all analog inputs; current
or voltage. For this example the 1746-NI4 with a 4 mA to 20 mA input
is used. Refer to the 1746-NI4 specifications in appendix A. These
specifications represent the worst case values. The overall error for the
NI4, which is guaranteed to be not more than ± 0.365% includes
non-linearity, repeatability, offset error and gain error at a given
temperature.
The overall error of ± 0.365% at 20 mA equates to ± 60 LSB of error, or
a code range of 16324 to 16444. Any value in this range is returned by
an analog input channel at 20 mA. The expected nominal value at 20
mA is 16384. After performing a software calibration the overall error
is reduced to 3 LSB, or a code range of 16381 to 16387.
The graph on the next page shows the linear relationship between the
input value and the resulting scaled value. The values in this graph are
from the example program.