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62
Calibrate the Module
Calibrate an Analog Input
Channel
Publication 1746-UM009B-EN-P - September 2007
We provide an example calibration program and a calibration
procedure to show you how to calibrate an analog input channel.
This example assumes an analog output of 4...20 mA from a
transducer. The corresponding decimal code that the module would
write into the processor's input image table would be 409 at 4 mA and
2047 at 20 mA if the overall error of an input channel were zero.
However, the overall error of ± 0.510% at 20 mA equates to ± 21 LSB
of error, or a code range of 2026...2068. In other words, the value that
the module transfers to the data table for a full scale sensor signal of
20 mA could be any value within the range of 2026...2068. Calibration
should reduce the overall error to less than ± 6 LSB, or a code range
of 2041...2053 for the error of the 20 mA signal.
Code Ranges
With this
For an uncalibrated channel,
full-scale
the corresponding output
sensor output
would have this range of error
20 mA > >
> > > 2047 > > > >
Example Calibration Program
Complete these tasks to maintain calibrated inputs for each channel.
• Add a calibration program for each channel to your application
logic.
• Calibrate each channel.
• Enable the Convert Enable rung (rung 2:4) during runtime.
The calibration program requires three external inputs to calibrate
each channel.
• Lo captures the low calibration value (calibration procedure,
step 3).
• Hi captures the high calibration value (calibration procedure,
step 4).
• Cal scales the Hi and Lo values to provide the slope and offset
(step 5).
For a calibrated channel,
the corresponding output
would have this range of error
2068
2026
2053
> > >
2047> > >
2041
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