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Calibration Tradeoffs
Calibrating an
Analog Input Channel
Calibrating the Module
This appendix helps you calibrate the module's analog input channels
to increase the expected accuracy from
The combination of calibration program and procedure is designed to
reduce offset and gain errors by:
scaling the values read during calibration
applying them during runtime
We present example computations and ladder logic for your reference.
Operating a calibrated module requires the addition of the calibration
program for each calibrated input channel. Scanning the calibration
program increases the program scan time during runtime, slowing the
module's response. If the overall channel error of
o
at 77
F is acceptable to your application, you need not calibrate. If you
require a calibrated input channel, consider recalibrating every time you
change the input sensor and/or the analog module.
To illustrate how to calibrate an analog input channel, we present:
an example calibration program
a calibration procedure
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
to 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 to 2068. Calibration should reduce the overall error to less than
6 LSB, or a code range of 2041 to 2053 for the error of the 20 mA signal.
With this
For an uncalibrated channel,
full-scale
the corresponding output
sensor output:
would have this range of error:
20 mA > >
> > > 2047 > > > >
Chapter
6
21 LSB of error to
21 LSB of error, or a code range of 2026
For a calibrated channel,
the corresponding output
would have this range of error:
2068
> > > 2047
2026
6 LSB.
0.510% of full scale
2053
2041
6–1
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