Optional Analog Input Software Calibration
F–3
Calculating the Software Calibration
Use the following equation to perform the software calibration:
Scaled Value = (input value x slope) + offset
Slope = (scaled max. – scaled min.) / (input max. – input min.)
Offset = Scaled min. – (input min. x slope)
Calibration Procedure
1.
Heat up / cool down your MicroLogix 1000 system to the temperature in which
it will normally be operating.
2.
Determine the scaled high and low values you wish to use in your application.
In this example, scaled high value (which corresponds to 20 mA) is 31207 and
scaled low value (which corresponds to 4 mA) is 6241.
3.
Using an analog calibration source connected to the analog input channel or
your system’s input device placed at the 4 mA position, capture the low value
by setting and then resetting the CAL_LO_ENABLE bit. Ensure that your low
value lies within the conversion range of your analog input.
4.
Using an analog calibration source connected to the analog input channel or
your system’s input device placed at the 20 mA position, capture the high value
by setting and then resetting the CAL_HI_ENABLE bit. Ensure that your high
value lies within the conversion range of your analog input.
5.
Set and then reset the CALIBRATE bit. This causes the MicroLogix to
calculate the slope and offset values used to perform the error correction to the
analog input.
The analog channel is now calibrated to
±
5 LSB at the calibration temperature.
The recommended calibration period is once every 6 months. If an application has a
wide range of operating temperatures, a software calibration should be performed
every 3 to 4 months.
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