Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 500)

F Appendix
F–1
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.
Software calibration reduces the error at a given temperature by scaling the
values read at calibration time.
The following procedure can be adapted to all analog inputs; current or
voltage. For this example, the 1761-L20BWA-5A with a 4 mA to 20 mA
input is used. The overall error for the MicroLogix 1000 is guaranteed to be
not more than
±
0.525 at 25
_C.
The overall error of
±
0.525% at 20 mA equates to
±
164 LSB of error, or a
code range of 31043 to 31371. Any value in this range is returned by an
analog input channel at 20 mA. The expected nominal value at 20 mA is
31207. After performing a software calibration, the overall error is reduced
to 5 LSB (0.018%), or a code range of 31202 to 31212.
The graph shown below shows the linear relationship between the input
value and the resulting scaled value. The values in this graph are from the
example program.
Figure A.1
Scaled
Value
Input Value
20 mA = 31207
(scale Hi)
4 mA = 6241
(scale low)
Low Value from card Hi Value from card
6292 31352
Scaled Value vs. Input Value
Calibrating an Analog Input
Channel