Manual

Publication 1756-UM009C-EN-P - December 2010 361
Appendix E
Additional Specification Information
This appendix offers additional calibration information that may assist you in
using the ControlLogix analog I/O module.
Analog to Digital (A/D)
Converter Accuracy
There are two types of calibration that occur on a ControlLogix
analog I/O module.
• The user-directed and user-performed calibration process described in
Chapter 11 . This type of calibration occurs only when you determine it is
necessary and involves an external calibration instrument like those
listed on
page 234 .
• A self-calibration process that takes place internally on ControlLogix
analog I/O modules when either of the following events occurs:
– Module power is cycled.
– You begin the user calibration described in
Chapter 11 .
The ‘A/D self-calibration’ feature maintains the accuracy of the A/D
convertor found on all 1756 isolated analog modules. This feature
executes each time the module cycles power or when a self-calibration
cycle is initiated.
The self-calibration compensates for inaccuracies of the on-board
reference signal and the A/D convertor only. In other words, the
self-calibration feature makes sure that the A/D convertor itself is
accurate with respect to its on-board voltage reference that is used for a
conversion of the input signal. Together with user calibration, the
module’s total accuracy is maintained.
Topic Page
Analog to Digital (A/D) Converter Accuracy 361
Calibrated Accuracy 362
Error Calculated Over Hardware Range 363
How Operating Temperature Changes Affect Module Accuracy 363
RTD and Thermocouple Error Calculations 365
Thermocouple Resolution 373