Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module User Manual - Manual (Page 84)

Publication 1746-UM022B-EN-P - January 2005
A-4 Module Specifications
Overall Accuracy
The accuracy of the module is determined by many aspects of the
hardware and software functionality of the module. The following
discussion explains what the user can expect in terms of accuracy
based on the thermocouple and millivolt inputs for the 1746-NT8
module.
The accuracies specified as follows include errors due to the cold
junction compensation for thermocouples and hardware and software
errors associated with the system. The hardware and software errors
include calibration of the system and non-linearity of the ADC. For the
sake of the calculations, the resolution of the ADC was assumed to be
at least 16 bits (use of the 10 Hz, 50 Hz, and 60 Hz filter frequencies).
Millivolt
For millivolt inputs, the error is ±30 uV typical at 25°C, and ±120 uV
maximum over temperature for the 10 Hz, 50 Hz, and 60 Hz filter
frequencies. The 250 Hz filter frequency accuracy is highly dependent
upon operating environment and may be worse in noisy
environments.
As with any high precision analog input device, system grounding
does affect the accuracy of the readings. Care should be taken to
ensure that the proper filter frequency has been selected based on the
environmental conditions in which the module is to be used.
CJC compensation does not affect the millivolt inputs in terms of
accuracy.
The following diagrams are provided to give a measure of “system”
accuracy using test data from a single test module. The tests recorded
deviation between measured and expected values. This data was
taken over an entire range of the thermocouple (or millivolt range, as
applicable) and over the module’s temperature range (0 - 60°C). The
maximum deviation for each thermocouple temperature (or millivolt
range) was plotted.
TIP
The 250 Hz frequency should not be applied to
thermocouple inputs (See table on page 3-4).