Rockwell Automation 1746-NR4 SLC RTD/Resistance Input Module User Manual - Manual (Page 47)

Publication 1746-UM008B-EN-P - December 2006
Install and Wire the Module 47
Calibration The accuracy of a system that uses the RTD module is determined by:
• the accuracy of the RTD.
• resistance mismatch of the cable wires that connect the RTD to
the module.
• the accuracy of the RTD module.
For optimal performance at the customer site, the RTD module is
calibrated at the factory prior to shipment. In addition, a
self-calibration feature, called autocalibration, further ensures that the
module performs to specification over the life of the product.
Factory Calibration
The four-pin calibration connector, on the RTD module circuit board,
is used for factory setup only.
Auto-calibration
When a channel becomes enabled, the module configures the channel
and performs an auto-calibration on the channel. The channel is
selected, the excitation current is turned off, and the three input lines
for the channel are connected to analog common. The module’s A/D
converters are configured for the proper gain and filter frequency that
is appropriate for your RTD configuration. Auto-calibration performs
an A/D conversion on the zero voltage (analog common) and the
full-scale voltage (A/D reference voltage) on the following signals:
• Lead wire signal
• RTD/resistance signal
• Excitation current signal
These conversions generate offset (zero reference) and full scale (span
reference) coefficients that are saved and used by the module to
perform future A/D conversions on this channel.
IMPORTANT
Channel calibration time is shown in the Channel Calibration
Time table.