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2
GFK-1206D
June 1997
Module Operation
The RTD Input Module accepts four inputs from three-wire or four-wire RTDs and converts
the input level to a digital value. Input spans support a variety of RTD types, as listed in the
Module Specifications section.
The analog/digital converter reads the voltage generated by each of the four inputs. The A/D
converter converts the differential measurement into a binary value (15 bits plus sign),
which is read by the block's microprocessor. The microprocessor sets the gain required in the
analog/digital converter for each individual input. The gain is based on the RTD type that
has been configured for that input.
The microprocessor selects the input by means of a solid state, optically-coupled multiplexer.
Three measurements are made for each input: Excitation current, RTD resistance including
field wire drop, and field wire drop. From these three measurements, the module calculates
the RTD resistance. A DC/DC converter circuit isolates the field side from the logic circuits.
Calibration
The module automatically performs A/D calibration at powerup. Automatic calibration is
then repeated every minute to compensate for changes in the ambient temperature.
The module stores the calibration constants for each gain in RAM memory. This data can be
read and changed if necessary.
2.5V
Excitation
Current
Signal
Conditioning
and
Multiplexer
A/D
2.5V
Reference
Analog Input Module
RTD Input Module
5V
+5 V
DC/DC
Regulator
–5 V
FLASH
Logic
Micro–
processor
Opto
6.5V
Serial Bus
46723
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