C-DIAS TEMPERATURE SENSING MODULE
CAI 888
19.04.2012
Page 11
Wiring Guidelines
The signals recorded by the analog modules are very small, as compared to the digital
signals. To ensure error-free operation, a careful wiring method must be followed.
•
The 0 V connection of the excitation voltage must be connected with the 0 V
assembly point over the shortest route possible.
•
The lines connected to the source of the analog signals must be as short as possible
and parallel wiring to digital signal lines must be avoided.
•
The signal lines should be shielded or at least twisted pair wires.
General Information on the Contact Point
Temperature measurement using thermocouples is based on the temperature-dependent
voltage, which is generated through the combination of two conductors from different
metals (alloys); this is called the Seebeck effect.
This voltage exists therefore not only at the measurement point (where it is desired) but
also at the coupling between the Thermocouple conductors and the copper connection.
The thermo voltage at this location is not desired, however, it is unavoidable. This voltage
falsifies the measurement value by that of the temperature at the coupling!
An exact measurement is therefore only possible if the voltage corresponding to
temperature of the cold junction is measured and added the voltage at the measurement
point. All required circuits are integrated in the analog card. The HW class runs the
compensation.
Thermocouples should therefore be connected directly to the connector plug on analog
card (over compensating circuits when possible).
For thermocouple applications where the compensating circuit is not connected to the
control, the comparison measurement must be made at the junction of the thermocouple
(or compensating circuit) and the copper conductor.