Siemens Simatic S7-400 - Manual (Page 271)

Analog Modules
5-55
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Theory of Operation of Internal Compensation
With internal compensation, you can establish the reference point across the
terminals of the analog input modules. In this case, you must run the compensating
lines right up to the analog module. The internal temperature sensor acquires the
temperature of the module and supplies a compensation voltage.
Note that internal compensation is not as accurate as external compensation.
Theory of Operation of External Compensation with Compensating Box
If you employ external compensation, the temperature of the reference junction of
the thermocouples is taken into account via a compensating box, for example.
The compensating box contains a bridge circuit calibrated for a definite reference
junction temperature. The reference junction is formed by the connections for the
ends of the thermocouple’s compensating leads.
If the actual temperature deviates from the compensating temperature, the
temperature-sensitive bridge resistance changes. This results in a positive or
negative compensating voltage, which is added to the thermo-e.m.f.
Abbreviations and Mnemonics Used in the Figures Below
The abbreviations and mnemonics used in the figures below have the following
meanings:
M
+
: Measuring line (positive)
M
–
: Measuring line (negative)
I
C+
: Constant-current lead (positive)
I
C–
: Constant-current lead (negative)
Note
The necessary connecting cables, which result from the potential connection of the
analog input module and the sensors, are not drawn in the figures shown below.
In other words, you must continue to take note of and implement Section 5.8 with
its generally applicable information for connecting sensors.