LMV851 Datasheet, PDF (18/22 Pages) National Semiconductor (TI) – 8 MHz Low Power CMOS, EMI Hardened Operational Amplifiers

Unwanted Thermocouple Effect
At the point where the thermocouple wires are connected to
the circuit, usually copper wires or traces, an unwanted ther-
mocouple effect will occur.
At this connection, this could be the connector on a PCB, the
thermocouple wiring forms a second thermocouple with the
connector. This second thermocouple disturbs the measure-
ments from the intended thermocouple.
Using an isothermal block as a reference enables correction
for this unwanted thermocouple effect. An isothermal block is
a good heat conductor. This means that the two thermocouple
connections both have the same temperature. The tempera-
ture of the isothermal block can be measured, and thereby
the temperature of the thermocouple connections. This is
usually called the cold junction reference temperature.
In the example, an LM35 is used to measure this temperature.
This semiconductor temperature sensor can accurately mea-
sure temperatures from −55°C to 150°C.
The two channel ADC in this example also converts the signal
from the LM35 to a digital signal. Now the microprocessor can
compensate the amplified thermocouple signal, for the un-
wanted thermocouple effect.
FIGURE 9. Thermocouple Read Out Circuit
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