Temperature is the most frequently measured physical parameter. However, the techniques of temperature measurement are grossly misunderstood, often resulting in serious inaccuracies or meaningless data.
Thermocouples are basically two wires made of different metals connected at one end (hot). The voltage difference that appears on the other end (cold) is directly proportional to the temperature difference between the two ends.
For accurate measurement of hot junction temperature, the cold junction temperature must be know, or alternatively the thermocouple could be “fooled” into thinking that the cold junction is at 0 degrees C, by backward it.
Exactly how this is done and how it is implemented on the AD594 and AD 595 TC amplifiers is inside this document. Also available are some PCB layout examples and guidelines on how to get the reference precision from these devices.