Cold Junction Compensation

This is a temperature measurement circuit using thermocouple. This is a low-frequencyapplication. It will allow the OPA335 to be switched directly into the signal path. Thiscircuit has low noise, high accuracy, and low drift. Here is the circuit :

Temperature Measurement via Thermocouple circuit schematic diagram

The 4.096-V bridge supply is provided by a precision voltage reference. A negativetemperature coefficient of diode D1′s forward voltage is -2 mV/ºC. It also provides cold junction compensation via the resistor network R1 to R3. R6 is used to obtain the zero adjustment for a defined minimum temperature. The gain for the output amplifier is set by R8 and R7.

This circuit has 16-bit or better accuracy at high gain in low-voltage applications because the single-supply amplifier provide an open loop gain of 130dB. This circuit has an auto zeroing, that is used to provide typical values of 1 µV of input offset and 20 nV/ºC of offset drift over temperature and remove 1/f noise.

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Laws of Thermocouple

The Seebeck Effect A German physician turned physicist named Thomas Johann Seebeck took two different metals, with one at a higher temperature than the other, and made a series circuit by joining them together to form a junction. He found that by doing so he was able to generate an electromotive force (emf). Emfs are voltages. Seebeck found that the larger the temperature differences between the metals, the higher the generated voltage, regardless of their shapes. His discovery is called the Seebeck effect, and it is the basis of all thermocouples. Background Seebeck, H.G. Magnus, and A.C. Becquerel proposed the empirical rules of thermoelectric circuits. Lord Kelvin explained their thermodynamic basis, and W.F. Roesser compiled them into a set of three fundamental laws. They have all been verified experimentally. The second law is sometimes broken up into three parts by modern day researchers, to give a total number of five, but Roesser's are still the standar

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