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AD7124 cold junction compensation (thermocouple)

Question asked by JulianoCioffi on Aug 4, 2015
Latest reply on Aug 6, 2015 by JulianoCioffi

Hello support, first of all I would like to thank you for all the help I am getting inside the EZ.

 

I have a questioning from the customer:

 

Can the internal temperature sensor of the part be used for cold junction compensation using thermocouple?

 

This is the answer I gave him:

 

The internal temperature sensor has approximately ±0.5°C typical precision. It will consume anyway two analog inputs for momentarily reading the temperature. Since it is placed near the junction and it works in the range of the converter temperature operating parameters it might be done. You still have to consider that it measures the die temperature and it is affected by the thermal resistance of the package which in the worst case is 32.5C/W. Also the soldering process would affect it. I could not find any indication from ADI using this configuration. I am looking for an expertise to answer this question with more property. Based in the presentation and datasheet they recommend using a RTD or a thermistor for optimal precision.


These are the specifications:

 

Maximum temperature of thermocouple slightly over 200°C, say, 250°C.

Temperature range is -40°C to 250°C, to be adjusted according to thermocouple type, etc.

Critical temperature range not specified, probably best accuracy would be expected from 20°C to 70°C. If possible, accuracy around +- 1.5°C would be desired.

Cold junction will be placed inside a VFD, as an add-on board to the control. Temperature on this board should be from -20°C to 70°C.

 

Thanks!

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