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LTC2983 Thermocouple temperature offset

Greetings, 

I´m currently building a little measurement system with, among others, a LTC2983 for measuring 20 Type K thermocouples single wired.

I have the issue, that all channels seem to have a offset of about 6 to 7 deegree. I get similar results (within 0.3 degrees) for all channels. 

For the cold junction reference, I´m using a 2N3904 transistor, which seems to work accurate.

The spi communication is working fine, so I guess that the offset is caused by the layout of the pcb...

As shown in the shematics, all channels have 1k resistors in serial (signal and ground) and a 0.01uf capacitors tied to ground. the com pin is connected to overall ground 

of the system. In addition, is used the SP3004-04YTG diode arrays to protect each channel. 

Do you have any idea what could cause my issue? 

Thanks in advance!

Best regards

Florian 

Top Replies

  • Hi Florian,

    Apologies for missing this. LTC2983 has this demonstration software that will help configure, program and run the LTC2983. This software will help you check any configuration errors in your setup.  Please refer to this demo software manual: https://www.analog.com/media/en/evaluation-boards-kits/evaluation-software/2983dsmf.pdf 

    Regards,
    Mae

  • Dear Mae,
    thanks for your reply, but unfortunately it doesnt answer any of my questions. How should this software help me with my issue?
    As I wrote, I think this is a hardware based problem.

    I hav read in the manual, that there could be a shift of the coefficients due to the soldering process of the chip on the pcb. Is this an effect you have already experienced?

    I would be pleased about an answer to my questions.

    Best regards

    Florian

  • Hi  , 
        The most likely issue is that there is about 6-7deg C of temperature gradient or difference between the 2N3904 and the thermocouple connector(s). Since the thermocouple voltage is proportional to the temperature gradient along it's length (from tip to connector), the cold-junction compensation sensor must be isothermal to the thermocouple-board junction. Our demo boards use a copper clip to hold the transistor to the thermocouple jack. 
         Other things that can cause an offset like that would be leakage currents causing a voltage across the filter resistors - 1nA * 1kΩ = 1μV ; ESD diodes tend to have some leakage depending on bias. 
         Reflow soldering could cause the internal reference to shift slightly but that would cause an offset proportional to input voltage, which in the case of thermocouples is small compared to the reference. 

    -Logan