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ADS1118: temperature will not be stable when there is touch between the K type thermocouple and case

Howard Zou
Genius 12365 points
Part Number: ADS1118

Hi,

my customer build a board with ADS1118 and thermocouple for temperature sensing.

The board is placed in a case and this case is connected to earth. 

When the K type thermocouple has no touch with the case, it can work well and the temperature reading is correct.

When the thermocouple has touch with the case, the temperature for both the cold junction and the thermocouple will not be stable, variation between 30-50C.

They have tried to connect the thermocouple to other metal it will not have the same problem.

  • 0
    TI__Guru* 92655 points

    Howard,


    If the customer is using an exposed thermocouple, the thermocouple itself is a part of the circuit and can therefore conduct electricity into the rest of the circuit.

    I haven't seen the schematic for your customer, so I don't know how they have their circuit set up, and how they have the thermocouple connected to the rest of the circuit. However, I would guess (and as you and your customer probably guessed too) that the connection from the case's earth ground through the thermocouple through the ADC to the ground of the ADC power is conducting some current that is disrupting the measurement.

    Can the customer use an ungrounded (or shielded) thermocouple for possible solution?

    One other thing they might try would be to connect the ground of the ADC power to the earth ground. This might prevent the ground loop from disrupting the measurement.

    Can they provide a schematic?


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points

    Howard,

    I haven't heard from you for a while so I thought I'd check to see if your customer had responded. 

    Much of the problem I think is from the fact that they have a grounded thermocouple, where the thermocouple itself can make electrical contact to earth ground and disrupt the measurement. In this case there may be a ground loop where current flows from the circuit ground to earth ground. 

    Hopefully, your customer now understands the problem and has a solution. I'll close this post for now, but if they continue to have problems, post back and we can continue to work on this thread.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Joseph,

    sorry for the late response.

    We have identified the cause is that the switching power supply ground is connected to earth ground directly, Which means that GND of ADS1118 is in fact directly connected to earth ground which causes the problem, thanks.