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    Thermocouple output voltage isolation in temparature transmitter (4-20mA)

    Hi...
    I need to provide isolation for thermocouple for temparature transmission (4-20mA). Means i have to provide separate ground for thermocouple.. I selected analog devices ADum1100 digital isolator.
    The only source is +24V which i am giving for XTR116 which outputs 4-20mA.
    Problem is Isolation means i need to separate ground of thermocouple (-ve) from remaining grounds. If i use a digital isolator for isolation where +5V required for its operation is supplied by XTR116, isolation between thermocouple ground and other grounds wont happen? Please suggest me some ideas ....

    If still information is required ask me i will post....


    Thanks in advance...

    •   Alt10th December 2013, 13:24

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    Re: Thermocouple output voltage isolation in temparature transmitter (4-20mA)

    The thermocouple is an analog device (not to be confused with Analog Devices), a DIGITAL isolator will do you absolutely no good. Also, you don't need to ground the thermocouple, you need to actually use a differential input device. There are LOTS of solutions, here's one: AD8494.



    •   Alt10th December 2013, 15:01

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  3. #3
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    Re: Thermocouple output voltage isolation in temparature transmitter (4-20mA)

    Quote Originally Posted by barry View Post
    The thermocouple is an analog device (not to be confused with Analog Devices), a DIGITAL isolator will do you absolutely no good. Also, you don't need to ground the thermocouple, you need to actually use a differential input device. There are LOTS of solutions, here's one: AD8494.

    Dear sir if i am using digital isolator i can interface this between my pic microcontroller (ADC) and DAC..

    Direct thermocouple interface ICs are available but which are costly.....

    Do AD8494 provides ground isolation? I studied its datasheet, they provide input filtering and amplification.. but not ground isolation...

    or
    Do common-mode rejection feature of ad8494 is same as ground isolation? i think not...



    •   Alt11th December 2013, 06:51

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  4. #4
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    Re: Thermocouple output voltage isolation in temparature transmitter (4-20mA)

    I don't understand. What are you isolating from what? Is the thermocouple grounded? What do you mean by "if i am using digital isolator i can interface this between my pic microcontroller (ADC) and DAC"? Your OP said nothing about a PIC. Where is the PIC in your signal chain? Do you want to isolate your PIC from ground?

    Please clarify. A schematic or block diagram would help a lot.



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