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TMP117: Shared CJC sensor for two adjacent TCs?

SmartEnrg
Expert 1155 points
Part Number: TMP117
Other Parts Discussed in Thread: LMT01, TIDA-010019, ADS124S08

Hi,

I would like to learn if I can use one TMP117 as cold junction sensor placed (right in the middle of and on the bottom side of PCB) between two "co-joined" TC connectors?

Such that this one, common TMP117 acts as a CJ sensor for two TCs (connectors) placed right next to each other?

Do you see any issues or recommend any best practice guidelines for such placement?

Please advise. Thanks.

  • 0
    TI__Guru* 91206 points

    Dear SmartEnrg -

    Please see this reference design for schematic, layout, description of use and firmware example ==> https://www.ti.com/tool/TIDA-010019 

    Here, among a few examples, a TMP117 is used in conjunction with an LMT01 to perform CJC.   

  • 0 in reply to Josh Wyatt
    Expert 1155 points

    Thanks Josh,

    I did visit TIDA-010019 and found it very useful overall, but couldn't find answer to my specific question, which is - 
    can I use one TMP117 as a common CJ sensor for two TCs placed right next to each other (TMP117 placed in-
    between the two TC connectors on the bottom side of the PCB)? Do you see any issues in such design? Thanks.

  • 0 in reply to SmartEnrg
    TI__Guru* 91206 points

    Dear SmartEnrg - 

    I have not seen any CJC done with a single sensor, I think for technically correct CJC implementation, you need (at least) two - one "cold" (which is reference) and one "hot" sensor. 

  • 0 in reply to Josh Wyatt
    Expert 1155 points

    Josh,

    There are hot (TC/ ADS124S08) and cold (CJC/ TMP117) in my case too. Sorry for the confusion, let me restate/ reword my question.

    Currently what I have (following "traditional" design) is illustrated below (J1 = Thermocouple connector), U3 is TMP117 (CJC sensor))

    What I am looking to do or questioning if it's doable or not, is as follows (J1=thermocouple#1, J1=thermocouple#2, U3=TMP117) - 
    question being can I use one TMP117 to read cold junction temp as use that as reference that's common to two thermocouples 
    attached to J1 and J2? Of course reading each set one at a time. Will the common U3 placed in between J1, J2 serve as accurate 
    reference for cold junction compensation purpose for both TCs - or TMP117 has to be dedicated to each TC as shown above? 
     

  • +1
    TI__Mastermind 41325 points

    Hi SmartEnrg,

    It should be feasible to use a single reference sensor for two cold junctions. It comes down to system design and system error you're willing to tolerate. I have seen products in the market with only one reference sensor for 8 cold junction terminals. There was a metal plate that helped to thermally tie the reference sensor to the terminals.

    ren