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XTR111: Input Isolation Clarification Question

AJ
Intellectual 570 points
Part Number: XTR111
Other Parts Discussed in Thread: XTR117, TIDA-01536

Hi Support Team,

I have browsed few threads on 4-20mA loop transmitter on e2e forum 

from this post  and this post , my understanding a 3-wire transmitter (XTR111) is used for cases where local supply is available
i.e. the ground of the DAC needs to be tied to the same ground as the supply,

For a 2-wire transmitter (XTR117) requires input isolation, i.e. DAC ground float on IRET and can not be tied to system ground. 


Perhaps, I am missing something but the TIDA-01536 goes against that. I don't see how isolation is relevant here if the XTR111 is powered form local supply?

In our application we need a 4-20mA transmitter, we are providing local 24V supply so I was going with XTR111 but now I'm confused?!

  • 0
    TI__Mastermind 32698 points

    Hello,

    You are correct that the XTR111 is used in applications where a local supply is available. However, there are some cases where an input isolated 3-wire transmitter is required. For example, if the Digital Controller is controlling multiple analog outputs that are not referred to the same ground, or the sensor you're using is referred to a different ground, input isolation is necessary. It looks like this reference design is providing a generic input isolated solution that would work for any of these cases. For you application specifically, if you can confirm that all of your parts can share one system ground without conflict, then you do not need the input isolation. The reference design has a section explaining the isolation: 

  • 0 in reply to Katlynne Jones
    Intellectual 570 points

    Hello Katlynne, 

    Thank you for the prompt response. 

    In our system, we have multiple analog input types and a single 4-20mA output.
    The idea is that any analog input can be mapped to the 4-20mA transmitter, i.e. flow sensor controls valve 


    The system is battery powered, but also accepts external power for charging, i.e. AC/DC adapter and PoE (isolated fly-back)
    Currently, there is no isolation. Both DGND and AGND are tied via 0R ohm. 

    The system provides loop power to field sensor and also powers the transmitter. 
    I would like to confirm if this setup works as shown.

    #1 Do you see potential issues here and if isolation would be required? 
    #2 If we must isolate, is it sufficient to isolate the XTR111 block only or do we need to isolate the loop supply and ADC ground as well?

    Thank you
    AJ

  • 0 in reply to AJ
    TI__Mastermind 32698 points

    Hi AJ,

    It looks like everything is referred to the same ground so you do not need any isolation. You may consider isolation if your connector is a long cable in a noisy environment that may be susceptible to EMI or any overvoltage condition. Or if there is a chance that AGND and DGND could develop a potential due to a transient spike in the system. 

    To your second question, if your application did required isolation, the isolation would go wherever there is a difference in ground potentials. In the reference design, the XTR111 supplied voltage to the DAC and they share a ground, but the difference in ground was between the DAC and the Digital Controller interface. So that is where the digital isolator was placed. 

    Let me know if you have any other questions,

  • 0 in reply to Katlynne Jones
    Intellectual 570 points

    Hi Katlynne

    Thank you for the clarification.

    So if I understand correctly, in the case of isolated XTR111 reference design, the 24v would be provided from an external power supply or a local supply that must be isolated.

    Best,

    AJ

  • 0 in reply to AJ
    TI__Mastermind 32698 points

    Hi AJ,

    I think the simplest way to understand it is that you need isolation if two parts of a circuit cannot share a ground. Any parts using the 24V supply and ground need to be isolated from parts in the circuit that use a different supply and ground in the case that these grounds cannot be connected. Because we don't know what other analog output modules may be connected to the Digital Controller in the reference design (possibly other independent 4-20mA loops that may be referred to a different ground), the digital isolation offers flexibility for the circuit to function correctly regardless of what else the Digital Controller is connected to. 

  • 0 in reply to Katlynne Jones
    Intellectual 570 points

    Katylnne, understood. Thanks for your support!