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Generate 4-20mA signal without loop power with XTR115U

Kenneth Jones
Prodigy 30 points
Other Parts Discussed in Thread: XTR115, XTR111, XTR112, XTR105

Is it possible to simply generate a 4-20mA current signal with XTR115U without wiring it in a 4-20mA loop?  I have a project where the customer would like to design a single PCB that is capable of handling 2 wire sensors(+, mA) and 3 wire sensors (+,-,mA).  Currently, I've have 2 wire mode working on my prototype board.  In 2 wire mode the XTR115U has a separate reference from the 4-20 loop and the references are connected at single point through a 25 ohm resistor (first figure in the datasheet connects Iret to Iout through 25 ohm resistor). When in 3 wire mode, I would like to just feed Iin with a voltage and have the XTR115U generate a 4-20mA signal and output it through Iout.  In my attempt for  3 wire mode I left  Iret unconnected from  the reference and connected  Iout to the reference, but I'm only getting 2.97mA through Iout with a Iin voltage of 400mV.  Am I missing something here in my attempt or is not possible for the XTR115U to genereate 4-20mA without being looped powered?

  • 0
    TI__Mastermind 27715 points

    Kenneth,

    Thank you for your interest in the XTR115, and welcome to the TI E2E forums.

    Because of the way the internals of the XTR115 operate, it is not possible for it to function without being loop powered. The operation you describe in your 3-wire mode requires a 3-wire transmitter such as the XTR111, which accepts a ground-referenced voltage input and converts to a current output based on an external resistor Rset. This configuration is also known as a voltage-to-current converter, since the transmitter is not dependent on a true current "loop" in order to regulate the output current.

    When you attempted the "3-wire mode" configuration, where did you reference the voltage input?

    Best regards,

    Ian Williams
    Linear Applications Engineer
    High Performance Linear

     

  • 0 in reply to Ian Williams
    Prodigy 30 points

    In my attempt, the voltage input reference was left open or "floating".  Now that I think about this, this was a pretty poor attempt.  Is there a part offered that is capable of performing both function of the XTR111 and XTR115?

  • 0 in reply to Kenneth Jones
    TI__Mastermind 27715 points

    Kenneth,

    What's the customer's application? Is the input sensor an RTD or a similar device which has a varying resistance output?

    Regards,

    Ian

  • 0 in reply to Ian Williams
    Prodigy 30 points

    Customer's sensor are either 2 wire loop powered or 3 wire.

  • 0 in reply to Kenneth Jones
    TI__Mastermind 27715 points

    Kenneth,

    If the customer's sensor is either a 2-wire or 3-wire connection, then the XTR105, XTR112, or XTR114 are suitable devices. They are the same device except for the amount of current used for sensor excitation. The currents are as follows:

    • XTR105: 800uA
    • XTR112: 250uA
    • XTR114: 100uA

    This current is used to develop a voltage drop across the sensor which will become the input to the XTR, so the proper part would be selected depending on the resistance range of the customer's sensor.

    All of these devices are 2-wire (loop-powered).

    Regards,

    Ian Williams