• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 47 subscribers
  • Views 1232 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

XTR117: Locally powered device sending 4-20mA process loop with DAC7612U

Tad Errey
Prodigy 40 points
Part Number: XTR117
Other Parts Discussed in Thread: XTR111, XTR300

Hi there,

I'm looking to drive two process loops from a microcontroller, using a DAC7612U to generate the voltage and then feeding a pair of XTR117's to convert to 4-20mA. All power is supplied locally; in fact we actually need to supply 24V loop power from this device.

This is obviously very different to the application circuits of the XTR117; which all take loop power and supply the control circuitry.

Do you have a recommended circuit for this? In the 'XTR117 and Arduino PWM analog interfacing' thread you employ a considerable amount of filtering and isolation on the control side of the XTR117. Is this as necessary if we're supplying the loop power? I use a low-ripple DC-DC converter with primary side L-C filter to supply 5V to the microcontroller and 2ch DAC.

Any help you can offer would be much appreciated.

  • 0
    TI__Mastermind 23050 points
    Hi Tad,

    I am not entirely clear on the distinction you are making in how you are sourcing the XTR117 loop or what you mean by "we actually need to supply 24V loop power from this device." I understand you have a local supply as opposed to a supply out at your receiver, but the same connections must be made to complete the loop.

    Regardless of how the XTR117 is sourced, the power supply and IRET can never be referenced to the same ground, because IRET must be able to float relative to the loop supply. If you had isolation between the loop supply and the supply for the rest of your input circuitry that would be okay. However, there is another issue, which is that a multi-channel DAC cannot drive multiple XTRs without isolation between the DAC outputs and XTR inputs. This is because the DAC channels are referenced to the same ground and driving the XTRs from this effectively shorts the IRET pins together, and the input circuitry of each device must be able to float independently.

    For this application you may actually want to consider using a 3-wire device, such as the XTR111 or XTR300. These devices are intended for use with a local power supply and don't require isolation between the input and supply so you wouldn't have to worry about any of the issues I mentioned above.
  • 0 in reply to Zak Kaye
    Prodigy 40 points

    Hi Zak,

    That is helpful, thanks for pointing me in a better direction. It's been hard to find a good circuit design for what is effectively a current loop simulator.. 90% of what is online is for a remote loop-powered remote sensor.

    The application circuit in figure 3 of the  XTR300 datasheet looks to be simplest, though capacitor Cc doesn't have a value, and how it is powered is unclear to me. Do I simply supply 24V to V+ and GND  to V- & DGND, and it then provides the 24V loop power from the DRV output?

    Thanks again for your assistance!

  • 0 in reply to Tad Errey
    Prodigy 40 points

    Hi Zak,

    After further reading, I realise the XTR300 is dual supply where as the XTR111 is single supply. I'll try the XTR111 and see how I go.

    Many thanks..

  • 0 in reply to Tad Errey
    TI__Mastermind 23050 points
    Hey Tad,

    Hope the XTR111 is working well for you, let me know if you need any assistance!