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XTR117: Using two xtr117 from a single dac with common ground

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Replies: 7

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Part Number: XTR117

Hi Support Team

I have a customer designing a circuit that is using two xtr117 being driven from a common dac. To make matters worse, the ground on the dac is referenced back to the VLoop ground. All bad, but I think I may have a solution. Can i use a high common mode difference amp powered individually off each XTR to translate from the dac output referenced to ground to the xtr input? Something like the ina149. Will design using a r2ri/o amp. 

Any thoughts?

Regards Jeff Coletti

  • Hi Jeff,

    You can isolate the input of each XTR117 with an ISO7310. Please refer to this blog post for more details. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

    Learn more about XTR products here

  • In reply to Katlynne Jones:

    Hi Katlynne

    That is not what I asked. I have a single micro with an internal dual dac. Each dac channel is driving an XTR117. the VLOOP is also supplied to the unit with the XTR's for power.  I know I need separation between the drive and the XTR. My question is can I use a high common mode voltage amplifier (INA149)between the dac output and the xtr input? This pcb is too small for an iso amplifier. Note the INA149 is powered by the xtr117. Also, I need a resistor on the xtr input.

    Thanks

    Jeff Coletti

  • In reply to Jeff Coletti:

    Hi Jeff,

    Your schematic didn't come through. Can you repost using the Insert Media button. 

    How is the micro being powered? I do not believe what you are proposing will work, as using multiple loop powered XTR devices requires galvanic isolation. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

    Learn more about XTR products here

  • In reply to Katlynne Jones:

    Hi Jeff, 

    I understand what you are trying to do now, and it does look like a viable solution. However, when being supplied with the XTR117's 5V regulator, the common mode of the INA149 becomes +25V and -20V. You would have to make sure that the difference in potential on IRET is always only inside the +25V and -20V range with respect to the microcontroller GND. Also, the INA149 output swing will be limited to 1.5 and 3.5V, when powered with unipolar +5V, and therefore only a DAC voltage in the range of 1.5V to 3.5V will be inside the linear range of the INA149.

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

    Learn more about XTR products here

  • In reply to Katlynne Jones:

    Hi Katlynne

    Agree on the ina149, used that part more of the architecture/concept, will make a high cm amp with a discrete amp, probably opa333/379tlv333. Would be nice if we have a newer version of the ina149 that operated off 3v w rro

    Thanks

    Jeff

  • In reply to Jeff Coletti:

    Hi Jeff,

    Please let us know if you have any other questions as you move forward with this design.

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

    Learn more about XTR products here

  • In reply to Katlynne Jones:

    Hi Jeff,

    Another comment. If the micro is referenced to the Vloop ground, you can use a 3-wire transmitter such as the XTR111. Have you considered this? If you are able to do this then you wouldn't have the issues with the isolation. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

    Learn more about XTR products here

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