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XTR116: Multiple converters on one isolated I2C bus.

John Kovach
Prodigy 170 points
Part Number: XTR116
Other Parts Discussed in Thread: XTR111,

I have been using XTR111 IC's on two products we manufacture, one product has one XTR111 the other has two XTR111 both are controlled by a MCU that has analog outputs.  We are having many problems with between the power supply for the MCU and the power used to drive the 4-20mA loops, which comes from a PLC or DCS. Without isolation I am having to pull a ground wire from the 4-20 input module to my 4-20 transmitter.  Because of this I want to isolate the MCU power from the power from the 4-20mA module(s) on the controller.  If I can do this I will expand the number of IC's to 6 per board.

I see three choices, have one isolated I2C bus powered by one of multiple XTR116 IC's,   use multiple I2C isolators with each isolator powered by the Vreg and the DAC powered by the Vref of each XTR116, or bring isolated power from the MCU for the I2C and power each DAC with the Vreg of each XTR116.

I would prefer the first option:

Is the above workable? If not what are the alternatives. 

  • 0
    TI__Expert 5830 points
    Hello John,

    Our resident expert on XTR devices is out of town at the moment. I will look into this for you.

    Best regards,

    Errol Leon
    Texas Instruments
    Precision Amplifier Applications Engineer
  • 0
    TI__Mastermind 23050 points
    Hi John,

    Before I can go into detail I need to clarify a few things and make sure I understand what you want to accomplish. Is your goal just to isolate the MCU from the 4-20mA transmitters to protect the MCU in the event of transmitter failures? Do you also want these channels to be isolated from one another to prevent damage to others in the event of a damaging transient on one channel?

    The most important thing to understand here is that the XTR116 is a 2-wire device, while the XTR111 is 3-wire. These are very different from a design standpoint. The inputs to a 2-wire device must be allowed to float relative to the IRET pin, and this reference cannot be shared with any other 2-wire device. As such, the solution you have shown above will not function properly with an XTR116, though it would be a viable solution using the XTR111 ( you wouldn't even need a second DAC if you were using the XTR111). Powering each DAC with the onboard regulator of the respective XTR116 with isolated I2C lines for each of the DACs would work, you just have to make sure that the total current consumption of each front end doesn't exceed 4mA, or you won't be able to reach a 4mA output current.
  • 0 in reply to Zak Kaye
    Prodigy 170 points

    Zak,

    Thank you for replying.

    We currently use the XTR111.  We sometime have issues with the power for the MCU being on a different ground than the power for the 4-20 loops.

    What if I did not use the VREG on either XTR116 but brought over isolated power from the MCU PS?

    Is the fact that I-RTN touches ground the issue.  Is it supposed to float?    If VREF of each XTR116 is supplying its own DAC131C081 would that not allow each loop to float?

    The other less favorable solution would be giving each loop its own ISO154x.  Would that work?

  • 0 in reply to Zak Kaye
    Prodigy 170 points

    I am not 100% clear when you use the term 2 and 3 wire. Right now when we use the XTR111 we need to bring a ground over from the 4-20mA module that is powering the XTR111 loop. I am trying to get away from this requirement. We do not have the option of powering the 4-20 loop locally.

  • 0 in reply to Zak Kaye
    Prodigy 170 points
    A better question on my part might be, is there a way to use the XTR111 or some other device, such that when power remotely, a third wire for ground is not required? Could you please be specific or supply a diagram. This is a large project for us and right now we are stuck.
  • 0 in reply to John Kovach
    TI__Mastermind 23050 points

    Hi John,

    There are a few different paths you could take. To start, I have attached a presentation that walks through the difference between 2 and 3 wire devices.4-20mA_Basics.ppt

    You pretty much hit the nail on the head though, the big difference is whether you have a local supply or a loop supply. 

    As I mentioned, if you want to use 2 wire devices, you have to make sure the IRETs of each device are not tied together, because they must be allowed to float independent of one another to maintain proper regulation. Even if you are driving each XTR116 with its own DAC supplied by the onboard reference/regulator, you would still have a common connection through the data line, so you would need to have an isolator for the data line of each channel. You may find this article on 2 wire isolation helpful: e2e.ti.com/.../2-wire-4-20ma-sensor-transmitters-background-and-common-issues-part-5

  • 0 in reply to Zak Kaye
    Prodigy 170 points

    Zak,

    Thank you for the Powerpoint presentation.

    It not only my current problem but also what I was seeing with my XTR111.  I made the application with with brute force trial and error engineering but without fully understanding why my readings were off without a common ground.

    Now I am more confident that when my proto board comes back it will work as intended.