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XTR117 with 0-3.3V from DAC?

shsjsn
Prodigy 50 points
Other Parts Discussed in Thread: XTR117, XTR115, XTR116, XTR111

Can I use the XTR117 to work with 0-3.3V (from uC DAC) to give me 4-20mA?

  • 0
    TI__Guru 62085 points

    Hello Sven,

    Yes, this will work.  Take a look at the reference design:  TIPD190 which features a +5V DAC and the XTR116.  The XTR116 provides a +4.096V reference to the DAC along with the +5V VREG output.  The same design principles can be used with a +3.3V DAC and the XTR117.  Consider using the XTR115 if you need a precision +2.5V reference for your MCU DAC.

    Be sure that the MCU and DAC that you'll be interfacing with the XTR device are fully powered between the VREG and IRET supply of the XTR and are not connected to any other potentials.  This is covered in detail in the following blogs, specifically blogs 2 and 4 but the others provide some intro and an application example.

    2-Wire Blog #2:  Why IRET can't connect to VLoop GND

    2-Wire Blog #3:  Why the IRET current must be less than 4mA

    2-Wire Blog #4:  Controlling 2-wire transmitters with externally powered sources

    2-Wire Blog #5:  Designing input isolated 2-wire sensor transmitters

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Hi Colin,

    Thanks alot for your detailed reply.

    Unfortunately, my MCU can only be configured to choose its internal 1.2V reference or the MCU supply voltage (3.3V). I am guessing, since I dont need to power any external devices with the 5V reg, I can leave the VREG pin unconnected. I guess I will operate the DAC with the 1.2V reference and connect Iret to my MCU ground and calculate values for Rin and Ros to yield the required Iout = 4-20mA. Does that seem fine?

  • 0 in reply to shsjsn
    TI__Guru 62085 points

    Hello Sven,

    What are you using to power the MCU if you're not using the VREG output from the XTR117?  As mentioned in the blog posts I reference above, the system will not function if you plan to power the MCU from a different power-supply/GND than the VREG/IRET outputs of the XTR117. 

    Could you put together a simple block diagram/drawing of the connections you intend to make in the circuit?  This will help us determine if the system will work as desired or not. 

    Our recommendation to power the MCU is to use a linear regulator (LDO) to convert the +5V VREG output to a +3.3V supply for the MCU. 

    We'll be able to tell after you provide your block diagram, but is it possible that a 3-wire transmitter, which has a local power-supply and GND, would be better suited for your design?  Take a look at the XTR111 for a 3-wire transmitter IC.

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Hi, If I connect the MCU ground to Iret, and power the MCU from an external source so not from the Vreg, wouldnt that achieve the same effect? I dont follow why the Vreg pin needs to be connected.

  • 0 in reply to shsjsn
    TI__Guru 62085 points

    Hi Sven,

    In a 2-wire sensor transmitter, the sensor, sensor conditioning circuitry, MCU, etc. must all be fully powered from the +Vloop supply and all of the operational and quiescent current of these circuit elements must return through IRET (2-wire GND).  Therefore, while you don't need to power the MCU from the VREG output of the XTR117, you do need to power it from an LDO that receives input power from the +12V Vloop supply and returns current to IRET. 

    In your drawing the main issue is that the GND for the MCU must be connected to the IRET pin of the XTR117 to properly interface the DAC output with the XTR117 input.  That same GND potential is also connected to the GND of the +3.3V power-supply for the MCU.  If the GND of the MCU power-supply has any potential relationship to the GND of the +12V Vloop supply (think EARTH GND) then the system will not work as explained in Blog #2 listed above. 

     

    These concepts are shown in the images below.  I think the first image shows a high level view of a system similar to yours with a PWM DAC instead of a voltage DAC.  As shown, if the "Sensor (MCU) GND" has any potential relationship to the Vloop GND, then the "Sensor GND" can not be connected to the "2-Wire Transmitter GND" (IRET) or the system will not function.  This is because the 2-wire GND (IRET) must float up and down relative to the VLoop GND as the output current changes the voltage across the Rload resistor.  If you short the 2-wire GND (IRET) to the Vloop GND then it can't float up and down as required (Blog#2).

     

    The solution is to add isolation in your system to isolate the Sensor/MCU GND from the 2-wire GND (IRET).  This is covered in detail in Blogs #4 and #5.

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Hi Collin, 

    I understand now. Thank you very much for your detailed reply. 

    Sven 

  • 0 in reply to shsjsn
    Prodigy 50 points

    Hi, you mentioned the XTR111 which has a local ground as well. Would that be better to use compared to the XTR11 and a digital isolator. Could I effectively replace the XTR117 with the XTR111 in the block diagram. Thanks!

  • 0 in reply to shsjsn
    TI__Guru 62085 points

    Hi Sven,

    It really depends on what type of transmitter you're trying to design and what you're going to connect it to.  Is your application for a sensor transmitter or a 3-wire analog output?  What are you connecting the 4-20mA output to?  If you'll tell us a little more about your system and provide a datasheet of what you're going to connect to the output of your circuit we can take  a look to see what looks more appropriate.