XTR111: 2-wire and 3-wire current transmitters in the same circuit (XTR111 and XTR117)

Hi Scott,

2 and 3-wire devices are functionally entirely different so it is not a good idea to try to interchange the two, and there usually isn't a reason to because they are used for different kinds of systems. For example, local supply vs. loop powered supply applications. 2-wire devices require that the input circuitry is floating relative to the loop supply, so you will generally power the DAC and sensor with the onboard regulator, or you must provide isolation on your inputs. This is not necessary for 3 wire devices and the input must be referenced to the supply ground, so you can see how these are incompatible.

You could perhaps get around this by designing a 2-wire system with the input DAC referenced to IRET, but then you could include a jumper that shorts this IRET trace to the supply ground if the XTR111 was to be used instead. You would need to make sure in this case that either the power supply for the DAC is isolated, or it is powered by the XTR111 Vreg when that device is used and the XTR117 Vref when that device is used.

Thanks Zak.  Agree with your points.  our goal is to offer one universal product for both types of customer.  The key point is it is one or the other, never both.  I have seen end products with 3 terminals (Supply, Gnd, Output for 3-wire mode, and Supply,Gnd for 2-wire loop mode), but i'm unsure how they accomplish this.  Jumper/switches are an ok solution for my product.  Currently i have a DAC referenced to both XTR111-gnd and XTR117-Iret.  In 2-wire mode, the customer wires to V+ and Io on the XTR117, while in 3-wire mode they wire to a separate supply/gnd/output.  One thing I discovered that led to 5 terminals was that the XTR117 part seems to sink a lot of current and potential damage if Io is unconnected.  Do you have any suggestions to prevent that?  I thought i read elsewhere that it isn't a good idea to short Iret to Io as you had mentioned. To reach loop powered <4mA, I use a SMPS and both positive supplies feed through diodes to that SMPS to create the 5V digitial supply.  Any issues with this approach?  Any better ideas?

Thanks!

Scott

Scott,

I see now, it was not clear to me that both of these devices will be on the board, even if only one is intended to be used. This is problematic for a few reasons:
1) The grounds of the 2 devices can't be tied together. When used in 3-wire mode the ground of the DAC needs to be tied to the same ground as the supply, but you must be able to disconnect this ground for 2-wire operation and have the DAC ground float on IRET. If you are tying the DAC to ground through a ground plane for example, then this could cause issues.
2) The other problem I see above is that even when the XTR117 is unpowered you are applying an input signal. If the device is left unpowered then this is going to bias the internal ESD diodes on the input amplifier and the device will attempt to power up through these diodes. Since there is no Io connection for current to flow through, you may get currents flowing through parasitic paths which is likely why you saw the increased current consumption.

I think the easiest solution would be to use a DPDT (double pole double throw) switch to switch the DAC output and ground connections depending on which device you intend to use. This way you aren't applying signals to the XTR117 when it is unused.

As long as your power supply scheme you mention above is referenced to IRET when using the XTR117, then I see no issue with this approach. You will also want to switch thesupply's ground reference when you use the XTR111. It may be easier to just power the DAC off of the onboard regulators since only one of them will ever be active and that way you don't have to adjust your ground connections.