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I'm trying to come up with a scheme for multiple loop powered 4-20mA transmitters controlled from seperate analogue voltages, which all have a common 0V reference. My inital thought was to use an instrumentation ampifier to provide isolation, however I'm going down a rabbit hole of datasheets and forums explaining problems with common mode input ranges of IAs, and keeping returns isolated.

My conclusion so far is the following but I've got no doubt I've missed something. Any advice as to if this will work appreciated.

enter image description here

I need the output current range to be 0 - 32mA (which the XTR117 says it can do). While my analogue controls can go up to 10V I'm only actually using it up to 6.4V to keep the scaling through the system as easy as possibl (5mA per Volt). I might change the scaling later, but I'm more interested in this conseptually at the moment to ensure this will actually work.

Will it work? What am I missing? Anything I should be concerned about?

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2 Answers 2

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If you intend to use a grounded sensor attached to 4-20 mA transmitter, I recommend that you use a 3-wire transmitter. This will allow all of your sensors, as well as the 4-20 mA receiver to share a common ground.

The set-up of a 3 wire 4-20 mA system looks like this:

(Image from instrumentationtools.com)

The obvious disadvantage of such a system is the extra wire required.

As before, there are a number of 3-wire 4-20 mA transmitter IC chips available from a variety of manufacturers. If I get a chance, I will add one or more examples.

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  • \$\begingroup\$ The problem is that your current loop ground is not the same as your sensor ground in a typical 2 wire system. I can get you part of the way to a solution by adding a precision current mirror, (op-amp, 2 resistors and 1 transistor) but I have to think some more to see if there is a good overall solution. \$\endgroup\$ Jul 16, 2023 at 19:12
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Generally that won't work because the receivers will have a common ground. However if you have something isolated on the receiving end (say, loop-powered indicators) and individual galvanically isolated loop supplies for each transmitter, then it should work.

Of course better commercial loop-powered or 4-wire transmitters will have galvanic isolation between input and output (and supply for 4-wire), for this and other important reasons. Then it will work. But it is not so easy to toss together a design from catalog ICs because of the required magnetics and the power supply constraints.

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