Spehro Pefhany wrote:

>> If I use the (-) side of the resistor as the ground reference, and it is
>> truly isolated on the transmit side then just hooking the (+) side to
>> the positive input of the opamp (configured as a vf) should work. If
>> its NOT isolated....then what?
> 
> Then you have a problem, or at least a more demanding specification.

I don't think you can generally count on it being isolated. It all depends
on the constraints you can put on the general case. The general case is
that both the sending unit and the unit you are sending to are on their own
potential, which may be way off yours. That's where all the instrumentation
amplifiers with 1+ kV common mode come from, and opto isolators...

So it depends on the constraints you can put on your ground potential/power
supply and the ground potentials of the connected units. But even if you
can count on a common ground potential, I don't think that all current
loops are ground referenced -- that's one of the reasons why they are
current loops, and not constant voltage signals. Normally, you'd use a
differential input good for 24 V common mode.

Gerhard
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