I have...what I am told....a 4-20mA current loop, two terminals at least so the assumption is that its a 'constant current source' in the reguards that any load placed on it won't affect the current being driven.  If thats the case, then the current loop itself should be isolated from the driving circuitry, but since I do not have a schematic thats kind of an unknown.  Lets assume it is.
 
So, I use a high precision 248ohm resistor across the two terminals that will generate a 0.992 to 4.96V over the range.  Next...getting it to the A/D of the PIC.  I planned on using a voltage follower, simple circuit using the MCP601 but as I started to sketch out the schematic, I wonder if this is really single ended.
 
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?  run both sides into a voltage follower and then a summing network?  Or run both sides into the opamp, but since one side of the resistor is at zero essentially as its the only load in the loop, that should be zero in theory.  So in either case, voltage follower should be fine with PIC and opamp ground reference attached to the (-) side of the load resistor.
 
 
 The second thought is that I haven't been able to see exactly what the output impedance of the MPC601, and wonder if I need to add a series resistor between it and the PIC.

		
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