XTR116 4-20 ma Transmitter Measuring Problem
You cannot draw unlimited current from the regulator in the XTR116 and still reach the lower limit of &lt; 4mA. It's just a linear regulator, after all. 

The XTR116 draws 0.3mA itself, so you have perhaps 3-3.5mA to play with. I suspect you are drawing much more than that.