4-20 mA PID controller PCB Layout
Two things that you should consider fixing:

your loop should not be interrupted by LM780x as you want the current to be driven by T1 only. So LM780x input should be in parallel of T1 and then feeding the rest of the board.
if you want the current output to be compensated for variations of your uC is to have the GND node connected to iRet as the datasheet mentions and not


IRET: All return current from IREG and IREF
 XTR115 datasheet - FIGURE 1.  

Otherwise A1 in the block diagram cannot compensate for variations across R2 which will be coming from U3 and DAC1 in your schematics.