The grounds on this chip are very particular and the information for injecting the FSK HART signal are very sparse. The HART subsystem in our meter draws about 30mA so it is impossible to make it loop powered, however FSK must be present at pin C2 in order to modulate the OUT pin. Also, we need to bring FSK in from the loop as well. The problem is that HART ground has to reference from somewhere. COMA & COMD are what I've chosen for now, however there is no way to create a system powered (3 wire) connection to run the 4-20mA and HART board.
What is the proper method for interfacing to HART FSK signals using this DAC? I'm already using the transformer on the SWI pins to isolate the meter, but HART requires more power to process than 3mA.