I have design AD5421 based 4-20mA loop powered system. The schematic is attached. I see following problem:
1. The D_VDD always outputs 4.2V but datasheet says 3.3V but can have 5.5V overdrive.
2. If REG_OUT pin floating instead of tying to D_VDD and IOD_VDD and measure voltage at REG_OUT then it measures 2.7V.
The REG_SEL0 and REG_SEL1 is tied to 3.3V and REG_SEL2 is tied to GND.
3. The range resistors are tied to ground to configure output to 4-2mA.
4. The ADuM1447 is disconnected from the system.