AD5421 Powering up



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.


Could anyone guide me what is wrong happening? Did I miss anything?



  • Hi,

    A few questions:

    1. When you took the above measurements what loop voltage did you use and how did you connected it?, can you mark this on the schematic?

    2. What load did you use and how did you connect it?, can you mark this on the schematic

    3. When you measured the voltage on the DVDD pin did you use the COM pin as the ground reference or the ground node of the power supply?

    4. Do you measure the expected alarm current of 3.2mA after power up (but before you programmed a code?)

    4. Can you measure 4mA output current for zero code and 20mA for code 0xFFFF?



  • Hi Michael,

    Thanks for your reply.

    To answer some your question, I did not programmed the Micro. So the data lines to DAC were pulled high.

    There are couple of things I found and I fixed it and now the DAC has correct voltage.

    1. Initially I tied REG_OUT and D_VDD  together.

    2. The reference to DAC had potential almost 1V above ground.



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