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AD8420 4-20mA Receiver grounding

Question asked by Robb-Engineer on Jul 13, 2017
Latest reply on Jul 20, 2017 by Robb-Engineer

I have a simple AD8420 configured as a 4-20mA receiver powered by +12v/GND .  The issue I have is that the output is only valid if I tie the return of the 4-20mA source (AI420_4_N) to the same ground as my board (DGND).  

I'm within the common mode requirements ie with 10mA input, I have 1.10V on "+" input and 0.78V on "-" input reference to my board ground, which is expected 0.43mV input voltage (across the 43 ohm resistor).  without a common ground, the output is ~150mV and slowly decays.  With the common ground I get my expected 1.93V out.

(These numbers are with R202 removed as I thought that could be causing some issues)

The reference design shows the common ground but I thought that was just for that externally powered loop design.  I've done a similar design with the AD8429 and didn't have this problem...

 

thanks

Schematic of 4-20mA receiver w/AD8420

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