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AD5421 always outputs alarm current after board modification

Question asked by aat on Feb 15, 2016
Latest reply on Feb 16, 2016 by harryh

I’m using an AD5421 in a product. I had a prototype PCB made that worked fine, copying the schematic from a previous product. I then had some production units made, where (as far as I can tell from analysing the gerbers) the only difference around the 4-20mA section was shifting a few tracks by a small amount to improve clearances. The AD5421 is not working on the new boards. I have one new board and one prototype board to compare.


As far as I can tell, there have been no other changes in the routing or the parts used to populate this part of the PCB. The only visual difference I can see to the parts is that the AD5421 is marked ‘#1438’ on the prototype board and ‘#1513’ on the new version. I don't know what number the other new version boards have.


The failure mode is that it always outputs the alarm current on the new boards, regardless of the DAC values sent to it. The fault register on SDO is showing no errors, and the Vloop low voltage bits come on as expected if I lower the voltage. The commands must be reaching the IC, because if I don’t send a command the SPI fault bit is set on the fault register, and I can send an 'initiate Vloop ADC measurement' command and see the Vloop voltage in the fault register.


The control circuitry and microcontroller are on a separate PCB, and the same control PCB works correctly when connected to the prototype board, so it can't be a firmware issue.


As I say, I can’t see any substantive changes to the parts or to the layout. From the symptoms shown above, can you recommend anything I can do to help diagnose the problem? For example, are there any particular components that might cause this behaviour? I presume there aren't any known issues with certain batches of the AD5421 that might cause this?