I've been using the AD5421BREZ for several years. It has always been spot-on 4mA to 20mA over several different board designs with default gain and offset values. Recently, I've gotten some boards back that are now reading 2.1mA to 19.1mA. I was able to increase the offset value to dial in 4mA with 0x0000 DAC, but the offset is maxed out by default and I'm not getting anything over 19.1mA (0xFFFF DAC). The parts appear to be labeled AD5421BREZS1. Boards are the same. I'm not reading any faults. VLOOP is good (24VDC). Load is a 250Ohm precision resistor. How do I make this work?
A little more information. I'm driving it with an ADum5411. In the failing units, the applied VLOOP is good, but the reported VLOOP is very low (1 to 3V). I am getting the undervoltage faults from the failing units. I checked each pin with a voltmeter and compared good to bad - there really is no discernable difference. I did notice that the IO_VDD pin was fluctuating in both units and remembered that I'm disabling the ADum5411 after I set the DAC. I stopped disabling the ADum5411, but the failing unit is still failing.
A little more information. I'm driving it with an ADum5411. In the failing units, the applied VLOOP is good, but the reported VLOOP is very low (1 to 3V). I am getting the undervoltage faults from the failing units. I checked each pin with a voltmeter and compared good to bad - there really is no discernable difference. I did notice that the IO_VDD pin was fluctuating in both units and remembered that I'm disabling the ADum5411 after I set the DAC. I stopped disabling the ADum5411, but the failing unit is still failing.
The AD5421BREZ is the working board. The other three with S1, S2, and S3 suffixes are all giving me problems. They were purchased by our contract manufacturer through Digikey.
