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AD5421 - Optional EMC Filter for 4-20mA Loop

Question asked by sridhar@arete on Sep 25, 2012
Latest reply on Oct 1, 2012 by sridhar@arete

Dear Analog team,


We are a design house based in India and we are currently developing a 4-20mA transmitter using AD5421 part.


I was going through the version E of the AD5421 datasheet and noted that you have indicated an optional EMC filter (on page 31) on REGOUT. We have a couple of questions on the same.


1. Could you please explain a potential use scenario that would require this filter?


2. Would you have any suggestions on the resistance value to be used in the EMC filter? As you may be aware, current budget that we have will be under 3.2mA and if we choose to configure REGOUT to output 3.3V, we need to minimize the voltage drop on REGOUT to enable other parts on the board to function to spec.


On AD5421, we have few other questions and would appreciate your help on the following.


3. We do not plan to use DVDD output from AD5421 DAC. This is just connected to 4.7uF and 0.1uF capacitors as stated in the datasheet. The load regulation for DVDD seems to be very poor and not sure what the intended use for this pin should be? Any inputs on this?


4. We are shorting the IODVDD, REG_SEL1, REG_SEL0 and SDO (since this is tri-state output) pins to REGOUT. REG_SEL2 pin is grounded (intent is to derive a 3.3V supply to power the rest of the system). Do you see any issues with this hook-up during power up?


5. What are the typical ESD considerations that we should be aware of for a loop powered 2-wire transmitter design? The concern that we are having is that most of the ESD current is possibly shunted through the 52 Ohm resistor - inside the DAC to the loop- pin. Is our understanding correct? If so, Can the 52ohm resistor handle the ESD currents? For that matter, any ESD events at the inputs of the transmitter (an example could be ESD event on the sensor inputs) is shunted through the ground which is actually the COM pin of the DAC. How is this handled inside the DAC?


Appreciate a quick response from you on the above questions. Thanks for your support.


With Best Regards,