AD5421 ILOOP CURRENT NOT BELOW 5.1 mA

Question

Dear Sir, we made the circuit attached using the AD5421. 
 The loop power is 24V and I use for testing a 120ohm resistor. 
 Setting of the device is done for 4 to 20mA. 
 SPI is decoupled using the ADUM 3151 with a clock of 78KHz. 
 Everything is working, the precision of the current is excellent but the problem is that it is not possible to go below 5mA. 
 I tried also to power the ADUM5131 using a separated L78L33 leaving unconnected REGOUT with the result the minimum current grown up to 8mA. 
 I kindly ask a support. 
 
 Thanks and best regards. PDF

iandal · Accepted Answer

Hi, 
 MARCOGESTRI said: Everything is working, the precision of the current is excellent but the problem is that it is not possible to go below 5mA. 
 This kind of issue comes when there are other parts in the loop that are consuming current. The common case is if the REGOUT is supplying currents higher than 3.15mA, the loop current can't reach the minimum 4mA. But in your case, even with regout unconnected, the loop current can't be brought down to the minimum and even increased. this might mean that other parts of the loop might be drawing current. 
 Could you check, when regout is connected to ADUM3151, how much current is flowing out of the regout pin? Are there other parts connected between V_LOOP and V420-OUT other than the 24V loopv and rload? 
 Best regards, 
 Ian