Help with 4-20mA System XTR116: AD623 Offset Issue at 0mV Input

[kemaltekbas]

Hi all,
I’m new to circuit design and working on a 4-20mA 2-wire loop-powered system to convert a 0-50mV sensor output using the XTR116 (simulated based on the datasheet since I couldn’t find a library). I’m stuck on a couple of issues and need help:

1. My sensor has a common mode voltage (50% of input). I added a voltage source at the AD623 input to simulate this. For 1-50mV inputs, I get 4.32-20mA output, which is great. But the AD623 output has an offset from the REF pin (tied to my simulated IRET). Oddly, the current through my 25kΩ resistors is always correct (e.g., 160µA at 50mV). Why is this happening?

1. At 0mV input, the AD623 output doesn’t go to 0V due to the REF pin offset, so I get 7mA instead of 4mA. How can I fix this to ensure 0mV gives 4mA?

Thanks for any help!

 

[Harald Kapp]

The 0 V (GND) reference in your circuit is "-" of the 24 V source V4 (right side, "earth" symbol"). Any voltage you measure is referenced to this potential. The supply of your opamp has its reference at U1(REF), the common connection of V3 and V5 Since there is a voltage difference between U1(REF) and GND, this voltage difference show as offset when probing e.g. the output of the AD623. If you need to check the output of the AD623 for correctness, you need to probe the voltage differential between pin 6 of the AD63 and U1(REF). Alternatively calculate V=V(pin6)-U1(REF).
Or make your life easy by moving the GND (earth symbol) from the right side to U1(RFE), making this the reference potential for the measuring circuit.