I tried the circuit shown below on breadboard and found that when I connect both the IC's input pin to the node 1, the op amp output is giving a different value. I believe this is due to the imbalanced input resistance on the IC (U3 & U6) pin. The AD736 has a very high input impedance. Is there a way to isolate the loads of the op amp from each other so that the correct voltage will be seen at each input pin of IC.
U3 is improperly connected for a non inverting buffer. It's output is shorted to U2 out and seems pointless.
You can disconnect these outputs from U2out to U3-in, but what you do with U3out is unknown.