Why don't we use PIN8 of ADA4530-1 as GUARD?

Question

I &rsquo; m reading PAGE35 of ADA4530-1 datasheet about GUARD. 
 &ldquo; In practice, the voltage across RSHUNT1 cannot be 0 V, the guard buffer offset voltage contributes to the difference in voltage potential between the A node and VGRD node. &rdquo; 
 And the voltage difference between A node and VGRD node will introduce current error. 
 My question is: W hy don &rsquo; t we use PIN8 as GUARD since the offset of ADA4530 itself is only 50uV, while the GUARD buffer voltage offset is 100uV which is larger.

DBautista · Accepted Answer

Hi justifice, 
 
 Good day! This was answered offline, but I'll post it here. The Rshunt is just the representation of the parasitic elements that may interact with high impedance sensor, low input bias current (of the ADA4530-1 itself), etc. and this may be the cause of the largest error in the system. 
 To answer your query, If we use pin 8 as the guard, the circuit will look like this: 
 
 Not only it may mess up the transfer function it will also affect the high input resistance of the ADA4530 which is one of the key parameters of the ADA4530-1. 
 Let me know if something is not clear and if you have more questions. 
 
 Best Regards, Dann)

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Why don't we use PIN8 of ADA4530-1 as GUARD?