AD629 Difference Amplifier

Question

The AD629 Difference Amplifier has a 380kohm feedback resistor (lets call it R2) connected between the output and the inverting pin of the amplifier. 
 The AD629 also has a 20kohm resistor (lets call it R4) connected between the non inverting pin of the amplifier and GND. 
 Difference Amplifiers normally have R2=R4. 
 Why is this not the case with the AD629?

maaarmas · Accepted Answer

Hi BlutoBlutarski , 
 I agree to what Achim said wherein the resistors are still match, it is just that the 380k and 20k ohm resistors are in parallel in the -IN. Resistor matching is done for the rejection of the common-mode signals while amplifying the differential input signals. 
 Thank you! 
 Best regards, 
 Mae)

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AD629 Difference Amplifier