cant understand this OPAMP configuration
Answer to the first circuit diagram:

The shown circuit is a unity-gain amplifier. 


What is the advantage of this circuit - if compared with the commonly known simple unity gainamp (opamp with 100% feedback)?
Answer: Bandwidth and stability margin !


In this configuration, you are allowed to use a broadband opamp which must not be compensated for unity-gain stability. Hence, it has a larger bandwidth. Stability is not a problem because there is not 100% feedback; the feedback factor is k=R43/(R43+R26) and can be made so small that the loop gain is in the stable region (The influence of R90 can be neglected due to its large value).

The trick of the circuit is that there is no direct dependency between closed-loop gain and loop gain (which determines stability proprties, like phase margin etc.). 

Analysis (Calculation): 

Vin=Vp; Vn=Vp (opamp ideal);

Therefore:  Vn=Vin >>> no current through R43 and no current through R26.

Result: Vout=Vn=Vin >>> Vout/Vin=1