Well the thing is I'm applying a step input for Vi. 0V to 40.96V using a microcontroller. I'm working with extremely high resistances. Here are the values:
Ri=10Tohm, Rf=10Gohm, Ci=3nF, Cf=47pF. The output of my opamp is taking a long time to settle because of that input capacitance but I don't have a clear picture in my head of what's the reason. The only thing I am capable of changing is the 47pF but not by much.