I have the attached datasheet for an OP AMP, in which I have to look for the input resistance of its. The OP-AMP will be used as a voltage follower with a 2.5V voltage reference (zener diode) connected to the non-inverter input. Please advice.

 

I think I really didn't asked the question. Practically the question is, how can I calculate the input resistance of the OP-AMP based on the datasheet and my configuration as voltage follower with a 2.5V zener connected to the non-inverter input?

 

I'll take a stab. The input bias current is ±400nA using a ±15V power supply. Applying Ohm's law, 15V/400nA gives a 37.5MΩ resistance. Perhaps there is more to it, but that's a start.

 

That's pretty much all there is to it. Without a diagram of the inputs you can only say that whatever is attached to the input must be able to give or take 400 na. If the input is 2.5V above ground and 400 na flows, that puts it at about 6.25 meg for this circuit. The only thing that matters is that the zener circuit can afford to give or take 400 na and still be stable. If you wanted to put a resistor in series with the 2.5 volt zener in order to get an inverting voltage gain circuit, this current would enter into the equation. What is the highest resistance I can use to only disturb the input voltage by .005 volts? .005/ 400 na = 12.5K