thank you. i'll make it more clear.
Input bias current is max 250nA. Now, like i said when using as inverter, i'll aplly +10v at - input. to get 250na, i'll need 10v/250na=50 Mohms ? is that true ? or because of the Rf resistor current will be too small after subtracting from input.
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There are two currents to consider. The current that flows through your Rf & Ri (which you can control by selecting resistors)
The second current is the one that the opamp will sink into its input terminals (250nA in your case). You dont need 50Mohms..
For example, consider inverting unity gain opamp.
Ri = 1K
Rf = 1K
If input = 10V, then current flow through Ri will be 10/Ri = 10mA.
If we assume opamp input current is small (like 250nA) then this 10mA
also flows through Rf causing a voltage drop of -(10mA * 1K) = -10V
which is the output you wanted. +10V in makes -10V out. You have an inverter of sorts.
Now seprately, consider that 250nA flowing through the 1K resistor, this will cause a voltage drop of 250uV through the 1K resistor and is a source of error. But this voltage error is so small, you might not care and so in that case, the input bias current didnt matter much to you.
The datasheets are not trying to tell you what resistors to use. Use whatever ones you want and then look at the datasheet to help calculate the effects.
As a completely seperate topic, yes you can use the Lm311 to make an inverter if you want.