How to design a high impedance buffer circuit?
Something like this will probably work for you: 

For R1 = 49.900K the ideal value of R2 would be 20.43904K  and R3 would be 89.331K approximately. 

The 4.096V source could be a 0.1% tolerance series reference that can sink or source current such as ADR3440. You can play with the resistor values to make them from standard values for R2/R3 as you like so long as the ratio of R1:R2:R3 is kept the same and the values are not too high or too low. 

There should be a bypass capacitor on the op-amp supply and input or output protection may be required in a real circuit. 

The op-amp has to be a rail-to-rail input and output type for +5V only supply. 







simulate this circuit &ndash; Schematic created using CircuitLab

Note that if you leave the input open you will get an output voltage of 3.333V.