We don't specify the input impedence directly, but as it is an input to an
operational Amplifier, its in the Mohm range. More significant is the input
bias current which for the AD8607 ( for example) is in the region of 50pA max
over the temp range from -40 to +85degC .
The input impedance in a real circuit will depend on the circuit configuration
- usually the input is bootstrapped by the negative feedback action of the op
amp and the actual change in input current with input voltage is very small.
Please remember that once parts are in feedback configuration. Rin and Rout
will get multiplied and divided down by the loop gain.