AD822's Input Impedance - best way to measure and variation from typical spec?

Q 

In the datasheet the AD822’s input impedance is a typical spec. Do you have any
more information regarding an estimated variation of this impedance? What's the
best way to measure this parameter?

 

A 

We do not measure the input impedance directly. An indirect way of inferring
the common mode input impedance is as follows:
. Input Impedance = Change in applied voltage / change in input current.

So you can estimate the input impedance from looking at Ibias versus VCM curve.
Of course, this simplistic model may not apply in all cases. Sometimes the
typical datasheet spec is also based on detailed knowledge of the process
corners (simulation results).

The input bias current and input resistance are DC specs. A simple model of the
op amp input can be something like attached:

Attachments: