Essentially, it is the change in voltage at the input divided by the change in
input bias current.
Delta Vcm/ Delta Ib = Input Impedance.
See Figure 9: Ibias vs. CMV in the AD8221 data sheet.
We see we go from 0V to 10V and there is a 20pA change. That is 500GOhms. We
specify 100GOhms because it is a nice number. It will never be 100GOhms or
less. The input impedance will be larger than 100GOhms, but it is an easy
number to remember so we just put that down on the data sheet.