Input Impedance
The concept of impedance generalizes Ohm’s law to AC circuit analysis. Unlike resistance, the impedance of an electric circuit is a function of frequency.
Impedance (Z) at different frequencies:
|ZC| = 1 / (ωC) = 1 / (2πfC)
where ω=2πf.
Figure 1 shows the impedance of an RC network changes over frequencies.
Figure 1
In the op-amp datasheet, there are two kinds of op-amp input impedance. One is common mode input impedance, and the other one is differential input impedance. Figure 2 shows the common mode input impedance model. For differential input impedance, it is the ratio of the differential input voltage change to the input bias current change.
Figure 2
Figure 3 shows an example of the MCP601 common and differential modes input impedance specification.
Figure 3