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.
![RC Impedance diagram](/xwiki/bin/download/products/amplifiers-linear/operational-amplifier-ics/introduction/input-impedance/WebHome/rcImpedance.png?width=600&height=180&rev=1.1)
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.
![Common Mode Input](/xwiki/bin/download/products/amplifiers-linear/operational-amplifier-ics/introduction/input-impedance/WebHome/CommonModeInput.png?width=600&height=204&rev=1.1)
Figure 2
Figure 3 shows an example of the MCP601 common and differential modes input impedance specification.
![Input Impedance specifications chart](/xwiki/bin/download/products/amplifiers-linear/operational-amplifier-ics/introduction/input-impedance/WebHome/ImputImpedanceSpecs.png?width=600&height=60&rev=1.1)
Figure 3