Input Impedance

Last modified by Microchip on 2023/11/09 08:59

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

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

Figure 2

Figure 3 shows an example of the MCP601 common and differential modes input impedance specification.

Input Impedance specifications chart

Figure 3