Input Capacitance Parameter (C_i)

Problem:
What is the meaning of the input capacitance parameter (C_i)?

Solution:
The input capacitance parameter, C_i, is defined as the capacitance between the input terminals of an op amp with either input grounded. It is expressed in units of F.

C_i is one of a group of parasitic elements affecting input impedance. The following figure shows a model of the resistance and capacitance between each input terminal and ground and between the two terminals. There is also parasitic inductance, but the effects are negligible at low frequency. Input impedance is a design issue when the source impedance is high. The input loads the source.

Input capacitance, C_i, is measured between the input terminals with either input grounded. c_i is usually a few pF. In the figure above, if V_p is grounded, then C_i = C_d || C_n.

Sometimes common-mode input capacitance, C_ic is specified. In the figure above, if V_p is shorted to V_n, then C_ic = C_p || C_n. C_ic is the input capacitance a common mode source would see referenced to ground.

Also refer to the following related links:
Op Amp Parameter Table
Op Amp Parameter Information

These pages were extracted from Op Amps For Everyone, Chapter 11, by Bruce Carter, Ron Mancini–Editor in Chief.
Copyright © 2000, Texas Instruments Incorporated