Common-Mode Input Capacitance Parameter (Cic or Ci(c))
What is the meaning of the common-mode input capacitance parameter (Cic or Ci(c))?
The common-mode input capacitance parameter, Cic or Ci(c), is defined as the input capacitance a common-mode source would see to ground. It is expressed in units of F.
Cic 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
Input impedance is a design issue when the source impedance is high. The input loads
Input capacitance, Ci, is measured between the input terminals with either input grounded.
ci is usually a few pF. In the figure above, if Vp is grounded, then Ci = Cd || Cn.
Sometimes common-mode input capacitance, Cic is specified. In the figure above, if Vp is
shorted to Vn, then Cic = Cp || Cn. Cic is the input capacitance a common mode source
would see when 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