(5.1)
where D
2
is the diameter of the antipad, D
1
the diameter of the via pad, T the thickness of
the PCB, and
r
the relative dielectric constant. The typical total capacitance of a through-
hole via is approximately 0.3 pF. It should be noted that the via model depicted in
Figure 5.1
assumes that half of this total capacitance is attributed to each pad.
The inductance of the vias is usually more important than the capacitance to digital
designers. The vias will add a small amount of series inductance to the system, which will
degrade the signal integrity and decrease the effect of decoupling capacitors. The via
inductance is modeled by a series inductor as shown in
Figure 5.1
and can be approximated
by [
Johnson and Graham, 1993
]
(5.2)
where h is the via length and d is the barrel diameter.
Summary :
(5.1) where D 2 is the diameter of the antipad, D 1 the diameter of the via pad, T the thickness of the PCB, and r the relative dielectric constant.
Tags :
diameter,capacitance,inductance,ias,pad,series,figure,total,typical,decrease,noted,dielectric,decoupling