[SI-LIST] Re: Indcutance role in EMI

Hi, U.K.U:
        Firstly, we should know that ferrite bead is intentionally designed
to have low impedance at low frequency and high impedance at high frequency
range. While the parasitic equivalent inductance in power/ground rails are
unwanted and inevitably, so designer always try to find strategy making them
as small as possible or compensate them. So it is ambiguous to say "the
inductor is behaving differently for the high frequency noise in the two
cases"
Based on the above characters, ferrite bead can be used in signal
transmission (almost energy approximately BELOW knee freuqncy can propagate
from source to destination due to the property --- low impedance at low
frequency, high frequency noise is suppressed due to high impedance,
several-k OHMs, in this frequency range). In some cases, such as connection
between analogy ground and digital ground, ferrite bead is utilized.
Designers always need and utilize this character.

As for inductance inducing voltage noise (ground bounce ::: v=L*di/dt) on
power/ground rails, it is always unwanted. Designer always want to remove
this character.
In normal case, this inductance is small, about several nH or so that
depends.

Long Yang
Dan Quayle  - "For NASA, space is still a high priority."

On Wed, Jul 30, 2008 at 7:17 PM, Umamaheswar U-TLS,Chennai <
umamaheswaruk@xxxxxx> wrote:

> Hi all,
>
> I have doubt regarding inductor functionality in power supplies and
> reducing EMI.
> 1.      We use common mode choke or ferrite bead to reduce the high
> frequency noise in the circuits
> 2.      But the when inductance present in the ground or at the IC pins
> ( lead inductance) it will cause ground bounce or induced voltage due to
> the flow of noise current.
> In the first case we can assume that inductor will act as open circuit
> or very high impendance for the high frequecy noise , so it is nopt
> allowing the high frequency noise.
> In the second case the noise current will flow through the inductance,
> due to the change in the current, V=di/dt  voltage get induced, this
> will result in ground bounce.
> My questions are: is my view correct in the both cases or not. If it is
> correct, why the inductor is behaving differently for the high frequency
> noise in the two cases.
>
> Thanks& Regards
> U.K.UMAMAHESWAR
>


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