[SI-LIST] Re: subscribe: the bypass and decouple capacitor

Zhoa, Vijay offers a pretty good description of the situation.  I would 
like to emphasize that both bypass and decoupling capacitors function as 
bypass capacitors.  Each shunts localized high frequency current across its 
terminals.  And as such, each presents a reduced impedance to local high 
frequency noise currents that in turn reduces local high frequency voltage 
noise.

The main difference is that in a decoupling application, series isolation 
impedance is used to form a LPF that isolates the nodes on either side of 
the series impedance from each other, whereas the bypass application 
operates without that isolation.  Although if you evaluate a modern PCB you 
will find bypass networks act more and more like decoupling networks, 
simply due to spreading inductance in the PCB planes.  I have a couple of 
slides on this in my DesignCon 2005 paper.  If for example we are designing 
a compound amplifier, decoupling each stage may prove crucial to stability.

Two decades ago we pretty much only had decoupling networks.  Then with the 
advent of multilayer PCBs, the series isolation elements were removed and 
ended-up with bypass networks.  You can see championing of the bypass 
versus decoupling concept in Dr. Johnson's seminal book "High Speed Digital 
Design a Handbook of Black Magic".  In that book and very appropriately for 
the times, Dr. Johnson strongly advocated global bypass as a way to reduce 
cost and improve performance of the PDS.  When Tr could be measured in 
inches instead of mm's, this made good sense.   Times have again changed 
and with the current ICs and PCB materials, localized decoupling offers 
some compelling advantages.  I urge clients to evaluate these benefits 
carefully.

Steve.


At 10:06 AM 1/27/2005 +0530, Vijay S CHACHRA wrote:
>Hello,
>  There is a definite conceptual difference between Bypassing and =
>Decoupling
>capacitors.
>
>Bypassing Capacitors: These are used when reduction in high frequency
>current flow along high impedance path is required.This reduction is
>achieved by shunting the path with capacitors.The introduction of these
>capacitors reduces noise on the power-supply lines.
>
>Decoupling Capacitors:The decoupling capacitors are used to achieve
>isolation between two circuits on a common line. The most common =
>application
>scenario for decoupling capacitors is when the designer wants to isolate =
>one
>circuit from the noise of an adjoining circuit when both the circuits =
>share
>the circuit trace between them.
>Decoupling capacitors introduce high impedance element in series with
>power-supply and also act as Low-pass filters and hence attenuate the
>high-frequency content of the flowing current.
>
>Hope this helps.
>
>Thanks and Regards,
>Vijay.C
>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
>On
>Behalf Of Zhao Yu
>Sent: Thursday, January 27, 2005 9:06 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] subscribe: the bypass and decouple capacitor
>
>
>=20
>dear sir:=20
>
>     I confront some problem in my design. Are there =3D any differences
>betweenthe two concept bypass capacitor and decouple capac=3D itor? =
>Before i
>concider the two concept as the same one, but recently i rea=3D d a book =
>which
>divides them to different concept. what is the very truth? A=3D re they
>different?
>
>BEST WISHES
>--=3D20=20
>
>___________________________________________________________
>Sign-up f=3D or Ads Free at Mail.com =
>http://www.mail.com/?sr=3D3Dsignup[1]=20
>
>--- Links ---
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