[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
- To: weirsp@xxxxxxxxxx, a.ingraham@xxxxxxxx, si-list@xxxxxxxxxxxxx, reanderson@xxxxxxxxxxxxx
- Date: Thu, 26 Aug 2004 02:03:12 +0000
Here are my 2 cents on Bypass vs. Decoupling cap:
The meaning of bypass is "(an alternate) passage". When a capacitor is used
on a signal line (say clock line) to ground it bypasses certain frequency
range to ground instead of carrying it to the other end. It provides a low
impedance path to higher frequencies. The harmonics of the fundamental
frequency are "bypassed".
When a capacitor is used in a Power Delivery Netwok, it bypasses as well as
decoples. The literally meaning of "to decouple" is "to separate". DC point
of view the capacitor in the PDN decouples power from ground. AC point of
view, it bypasses. It means it provide an alternate path to high frequency
signals.
Thanks,
Vishram Pandit
Intel Corporation
>From: steve weir <weirsp@xxxxxxxxxx> >Reply-To: weirsp@xxxxxxxxxx >To:
a.ingraham@xxxxxxxx, <si-list@xxxxxxxxxxxxx> >Subject: [SI-LIST] Re: Bypass
vs Decoupling capacitors >Date: Wed, 25 Aug 2004 09:10:46 -0700 >>Andy,
>>Decoupling, as in anti-coupling, is used to isolate circuits. Back in the
>DTL / RTL days with point to point, or two-sided board wiring, that
>isolation was important, and we had ferrite beads on boards and at the
>power entry to boards, and between the analog and digital power feeds. The
>decoupling network was a LPF, much as Ray described. >>The advent of
low-impedance planes did away with the need for series >isolation in most
digital circuits, so with the series element gone, all >that was left of the
decoupling network was the shunt capacitor on the load >side which looks and
acts like a bypass capacitor, because it is one. But >the decoupling term
got carried forward. >>Since, I am a fan of putting the series impedance
backin decoupling >networks as a way to dramatically reduce cost and improve
EMC performance, >I like to use the terms the way they were 30 - 35 years
ago. >>Regards, >>>Steve. >>At 11:42 AM 8/25/2004 -0400, Andrew Ingraham
wrote: >>>Bypass caps are used to eliminate (short out) resistors during ac
>>>operation. An example would be to bypass an emitter resistor in order
>>>to increase the voltage gain of an amplifier. >>>>>>Coupling caps are
usedto block the direct current, but still allow ac >>>signal to pass. An
example would be to couple multiple stages of an >>>amplifier. >>>>Spoken by
what must be an analog guy! Who else would have remembered >>bypassing
cathode or emitter resistors, and inter-stage coupling capacitors?
>>>>Somewhere along the way, the term "decoupling" (as in decoupling
capacitors) >>seems to have taken on some of the meaning of "coupling", in
that decoupling >>capacitors actually couple some current (usually to ground
in modern digital >>use) ... thereby doing something similar to what Steve
Weir said, which is >>to isloate one device from another, or a load from a
power source. I used >>to wonder why a capacitor, whose function is to
couple signal, would be >>called a decoupling device when it itself does
nothing of the sort. But >>that's what we call it these days. >>>>Regards,
>>Andy
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