[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: steve weir <weirsp@xxxxxxxxxx>
- To: a.ingraham@xxxxxxxx, <si-list@xxxxxxxxxxxxx>
- 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 back in 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 used to 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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