Re: Re: Bypass thoughts

Peter Hallenbeck wrote:
>   I think we're in the same ballpark here.  Next time I have a small test
> board for some chip, I'll play with the "two of the same size in parallel"
> vs. "one cap, and another at 1/100 it's value" approach.  I'll also take a
> board with a HCS08 on it (which has only 3 bypass caps) and change out the 1
> nF to a 100nF so all three are the same and make some measurements on it.
>   I know the "spin frequencies" for caps (the self resonant point) has got
> much better (higher) with time, and there's less lot to lot variations.  My
> production house does much better with 0805 than 0603, and so it's possible
> that when they get more accustom to 0603 that the smaller size will change
> things too.  I started this practice back in the 0612 .1 uF days, and am
> probably guilty of not re-thinking / re-verifying the approach.  So I
> appreciate your email.  Bypass design is part of every PCB design, and it's
> easy to forget that and get stuck in a rut.
>   The key here is knowing about the impedance 'V' graph and the self
> resonant point (if working with frequencies above 500 MHz).  That and some
> knowledge about PCB layout and you're good to go.  I might also be overly
> concerned about emissions because the product I'm working on has 120
> unbalanced, unshielded, single ended wire coming out of it that are as
> little as 1 foot and as long as 180 feet long.  It's a consumer produce, so
> it must pass (and did pass) the tougher emissions requirements.  The
> daughter board the 5282 is on has more $$s in the EMI suppression than in
> the micro, and it's a multi-layer PCB.
> 
>   All this said, tacking on some more caps is a small time hit that can
> eliminate a possible design problem on a new PCB layout.  If it makes a
> difference, then all these issues being discussed here can be worked
> through.
> 

There was one other common myth about bypassing that I forgot to mention 
- placement.  People often think it's important to get the bypass caps 
as close as possible to the chip in question, and aim to put them on the 
back side of the chip if possible.  There are three things to consider 
about placement - electrical distance to the chip, impedance in the path 
from the cap to the chip, and impedance from the supply (or bulk cap) to 
the bypass cap.  The electrical distance should be under a quarter 
wavelength between the cap and the chip at the frequencies you are 
interested in.  For up to around 200 MHz (beyond which you need to take 
more care anyway), that's something like 3 or 4 inches, depending on the 
type of board.  In other words, there's no need to squash things up. 
For the impedance, you want the least possible impedance between the cap 
and the chip pin.  Aim for a mini power plane under the chip, extending 
out as far as the bypass caps - then the exact placement doesn't matter 
too much.  If you have no power plane available, and must use tracks, 
then you need the shortest possible track.  Finally, you need a good, 
low impedance (at lower frequencies) and low DC resistance path from the 
supply or bulk caps to the bypass caps - to "refill" the cap, if you like.
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