From: John Popelish
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Subject: Re: Bypass capacitor position
References: <3DB96152.1393B701@rica.net> <3DB9800A.780B223E@rica.net> <3DB9C8F4.5A42BD61@rica.net>
Date: Sat, 26 Oct 2002 03:20:49 GMT
NNTP-Posting-Date: Fri, 25 Oct 2002 23:20:49 EDT
Dave VanHorn wrote:
> John Popelish wrote:
> > But you still describe 3 reactances in series, not in a tee structure.
> > Here is a chip with a bypass capacitor and some trace inductance. How
> > is this a tee filter?
> > +5v-----------+---+
> > | |
> > [c ] I
> > [h ] |
> > [i ] C
> > [p ] |
> > [ ] I
> > | |
> > ground-----+------+
> I don't make any other connection to the chip's VCC lead. It goes to it's
> bypass, and nowhere else. So, you get a tee filter, with the chip as the
> output load, if you will..
Okay, I think I understand, now.
to + 5 end of capacitors for other chips
I [c ]
| [h ]
C [i ]
| [p ]
I [ ]
To ground end of capacitors for other chips
I think there is not much difference ot the difference.
> > You could reduce the noise between different sections if you gridded
> > the ground
> I could let return currents pick their own path instead of controlling them,
> and they might run by cable grounds, so that when I attach a cable, it
> becomes an antenna.
> Bleagh.. When my chopped stepper driver takes current from the main PS cap,
> it shall return it therefore, and directly, to whence it came. Neither
> shall it be allowed to wander hither and yon about the board, leaking off
> the foul vapours of EMI as it searches for a path home, for it's path shall
> be golden, broad, and well defined.
Do you carry this separation all the way back to the power regulator,
or just on the circuit board?
I put a bit of high frequency impedance (ferrite bead on lead) between
the positive supply and the local bypass capacitor for the stepper
drive, and keep the ground gridded. This also forces the local bypass
to carry almost all the high frequency currents, while keeping the
ground as clean and consistent as possible for the circuits
communicating with the drive circuits. I put the highest current
loads closest to the supply connector, so that any drop it causes
getting from supply ot board are common to all circuits.
> >and 5 volt tree structure into a mesh work. And lower the
> > emissions some more.
> I don't think so.
Won't know till we measure two otherwise similar designs. I would
love to have time to have a design contest. :)