[sdiy] PCB layout tips

[harrybissell]

Hi Scott...

Think of the supply lines and ground as being hot and cold water pipes...
and the ground as being a sewer pipe (which I had to snake tonight
in the dark and snow but I digress...)

You want to arrange those pipes so that the water draining from your
washing machine... does not back up into your sink.

ie.  You use the direction of the traces to make the current flow where you
want it to... and away from where you don't want it to.

Two power supply traces, run tightly together, or tightly together and near
ground (in Jim Patchell's example) have a lot of capacitance between them
(good ... eats noise) and very little inductance (bad... makes the current arrive

late... like having a too small water pipe...)

Having a separate power supply trace from a heavy load (like a bunch of LEDs)
and a separate trace from a sensitive circuit... like a preamp... assures that
the
heavy current draw will not make a voltage drop in the RESISTANCE of the
ground trace (its not zero ohms... too bad so sad...)... said voltage drop will
be amplified by the sensitive circuit.

These good or bad layout practices are affected by unintentional, often
overlooked
circuit elements called "parasitic" elements.  These small capacitances,
inductances... can make or break your performance.

So your "star" ground should force the currents to go back to the power supply...

with very small area enclosed by the 'loop' from the power and back to ground.

Usually the star ends at the main filter caps.  I keep the ground plane separate
from
the power ground... and let the ground plane carry only noise currents (picked up
from stray "parasitic" elements).

When I do a board... I lay out the power supplies before ANYTHING else. and
use LOTS of decoupling caps. Maybe even more than Jim Patchell  ;^P

H^) harry (expecting a reply from Jim soon....)

Scott Juskiw wrote:

> Regarding the ground plane and the star ground, (BTW this is an all
> analogue layout I'm doing)...
>
> Do I connect up the grounds to the chips (and other components) in a
> star, and _then_ fill up all the empty spaces with a ground plane? Is
> it OK to connect the ground plane to the nearest star ground point as
> long as I don't make any loops? Or is the ground plane one
> continguous sheet of copper that is completely separate from the
> star? Am I over-analyzing this?
>
> Thanks for your help, I don't want to screw up too badly on my first attempt.
>
> At 4:12 AM +0100 2002/12/16, Theo wrote:
> >I often do this, IMO it simply makes for the easiest layout.
> >However having a ground plane under your ICs may help prevent noise
> >problems.
> >Sometimes e.g. with higher resolution DACs having the ground plane under the
> >IC is often mandatory.
> >With double sided layouts my usual way is to have a ground plane under the
> >ICs on the component layer and the power lines on the solder side.
> >
> >As for a do and don't list:
> >- Use a "star" ground layout as much as possible
> >   (in practice more likely a mix between "star" and "not star")
> >- Do not mix digihell and Analogue, especially the grounds.
> >- Keep they traces short.
> >- Use a star ground layout.
> >- Don't run stuff in parallel that could influence each other.
> >    (extreme example; a digital clock parallel to microphone signal)
> >- From about 20Mhz and up keep digital traces that belong together the same
> >lenkt,
> >   even if this means you have to make a silly to and fro pattern with one.
> >- Did I already mention to use a "star" ground layout?
> >- Use ground planes.
> >- Don't make "loops", traces should NEVER come full circle
> >   (ok there _are_ exceptions, but few and are _exceptions_)
> >
> >Theo
> >