component layouts?

[Paul Perry]

An 'unwanted' signal can get into the wrong part of the 
circuit in 3 ways:

1. sharing wiring to the power supply (including earth
wiring). Avoid this by 'star' wiring, that is going directly to
a common PSU point, right where th power comes from.

2. electrostatic coupling. Runs of signals 'too close'.
Solve by rearranging the layout so the offending source
is out of harms way. In emergencies, i've done sheilding
with earthed brass shim. Sometimes using sheilded wire for 
one or more connections is needed. Of course you can do cute 
things with ground planes too, but remember a chopped-up 
ground plane isn't much use anymore.

3. electromagnetic coupling. For this to occur you need a 
'transmitting' loop and a 'receiving' loop.
Imagine the notorious 555 timer.When it switches, it draws AMPS.
So if the power supply driving it forms a loop, this generates 
a magnetic pulse. Which is picked up by any part of the 
circuit that forms a loop. 
The amount of signal picked up by the loop is a function of 
it's area, and how fast the magnetic field changes.
So you can get around this by keeping 'to and fro' currents
close together, and avoiding (wherever possible) risetimes 
that are faster than necessary.

So these 3 will give food for thought.
And from them you can derive other hints, for example it is
easier for a high impedance input to pick up interference.

Those decoupling capacitors are all supplying current to 
adjacent chips so that there is no sudden pulds of current sucked 
along the power rail, which would cause a voltage drop that would
be coupled into other chips.

If you want to tawl the net for hints, search for these terms:
ground plane PCB layout noise.
If you want to get right into it search for 'ground bounce' too.

there was good stuff last time I looked. Use google.

paul perry (not an engineer) Melbourne Australia
(but, I know not to use the auto-route function on my layout prog!)