Geoff Wootton wrote:
>
> I'm building a simple printed circuit board that uses a 16C84, couple
> of transisters + diodes and a crystal + 2 caps. I want this circuit to
> be as quiet as possible. 2 questions:
>
> 1) Is it ok to join up both ends of the earth (0v) rail or should the
>    earth rail be open ended.
>
> 2) Should I make the earth rail as wide as possible, filling as much
>    open space on the pcb as possible.
>
> Many thanks in advance for any help on the above.
>
>              Geoff.


If you have enough room to actually create a ground plane, go ahead and
do so.  If you have a ground trace running around hither and thither,
it's best not to make it into a complete loop, since this may increase
magnetic coupling, thus affect EMI susceptibility and emissions.  If you
are depending on a particular capacitor for most of your bypassing
capacity, consider using its ground terminal as the single point
connection for a number of ground traces, and put that cap roughly at
the geometric center of it all.

Wider traces have less inductance and resistance, so they are, of
course, preferred.

Traces carrying equal and opposite polarity currents are best positioned
parallel to one another on opposite sides of the board (or layer), since
doing so minimizes magnetic loop area.  The ground plane is a special
case of this.

However, a complete ground 'ring' around the perimeter of the board is
often desirable for ESD immunity.  It becomes the path of least
resistance for static discharge.  This ring may be connected to
protective ground (the green/yellow wire in the line cord) and/or to
your system ground at a single low impedance point.  In other words, the
ESD guard doesn't carry any circuit currents.

--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists