Steven;

I am going to go out on a limb and say that ground planes are overrated.  They
are good for three things;  overcoming electrostatic field interference, making
high density layouts possible, and compensating for an inability to route a
decent ground net.

Just a hypothetical: let's say you had four circuit pieces on a single board.
Further say that three are analog, and one is high current switching.  Further
say that none of them can withstand interference from the other.

The PSU is a split supply coming in at one corner of the board.  Route each sub
circuit in a clean manner, but return ground and power for each section
individually to the power entry point.  Do this for all four sections, and add
generous capacitor decoupling at the sub circuits.  Add both mono and
electrolytic caps at the power source.

Take this example one step further and split the grounds from each component if
necessary, maybe in a tree fashion.

Two case examples of where this helped.  I had one project where the display
board current passed through the same ground trace as the thermocouple
amplifier.  Someone noticed that when one LED went on and off (one of at least
12 segments on at any time) that the temperature went up and down one degree.
Sleuthing uncovered this current effect.  This is not to be confused with a
ground loop.  This is just a shared trace.  Cut the amplifier trace and routied
to the PSU, and problem dissappeared.

Another project had a 12 bit A/D converter, and clean values were critical.  I
was getting 3 bits of noise, and filtering the crap out of it to get semi-stable
values (sound familiar, anyone?).  The customer had some very seasoned analog
hands at their place, and they borrowed the unit. When it came back, it had a
nest of wires soldered all over the boards, but the converter gave me rock
steady readings to one bit without a filter.  (Using a good millivolt source).
Very few times in my endevors had I seen a 12 bit value sit on a bit.  And they
didn't use a ground plane, they just re-wired the supply and reference networks.

And on ground planes, don't forget that in the near field, an electromagnetic
field can be predominantly electrostatic or magnetic in nature.  A ground plane
is good for stopping an electrostatic field, but not a magnetic field.  A
switching supply or a motor ar great examples of predominantly magnetic fields
in the near field.  They will not be stopped by a ground plane.  Then the signal
gets picked up by loops.  The more circular in shape a loop is, the better the
pickup.  The more parallel and close the lines are, the less field you pick up.
Once again, it comes down to layout.


> Just a quick question about ground planes.  If you are using a run of the
> mill "Perf"board, they do not generally have "true" ground planes.  I have
> never really given it much thought, but how would you recommend duplicating
> a ground plane?  I typically tend to leave my leads extremely short and my
> prototyping area pretty compact to get rid of transient noise.