Re: gEDA-user: basic anti-EMI design q

[sdb@xxxxxxxxxx (Stuart Brorson)]

> > > If you split the plane, you run the risk of running tracks over
> > > slots and other GND structures in your board which can
> > > radiate/receive & can contribute to SI problems.
> > 
> > Actually, I think I can easily avoid this.  The I/O block is near the
> > edge anyway, so the only things that go there are the things that need
> > the isolation.  The 10baset is also not much of a problem, it's near
> > the P/S anyway and the gap would only surround the analog half of the
> > chip and the magnetics.
> 
> The reason to use one plane is that it is easy.  If you try to do a
> split plane on an analog/digital design of reasonable component
> count/complexity, you end up gerrymandering the two GND planes to pick
> up all the different analog & digital nets.  You also have to think
> harder during placement about how to place and orient each component
> to facilitate the two planes [1].  This takes time, and
> there is always the possiblity of an error in which some current takes
> a return path you didn't anticipate, and you end up with noise.
> 
> In any event, you'll need to
> classify each net as either analog or digital, and then make sure that
> the correct GND plane runs underneath it.  If you're worried about EMI
> pick-up, it's particularly important that all nets travel over their
> associated GND planes.  Don't allow any loops since they are the
> structures which are most sensitive to magnetic pick-up.

One other thing:  Even if you have the correct GND plane underneath
each and every track on your board, a split-plane design still has the
question of how the currents flow inside each mixed-signal component.
If I have an A/D or a PIC with some analog and some digital inputs,
how do I know that an analog signal current isn't feeding into a
digital path somewhere in the chip and getting dumped into DGND
instead of AGND, thereby creating the possibility of a big current
loop as the return current tries to find its way back along the DGND
plane to the AGND plane and then back to its source?

Stuart