[sdiy] Various GND planes in PCB layout..

[harrybissell]

One often overlooked point of ground and power planes... is that source and
return currents will take the same path (the magnetic fields will cause them to
run parallel on opposite sides of the board.

They are NOT evenly distributed across the plane.  The "islands" mentioned
will cause some of that current to be diverted on one side or another... making
the current flow in a larger loop. That will cause noise to be radiated.

A flip side of this is that by cutting the planes into sections, you can force
the
current to take ANOTHER path.  You can force the analog currents to go one
direction, and the digital another direction.

One tip for split planes... do not overlap them.  Keep analog power and ground
opposite each other, and don't extend either one over a digital plane area.

H^) harry

Jay Schwichtenberg wrote:

> Bert,
>
> If done right yes you can use the same analog ground. Typically where you
> have problems is where there isn't enough copper to get enough current to
> where it needs to go. So if you have 'strong/good' path and a 'weak/poor'
> path more current will go though the 'strong/good' path and this may
> unbalance the ground or power in portions of the circuitry. If you have a
> major amount of copper going around the board you will probably be fine.
> Here's another thing to watch out for. When you have something like your
> digital section in the middle of the analog section you got to make sure
> that the path around the digital island has enough copper to support the
> current. An example would be you have point A upstream in a river and point
> B downstream. Then put an island in the river. Depending on the size and
> placement of the island the water is going to flow around it differently.
> What you want to do is place the island so that you get even, smooth flow of
> water all around it.
>
> As the other person mentioned I would be weary of putting the inductor in.
> You have an active component in the ground that can change properties on you
> and thus change the ground plane. The only place I can really remember
> seeing inductors on both sides of power is on AC lines for noise
> suppression.
>
> Hard core PCB layout is some pretty deep dark magic. If you haven't ever
> seen any RF stuff you should see what the RF guys do. There the PCB is
> actually part of the circuit. Pretty much for what we do here just sticking
> to basics as far as PCB and circuit design will carry us a long way.
>
> Jay S.
>
> > -----Original Message-----
> > From: owner-synth-diy at dropmix.xs4all.nl
> > [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Bert
> > Schiettecatte
> > Sent: Thursday, September 01, 2005 12:58 AM
> > To: jays at aracnet.com; synth-diy at dropmix.xs4all.nl
> > Subject: RE: [sdiy] Various GND planes in PCB layout..
> >
> >
> > Hi Jay,
> >
> > > I don't know if I would put an inductor on the ground. What
> > > I've done before when working with audio converters is the following:
> > >
> > > 1) Connect the digital and analog ground at one and only one
> > > point. Make sure the trace is big enough to handle the
> > > current. Put this near where the power comes into the board
> > > and before it is used by the digital section.
> > >
> > > 2) The system that I was working with shared +5V for both the
> > > analog and digital components on the board. The primary was
> > > analog +5V. To get the digital +5V we would use either a
> > > ferrite or a small value resistor (10 to 20 ohms) between the
> > > analog and digital +5V. Then we would bypass the +5 at the
> > > resistor or ferrite with a larger value cap (4.7uf to 22uf)
> > > and each digital chip had a smaller 0.1uf bypass cap. Also
> > > make sure the resistor/ferrite can handle the current.
> > >
> > > Might want to get some of the application notes and/or audio
> > > converter data sheets from Analog Devices, Crystal or AKM and
> > > check out their PCB layout notes.
> >
> > Thanks for the tips. So if there are several different audio
> > Circuits distributed around the PCB and around the digital
> > Section (which is in the middle of the board), do they all have
> > To share the same analog ground copper pour on the top? I also
> > have a Ground plane since this is a 4 layer board, but I'm not
> > sure I  Should split that up into an analog ground section and
> > a digital Ground.
> >
> > Since the analog sections are all around the digital section
> > In a circle, if I all give them the same analog ground copper
> > Pour on the top layer, they will form 75% of a full circle
> > Around the digital section. People told me making loops with
> > Pcb traces is a bad idea so that's why the idea came up to give
> > Each analog section its own ground copper pour on the top layer.
> >
> > Thanks!
> > Bert
> >