I'd connect them close to where the power comes in, using a star ground
approach. Sometimes other places are better, like the point where the
signals using the common ground cross, for example right under a micro if
you use the ADC.
Basically you don't want any noise introduced in the ground mess up the
other section, so they should only meet once.
You can use vias for a very small inductance (like down, short trace, up),
or ferrites for a larger one.
But think what an inductor does - it will allow your digital ground to
"noise about" even more since it is a higher impedance for high
frequencies. You might not always want that. Maybe it is better to tie
your digital ground solidly to your analog ground, at a point where
current in the digital ground can not change the potential of analog
ground.
It's hard to give a general statement, look at your circuit and imagine
the digital section putting noise on gnd, and connect accordingly.
Routing signals through a ground plane, well, you must take care that you
don't break the plane up where it matters or you no longer have a ground
plane. i like to route ground with sufficiently thick traces anyway, no
matter if i make a plane or not, to ensure i have at least the ground
paths i want. Watch out for capacitive coupling from ground to the
signals, especially if you have to get analog signals across digital
ground. Sometimes it is required to clear out the areas around and under
sensitive signals if you really can't have that capacitance to ground, or
maybe even guard them with some analog ground if noise is critical.
ST
On Mon, 27 Feb 2006 09:27:20 +0100, J <wyninejr@...> wrote:
> Hello,
>
>
> I'm designing a VERY small board (less than 1 square inch) and there
>
> is an analog part and a digital part. The board is going to be two
>
> sided and I plan to use surface mount technology and mount everything
>
> on the top side with a ground plane on the top side and route all
>
> signal lines on the botton side. I'm going to seperate the analog and
>
> Digital ground planes but my basic question is how? Should there be a
>
> common area in which you must go through a 180 degree bend to get to
>
> it from each side. Will that block out most interference signals. Or
>
> should they be seperated completly except through ferrite beads?
>
> Also, is it a good idea to route my signal lines through the common
>
> ground plane that "S"'s it's way through to the other ground plane.
>
> Any help is appreciated.
>
>
> Jimbo