piclist 2001\04\01\161334a >
Thread: Seperating grounds
face BY : David VanHorn email (remove spam text)

>This is one of those points with multiple opinions. As I mentioned, some
>people think this gnd pin is for your analog signals. OTOH, you have Dave's
>answer. In my case, I have always used a small gnd trace "island" encircling
>my xtal and bypass caps, which I have always tied to this gnd,

The ground ring isnt' a bad idea at all, but I'm not clear what ground
you're tying it to.
It should go to the digital ground, not the analog. (if they differentiate.

>  and my analog
>circuitry [opamps/etc] has a separate gnd trace that goes striaght back to
>the power injection point on the board.

I wouldn't think it was an analog ground, unless it was called out as such
in the datasheet.
The AVR parts are that way, AVCC and AGND actually power the port where the
ADCs are..
What I do is bring all the analog back to this AGND, then link AGND to GND
under the CPU with a zero ohm resistor.
This lets my cadware see AGND as a separate signal from GND, and it then
knows not to connect AGND to anything else.

>Dave has a slightly different take on this, but he has done a lot of
>testing, and his boards may be less noisy than mine.

The general idea is to let the bypass function as a "T" filter, so that
you're always using the PCB track impedances to steer HF currents where you
want them to go.

>IIRC, one thing they do recommend when making a single-point connection
>between analog and digital gnds, when made "under" the A/D, is to make
>that trace thin - so DC currents will pass, but digital noise will be
>somewhat blocked.

You could use a ferrite bead for this, but I've never had to go that far.

>Is your head spinning yet? [it should be - this stuff is complicated,
>and there are more hard questions than easy answers :)].

Yes. You really have to get into it and look at all angles, and develop an
understanding of the whole picture.

Dave's Engineering Page: http://www.dvanhorn.org
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

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