Is your analog section seperate from the digital section electrically
(as much as possible?).  Try to seperate out that section, and have its
ground and power planes only connected to the ground and power planes of
the rest of the board at one point, preferably connected by
filters/chokes, and as close to the source of power as possible.  Points
where the signals must cross this boundary can also be choked.  Then you
can always try metal shields and such...

-Adam

Wynn Rostek wrote:

>The first version of our product that I designed a 16F877 into was very quiet. The second version I'm having some trouble with. It seems that I'm getting digital noise into the analog section which gets aliased down to the audio band and shows up as noise. It's not much, only a few millivolts but the audio signals I'm working with are only a few tens of millivolts, so it ruins the signal to noise ratio.
>
>Other than adding more bypass caps (which we've tried) does anyone have any tricks? This is a four layer board with ground planes on one layer, and power planes on another. The two outer layers are signals. It's mostly surface mount and about 4 inches by 6 inches and fairly full. (So I can't go for more physical seperation.)
>
>Any ideas greatly appreciated.
>
>TIA
>
>Wynn Rostek
>
>--
>http://www.piclist.com hint: The PICList is archived three different
>ways.  See http://www.piclist.com/#archives for details.
>
>
>
>
>

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu