At 02:19 PM 5/29/01 -0400, you 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.

Using the same ground plane for digital and analog can lead to problems if
there are large digital
currents flowing. I generally try to split the ground plane (particularly)
on a 4-layer board
in such circumstances. This isn't much comfort to you with the layout
already done, but maybe you
can isolate some high curent pin(s) and connect them with wire to see if
that's the problem. Splitting
the ground plane can lead to increases in EMI, depending on how the high
speed digital signal layers
are routed (try to avoid running them over the gap).

Best regards,
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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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