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.
>

Wynn, I have had a similar problem in a project, but in my case I have
1 Mhz BW, high-impedances, and 12-bit conversion. I have had trouble
getting the noise below a couple of mV in this instance.

You are only doing 10-bit conversions in the audio band, so it would seem
that noise should not be such a big problem.

One thing that really affects this is the impedances in your ckt.
Use the lowest value resistances possible, < 10K instead of > 100k, etc.
This is something doable without redesigning the pcb.

Also, if using analog switches to change gain, switch signals, etc,
check that something is not floating and picking up noise through
the ether [non-technical term].

As others have suggested, the single best thing is probably physical
and electrical separation between analog and digital sections, separate
v.regs and busses for analog, one point gnd-plane connection between
analog and digital, non-overlapping gnd planes.

If you are having a noise problem, given what you described, you
might check you do not have a basic layout error on the pcb
- gnd loop, non-terminated trace, etc.

hope this helps,
- dan michaels
www.oricomtech.com
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