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

First, separate your analog and digital grounds, and join them only at one
point (likely at the uP)
Second, feed your opamp power through 10 ohm resistors, with local bypasses
to analog ground.
This assumes you don't have heavy loads on the opamp. If so, adjust the
resistors accordingly.

Hard to say more without seeing the circuit exactly.

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