What frequency is the ripple? You may need to use a small, moderate frequency
cap. in addition to the big 680 uF one to get rid of it, and make sure the cap.
indeed acts like a cap. at the frequencies you're trying to suppress. (If
you're above the resonant frequency of the cap., it will look inductive.)
My experience with Grounds in mixed signal systems indicates that it's not
generally helpful to separate the Grounds for analog and digital, because the
result is increased Ground impedance and therefore more noise. Also, make sure
that your ferrite bead has maximum attenuation in the frequency range of the
ripple - their characteristics vary significantly based on which bead you
choose.
Good luck,
Jay Diepenbrock
Advisory Engineer
Interconnect Technology & Qualification
IBM Global Procurement, B8UA/061, RTP, NC
Phone: 919-543-8804 Fax: 919-543-3642
Email: [email protected]
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