[sdiy] Analog & Digital Ground

[Joe Grisso]

     We do this a lot in most of the audio designs I've worked on. The way
we've handled it in the past is very straightforward. We have a digital
section and analog section, and separate the ground planes of each section
with a moat, and the power supply being in the center of the moat. This
preserves star grounding while isolating the noisy digital plane from the
quiet analog plane.
     Furthermore, we run supply rails for most of the analog since it's
fairly small and keeps noise coupling down, while we dedicate 1 or 2 plane
layers for the digital side. This design topology allowed me to develop a
3.5" x 5.5" board that has a 200K gate FPGA, a 600MHz PowerPC processor,
64MB of SDRAM and 8MB of flash as well as a 113dB DNR/SNR  and -98DB THD+N
analog audio subsystem all on the same board. Also, no. I didn't have to put
a faraday cage around the analog and/or digital to keep noise away. :-)

Best Regards,

Joe Grisso
Detachment 3 Enterprises

On 4/21/06, Seb Francis <seb at burnit.co.uk> wrote:
>
> ASSI wrote:
> > On Freitag, 21. April 2006 02:09, Seb Francis wrote:
> >
> >> Personally I'm going to try it with it in first.  There is local
> >> decoupling for the PIC power so any short current spikes should be
> >> able to come from here rather than from the PSU.
> >>
> >
> > Colin has it right, the ground return path to the PSU needs to be as low
> > inductance as you can help it (hence the need for ground _planes_
> > instead of ground lines in high-speed circuits). The ferrite beads on
> > ground connections may only come in handy in situations where you need
> > multiple connections between analog and digital ground for local
> > referencing (like if you have multiple ADC/DAC) or when there is
> > otherwise no way to keep a strict star topology on your ground
> > distribution. Such connection must not carry any supply current anyway.
> >
> >
> I don't agree with you here, at least not for the application I am
> talking about.  We are talking about a single PIC IC here running at
> 4MHz with a low-ESR decoupling capacitor right next to it.  This is not
> a large very high speed digital circuit.
>
> Surely any fast current needs of the PIC are going to be met by the
> decoupling capacitor and having a ferrite bead in the ground connection
> just serves to ensure this, and to ensure that any current spikes are
> not coupled into the analog parts of the circuit (which is the majority
> of the board).
>
> Also, I don't understand how you can keep a strict star topology on your
> ground distribution *and* have a ground plane.
>
> Seb
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20060421/4c38c91c/attachment.htm>