[sdiy] Analog & Digital Ground

[Seb Francis]

Harry Bissell Jr wrote:
> Gaarrrr Ian, you should know better than that :^P
>
> "what we have here is a failure to communicate"
>
> I understood the beads would be TWO beads, one in the
> power and one in the ground, (like Fig 15) and I
> disagreed with that.
>
> I do agree with Fig 16, if they were talking about
> both traces through the same bead.
>
> I was thinking of the beads that have a single
> conductor through them, like MOTM uses on their power
> supplies.
>
>   
Yes, this is what I was asking about: using the 'MOTM' style beads on 
the PCB power and ground rails ....

> I don't agree there should be a bead between the
> analog
> and digital grounds on the same board.
>
> If the example you're thinking of is Fig 21... this
> would be good for independant circuits in the same
> rack but i think it would be more trouble when you
> start to interconnect them.  A signal originating from
> one board has to reference through two beads (extra
> inductance) before returning to the source of the
> current. I think it will increase the signal
> distortion
> (but that remains to be seen)
>
>   
Now that Harry has enlightened me to think in terms of return path for 
currents, I can see that when sub-circuits interconnect the ground 
impedance between these should be low as possible.

> I'm not even sure I'd use the ferrite beads at all. I
> usually don't. If the issue is to reduce radiated
> EMI from the PIC, maybe they are needed to meet agency
> approval. Since as a rule, I do not let the FCC enter
> my home... moot point for me.
>
>   
No, I'm not letting the FCC in my house either ;)
I just don't like to see the little HF spikes that PICs tend to make, 
appearing all over my high-fidelity analog circuitry!
Probably they're not even audible, but they show up on the scope and 
it's just playing it safe to try and filter them.

Seb