[sdiy] Analog & Digital Ground

[Seb Francis]

Ian Fritz wrote:
> I'd go by what the bead experts say.  This is a *really* good document:
>
> <http://www.steward.com/Steward/pdfs/emi/technical/Use%20of%20Ferrites%20in%20EMI.pdf#search='ferrites%20in%20EMI'> 
>
>
Thanks Ian, this is a useful document.

> (Sorry, Harry)
>

Do you imply by this that the document contradicts what Harry said?  As 
far as I could see it was actually saying the same thing as him.

In figure 20 (shows how ferrite beads and decoupling caps are used in 
combination to localise high frequency circuits) there is only a bead 
shown on the power rail and not on the ground.

The document is saying that if you have a DC (or LF AC) power connection 
it is better to have a single ferrite bead around both power and ground 
connections at once, but this is to cancel out the effect of DC bias 
currents on magnetic saturation (and hence get more HF loss from the 
bead).  This doesn't really apply to low current PCB power rails (and 
I've never seen a device that can be used to pass 2 PCB power traces 
through a single bead).

Seb