[sdiy] Analog & Digital Ground

[Ian Fritz]

At 10:50 AM 4/11/06, Seb Francis wrote:

>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.

Well, Harry has come right out and said that he doesn't think you should 
use beads at all.  Draw your own conclusion.  :-)

I have found that they can help in some situations.


>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.

Figs 15, 16, 19, 21 and 24 all show a bead in the ground line.  (The 
implication is that fig 15 is proper if there is not a large dc 
component).  I think in terms of fig 21 for isolating separate circuits on 
a single board, but I haven't done comparative tests of different 
configurations.  Maybe it's an overkill.


>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).

Wait!  It also says use a single bead around everything to kill CM spikes 
(fig 24).  One bead may be fine in your application.  If you need more you 
can put two (or three) wires through a single bead and solder them to the PCB.

Anyway, let us know how this works out in your application.

   Ian