SV: Re: [sdiy] Analog & Digital Ground

[karl dalen]

> 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. 

This is the whole ide about beads, EMC and EMI supression
on power supply lines and on signal lines "to break the antenna".
Beads at 20khz, i doubth its usefullness, whats the point there?

> to maximize capacitance and minimize inductance
> (parallel power and ground traces) and proper
> decoupling will do much more than the beads...

Agree, beads i use on noisy digital lines.

I have never seen beads as isolators on GND, but maybe
it can be useful since its impedance is low at DC and high
at RF. But maybe we just increase noise on the digital DC lines
and that is no good idea either.(bounching and accumulating the digital crap).

KD

> > >different things :-)
> > >
> > >Ken says yes isolate the digital parts via beads
> > (including on the ground).
> > >Harry says don't isolate the ground, only the power
> > rail supply to the 
> > >regulator for the digital parts.
> > >
> > >So, what to do ...
> > >
> > >As Harry points out, it's important not to share
> > ground traces between the 
> > >analog and digital stuff, but even if there are
> > separate traces back to 
> > >the PSU part of the board, is it not possible for
> > high frequency glitches 
> > >to be introduced from the digital ground into the
> > analog ground?  After 
> > >all, there are lots of low impedance power sources
> > (0.1uF decoupling caps) 
> > >all over the analog circuitry, so some current
> > might flow from these as 
> > >well as from the PSU components.
> > >
> > >I imagined that by putting a bead in between the 2
> > grounds and having 
> > >separate decoupling caps for the digital parts, the
> > high frequency digital 
> > >edge spikes would be mostly blocked from the analog
> > parts of the board.
> > >
> > >I take Harry's point that you do not want
> > inductance on the ground line, 
> > >but I thought maybe a very small amount of
> > inductance can be a good thing 
> > >for isolation of very high frequencies.
> > >
> > >Any comments from you electronics gurus out there
> > :) ... ??
> > >
> > >Seb
> > >
> > >
> > >
> > >harrybissell wrote:
> > >>Hi Seb...
> > >>
> > >>I would not put a bead in the ground line. You do
> > not want inductance
> > >>there (imho). Better to just separate the ground
> > traces so the PIC and
> > >>opto
> > >>current cannot cause voltage drops on the analog
> > circuits.
> > >>
> > >>The bead would be a good idea in the positive
> > supplt, where it will
> > >>decouple
> > >>the two supplies.
> > >>
> > >>H^) harry
> > >>
> > >>Seb Francis wrote:
> > >>
> > >>>Hi,
> > >>>
> > >>>I'm just laying out quite a large board which is
> > almost all analog,
> > >>>except for a PIC&opto for the MIDI interface.
> > >>>
> > >>>Normally I have totally separate analog and
> > digital grounds which are
> > >>>joined right the way back at the PSU (at the end
> > of the power lead), but
> > >>>in this case the PSU is on the same board.
> > >>>
> > >>>I am thinking that I should separate the PIC&opto
> > ground from everything
> > >>>else with a ferrite bead.  The power is already
> > separate (it has its own
> > >>>5V regulator).
> > >>>
> > >>>Does this sound sensible?  Also, should I hook
> > the 5V regulator to this
> > >>>digital ground as well?  And perhaps even connect
> > the power input to the
> > >>>5V regulator via another ferrite bead?
> > >>>
> > >>>Thanks,
> > >>>Seb
> > >>>
> > >>
> > >>
> > >>
> > 
> > 
> 
>