[sdiy] Analog & Digital Ground

[Seb Francis]

Thank you Ken and Harry for your replies.  Unfortunately they both say 
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
>>     
>
>
>