[sdiy] Analog & Digital Ground

[Ian Fritz]

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'>

(Sorry, Harry)

   Ian


At 06:15 AM 4/11/06, Seb Francis wrote:
>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
>>>
>>
>>
>>