[sdiy] Analog & Digital Ground

[Seb Francis]

I guess that by having a bead on just the power rail it will be enough 
to make the high frequency current want to flow via the local decoupling 
capacitor(s), and thus stay out of the analog parts of the circuit.  I 
will also do the power and ground traces as you suggest.

And I take your point that when the circuits are patched together (in my 
case it is where the gate signal from the PIC connects to trigger the 
analog envelope circuit) the current needs to return via the common 
ground and thus it may actually cause noise problems if this ground 
route is attenuated by a bead.

Thanks for your help,
Seb



Harry Bissell Jr wrote:
> Current will return to the source via the
> shortest route it can. If you keep the power and
> ground
> running to the (small) ditial section parallel to each
> other and keep a minimum of enclosed loop area
> (inductance) between them... 
>
> ... they really will not want to radiate TO the analog
> sections... they will want to keep within their own
> path.
>
> There could be some slight capacitive coupling, that
> you could minimize by placing a ground 'guard trace'
> between the digital and analog sections.
>
> Do not overlap the digital and analog ground areas.
>
> You will need to see if Ken's solution, or my solution
> works better. You cannot say for sure until you build
> it.  The dual beads would be best for isolating to
> independant circuits, but when you patch them together
> the beads in the ground may cause more interaction.
> OTOH beads are 'lossy' and only work at very high
> frequencies anyway...
>
> H^) harry
>
> --- Seb Francis <seb at burnit.co.uk> 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
>>>>     
>>>>         
>>>   
>>>       
>>
>>     
>
>
>
>