[sdiy] PCB layout rules

[Laurie Biddulph]

If a design uses DGND (Digital GND) and AGND (Analog GND) would it make 
sense to take ALL decoupling capacitors DGND (irrespective of whether they 
are analog or digital circuits) and thus leave the AGND purely as your 
`reference' GND? Or would the introduction of the DGND throughout the whole 
board cause more problems than it solves?

Best Regards

(Mr) Laurie Biddulph
Phone: +61 (0)2 4340 0938
Mobile: 0400 257 645

Elby Designs
ABN: 70 022 727 605
http://www.elby-designs.com

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----- Original Message ----- 
From: "Antti Huovilainen" <ajhuovil at cc.hut.fi>
To: "Seb Francis" <seb at burnit.co.uk>
Cc: "synth-DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, December 23, 2006 10:27 AM
Subject: Re: [sdiy] PCB layout rules


> On Fri, 22 Dec 2006, Seb Francis wrote:
>
>> Isn't that the point of the cap that the ground doesn't get 
>> contaminated - the
>> very fast current spikes are sourced/sinked from the capacitor not the 
>> ground
>> trace.
>
> No. The cap steadies the power supply lines, not the ground (ground is
> where everything is referenced to). And since it acts as short circuit for
> high freqs, a spike in the power supply line would get coupled through the
> cap to ground and possibly contaminate it for other signals.
>
> This is partly why I'm interested in the sense/force ground. Such
> decoupling caps would be connected to force ground while the sense ground
> would be used for reference (such as + input of inverting opamp).
>
> Antti
>
> "No boom today. Boom tomorrow. There's always a boom tomorrow"
>  -- Lt. Cmdr. Ivanova
>