[sdiy] Analog & Digital Ground

[Seb Francis]

ASSI wrote:
> On Freitag, 21. April 2006 02:09, Seb Francis wrote:
>   
>> Personally I'm going to try it with it in first.  There is local
>> decoupling for the PIC power so any short current spikes should be
>> able to come from here rather than from the PSU.
>>     
>
> Colin has it right, the ground return path to the PSU needs to be as low 
> inductance as you can help it (hence the need for ground _planes_ 
> instead of ground lines in high-speed circuits). The ferrite beads on 
> ground connections may only come in handy in situations where you need 
> multiple connections between analog and digital ground for local 
> referencing (like if you have multiple ADC/DAC) or when there is 
> otherwise no way to keep a strict star topology on your ground 
> distribution. Such connection must not carry any supply current anyway.
>
>   
I don't agree with you here, at least not for the application I am 
talking about.  We are talking about a single PIC IC here running at 
4MHz with a low-ESR decoupling capacitor right next to it.  This is not 
a large very high speed digital circuit.

Surely any fast current needs of the PIC are going to be met by the 
decoupling capacitor and having a ferrite bead in the ground connection 
just serves to ensure this, and to ensure that any current spikes are 
not coupled into the analog parts of the circuit (which is the majority 
of the board).

Also, I don't understand how you can keep a strict star topology on your 
ground distribution *and* have a ground plane.

Seb