Homebrew PCBs

I read some articles about this problem and all talk about star 
groundind, but nothing about power supply line.
Let's supose we have a simple transformer-bridge-cap-regulator power 
supply and two circuits, one analog and one digital. What is the best 
layout in this case. I mean, what is the principle? A longer power 
line and a shorter ground line for the digital section? It's OK to 
connect the digital power line at the same single point whith analog 
and the digital ground between transformer and bridge. 
Or simply two ground and two power lines conected at the power supply 
entry, after the filter cap? 

Adrian

On Fri, 25 Aug 2006 10:54:40 +0200, Adrian <adicont2@...> wrote:

> I read some articles about this problem and all talk about star
> groundind, but nothing about power supply line.
> Let's supose we have a simple transformer-bridge-cap-regulator power
> supply and two circuits, one analog and one digital. What is the best
> layout in this case. I mean, what is the principle? A longer power
> line and a shorter ground line for the digital section? It's OK to
> connect the digital power line at the same single point whith analog
> and the digital ground between transformer and bridge.
> Or simply two ground and two power lines conected at the power supply
> entry, after the filter cap?
> Adrian


Usually you have some sort of interface between digital and alalog, like a  
AD converter. Then you will connect the grounds right at that component.  
If you don't then right at the supply is fine. Power is not such a problem  
as long as you decouple things well. You may need inductors as additional  
filters for sensitive circuits, with caps both sides. The inductor blocks  
ac as you know.

When doing ground layout i like to think in voltage drops.

ST

When I say digital, I refer to a CMOS counter (RC clock) to control a 
multiplexer/demultiplexer IC to switch an analog signal. So only two 
digital ICs.
"When doing ground layout i like to think in voltage drops"
You refer to traces voltage drops?

Adrian



--- In Homebrew_PCBs@yahoogroups.com, "Stefan Trethan" 
<stefan_trethan@...> wrote:
> 
> Usually you have some sort of interface between digital and alalog, 
like a  
> AD converter. Then you will connect the grounds right at that 
component.  
> If you don't then right at the supply is fine. Power is not such a 
problem  
> as long as you decouple things well. You may need inductors as 
additional  
> filters for sensitive circuits, with caps both sides. The inductor 
blocks  

> ac as you know.
> When doing ground layout i like to think in voltage drops.
> ST

On Fri, 25 Aug 2006 19:40:50 +0200, Adrian <adicont2@...> wrote:

>
> "When doing ground layout i like to think in voltage drops"
> You refer to traces voltage drops?
> Adrian


No, i imagine where the currents go and if the voltage drop they cause is  
going to upset another circuit.
If voltage drop is not seen by a sensitive part of the circuit it does not  
matter.

The reason for star ground is that all the voltage drops are in parallel  
to each other, so none can be seen by any other part of the circuit. But a  
star branch to each and every connection is not always possible. So you  
must think if the voltage drop of one branch will be a problem for another  
branch, the drops will now be in series. You do not want "noisy" drops  
 from digital circuits to be seen by a analog component that is sensitive  
to the ground noise.

ST

Yes, now I understand. 
This is another ocasion when I read so many pages to understand almost 
nothing, and you give me the answer in few words :-) 
This is a real talent. Maybe you must take into consideration a 
teacher career ;-)


Adrian

--- In Homebrew_PCBs@yahoogroups.com, "Stefan Trethan" 
<stefan_trethan@...> wrote:

> 
> No, i imagine where the currents go and if the voltage drop they 
cause is  
> going to upset another circuit.
> If voltage drop is not seen by a sensitive part of the circuit it 
does not  
> matter.
> 
> The reason for star ground is that all the voltage drops are in 
parallel  
> to each other, so none can be seen by any other part of the circuit. 
But a  
> star branch to each and every connection is not always possible. So 
you  
> must think if the voltage drop of one branch will be a problem for 
another  
> branch, the drops will now be in series. You do not want "noisy" 
drops  
>  from digital circuits to be seen by a analog component that is 
sensitive  

> to the ground noise.
> 
> ST
>

On Fri, 25 Aug 2006 20:48:45 +0200, Adrian <adicont2@...> wrote:

> Yes, now I understand.
> This is another ocasion when I read so many pages to understand almost
> nothing, and you give me the answer in few words
> This is a real talent. Maybe you must take into consideration a
> teacher career
> Adrian


Well, there is much more to grounding that that (induction, emi,  
resistances,...)
But with this method and a healthy oversizing of all traces i usually get  
by without too much thinking.

ST