[sdiy] PCB layout rules

[harry bissell]

Hi Ingo (et al)

Ingo Debus wrote:
>
>
> Decoupling an opamp (running from dual supply) with a cap only from 
> supply+ to supply- and no cap to ground? I think I read somewhere that 
> this is a no-no, but I can't remember why. Anyone?
Its just not effective unless the load is referenced to one of those 
rails. (usually its ground referenced)

Of course if its a single supply then....    ;^P
> Anyway, some thoughts: yes, an opamp has no ground pin, but often the 
> load is connected to ground (or virtual ground, when driving another 
> inverting amp). In this case there's either varying current in the 
> +supply line or in the -supply, but never in both at the same time. 
> Just one cap across the supply lines would be a bad idea then, no?
>
> A related topic, some people say it's better to route the power like: 
> supply -> decoupling cap -> IC rather than supply -> IC -> decoupling 
> cap. Does anyone here follow this rule? It sometimes makes the layout 
> look funny.
Religiously (waves hand)
> I'd use the star grounding if there were two grounds... one carrying a 
> lot of current and another
uhh... not carrying a lot of current.  Running the panel LEDs back to 
the supply can cure a lot of noise
headaches.  I even like to run them with current from the main filter 
caps, pre-regulator... and drive
with transistors. The only current in the main circuit is the base 
current of the transistor. I run the transistor to
the power ground via a separate wire (trace).  The return current for 
the transistor has to take the long way
home... otoh its only some (hundreds of ) uA.

H^) harry