[sdiy] PCB layout rules

[Ingo Debus]

Am 23.12.2006 um 00:41 schrieb Antti Huovilainen:

> On Fri, 22 Dec 2006, John Luciani wrote:
>
>> The cap is there for stability. It delivers surge currents through  
>> a low
>> impedance path to the IC. Without the capacitor you would have the
>> series inductance of a much longer trace which would result in  
>> voltage
>> dips and ringing at the Vcc pin of your IC.
>
> Only if you had no cap there. I was speaking of the choice of cap  
> between
> V+ cap V- and V+ cap GND cap V-. The first cannot contaminate  
> ground, yet
> should guarantee stability (an opamp after all does not actually use a
> ground for anything).

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?
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.

> Of course you'd use tree/star ground then.

Talking about star versus bus routing: In Eagle there's a tool called  
length-freq-ri.ulp that calculates the total length of the trace for  
each signal on a PCB. It also calculates the maximum frequency for  
that signal from that length (just using f=c/l). Are these results  
relevant at all? Of course, when doing a star layout, the total  
length can get ridiciously long, because this program just adds all  
the traces of one signal. Makes me think of the stories in Bob  
Pease's book where people blindly trust the numbers they get from a  
computer program.

Ingo