[sdiy] PCB layout rules

[Batz Goodfortune]

Y-ellow Ingo, John, Antti 'n' all.
         If I may, throw the cat among the pigeons to some extent. For 
audio apps, I generally use a different scheme altogether. I've been doing 
it so long I can't think of the source I derived it from some 30 years ago 
maybe. However I suspect it was either Tim Orr (Practical Electronics/UK) 
and/or  David Tillbrook (ETI/AU) Both used similar schemes at various 
stages but often for different reasons.

Where super smooth decoupling for say, an op-amp is required, I use a small 
resistor between the power bus and the op-amp's power terminals. One on 
each supply of course. The resistor's value depends but is usually between 
10ohm and no more than 100. One can then either bridge the two power pins 
of the op-amp with a single electrolytic or use two electros to ground. It 
entirely depends on whether you think one side might get a bit more noisy 
than the other. Obviously this is basically a low-pass filter but more 
importantly, it acts as though each op-amp had it's own regulation. Not 
only does this help prevent supply glitches from jacking the op-amp but 
also any sudden current that the op-amp may pull won't manifest back onto 
the supply buss.

Tim Orr's reason for using it was to completely decouple the current 
produced by LED VU meters in a mixer. He went so far as to also decouple 
the LED meter in this way after rectifying and level shifting the signal so 
that the meter never dumpped current to ground (Zero volts) The latter 
however isn't very convenient and I prefer using an L/C arrangement to 
isolate the whole meter as a subsystem. But the point is that this can also 
be effective where you've got something sharing the same PSU that is really 
dirty.

Many people have commented on this technique over the years but it's just 
SOP for me. (Standard Operating Procedure) Someone once commented that they 
were going to try this to isolate some VCOs in an attempt to prevent 
self-syncing. Though I don't know if this was effective or not. I guess, in 
theory, it should be.

The scheme could be analogous to suspending a microphone on spring mounts 
to prevent bumps and fluctuations from being picked up by the mic. You're 
providing suspension for your op-amps. Your power supply road may be full 
of pot-holes but your op-amps will tend to just glide over them. The rule 
of thumb is small resistor/big cap. Try 10 ohm and 10~33uF. Or 100 Ohm and 
1~10uF. The main criteria for the cap is fitting it on the board without 
causing too much spread. However the resistor shouldn't be too large 
because that just means the op-amp ends up flolloping around like a fly in 
a spider's web. Though this may cause some fun linearity errors which I 
must make a note to try some day.

One last note. This does absolutely nothing for bad grounding. That's an 
entirely other issue.

Hope this helps.
Be absolutely icebox.

At 04:58 PM 12/25/06 +0100, Ingo Debus wrote:

>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
>

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