[sdiy] PCB layout question

harrybissell at wowway.com harrybissell at wowway.com
Tue Nov 4 19:17:40 CET 2008


YOU (Ian) don't need decoupling caps because your electrical layout is usually
pretty good.

You can argue (correctly) that using bypass caps to cover up a BAD
layout is not a good or proper use of decoupling.

I have found that even with a decent board there can be a lot of interaction
at the power supply pins... especially with those switching a lot of
current (555 'shoot-through' anyone ?)

I have found that too few decoupling caps is asking for trouble... too many
is just a waste of space and money.  I'd rather over-do it. Your mileage may

H^) harry

On Sat, 01 Nov 2008 08:22:51 -0600, Ian Fritz wrote
> The need for decoupling is usually vastly exaggerated in the diy 
> community.  Studies by semiconductor manufacturers from many years 
> ago show that decoupling caps distributed every few chips is plenty 
> adequate for most analog applications.  Even for switching 
> applications look carefully at the manufacturers data sheets for 
> decoupling requirements.  For example, the LM139 comparator data 
> sheet says "It is usually unnecessary to use a bypass capacitor 
> across the power supply lines".  The data sheet for the LM311 
> comparator says "The power supply bypass capacitors should be 
> located within a couple inches" of the chip.  And so on.
> Electrical disturbances travel at the speed of light, which is 3 cm 
> per 100 picoseconds.  So unless you are working with nanosecond 
> signals there is no need to chew your fingernails over bypassing.  
> Your decoupling caps could be anywhere on your little board -- if 
> you really think you need them.  :-)
>    Ian
> At 07:44 AM 11/1/2008, Derek Holzer wrote:
> >I've got a question about decoupling caps. I'm making my first attempts at 
> >a layout in Eagle, starting with a very simple circuit from Thomas Henry 
> >to add PWM to his XR-VCO. The layout is here:
> >
> >http://www.soundtransit.nl/temp/pwm_test.png
> >
> >My question: do decoupling caps need to be directly in line with the rest 
> >of the circuit? Note the position of C3, for example. Will the power to 
> >pin 8 of the IC be properly decoupled in this layout? Of course, any other 
> >suggestions are welcome too!
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Harry Bissell & Nora Abdullah 4eva

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