[sdiy] PCB layout question

Ian Fritz ijfritz at comcast.net
Sat Nov 1 15:22:51 CET 2008


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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