[sdiy] PCB layout question

mark verbos mverbos at earthlink.net
Sat Nov 1 20:01:05 CET 2008


Don't let Harry or Paul here you say that! ;)

I'm with Ian. I put some big electros at the power inlet and a few  
ceramics between chips here and there and it has always been fine.

Mark





On Nov 1, 2008, at 10:22 AM, 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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