[sdiy] PCB layout question
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.
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. :-)
> 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:
>> 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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