18 Jul
2007
18 Jul
'07
3:39 p.m.
I need some books and a double E degree. One more
question ...
I have the former, but not the latter :-)
Some of what I'm reading says put the capacitor near the power supply,
and other things are saying get it as close to the chip as possible.
(With a few rules, I'm simplifying.) If the bypass capacitor is being
used to filer the AC component, wouldn't the answer for a good design be
"do both"?
YEs.. Typically a 0.1uF per chip (maybe per power-coonnection on a chip)
and a larger capacitor near the power input.
Remember that capcaitors have self-inductance, whci hcan be considerable
for some types. That's why you don't use aluminium electrolytics for
decoupling at the IS pins.
As I've said to many designers starting out '0.1uF capacitors are cheap,
my time in finding a nasty logic glitch isn't'. In other words, it's hard
to over-decouple, if in doubt, add the capacitor. Maybe if you're in
production you want to save a few cents on each PCB (but to be honest,
I'd ratehr may a few _bucks_ more per PCB and have something that works
properly), but not for experimentaly designs.
-tony