17 Jul
2007
17 Jul
'07
4:16 p.m.
Tony Duell wrote:
I need some books and a double E degree. One more question ...
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"?
The board I posted about is not optimal in this regard. It's an ISA bus
to PCjr bus adapter, so there is no TTL logic anywhere on the board -
the individual ISA cards have the TTL (and hopefully bypass capacitors).
In this application was the designer just trying to 'clean up' the
voltage sources for the TTL on the cards a little bit?
Mike
Get the cap as
close to the chip as you can: think of the trace as a
(really low resistance) resistor: the resistance PS-to-cap is is much
larger than resistance cap-to-chip and bingo! low-pass filter.
Actually, it's the inductive reactance of the trace/PSU connections
that matters here. With high-frequency signals -- this doens't mean a
high clock rate necessarily, it means a fast ris-time edge -- you can
get significant voltage drop from very small 'inductors', like the
connction from the chip to the PSU.
-tony