🚀 go-pugleaf

> Do despiking caps need to go directly between the VCC and GND inputs
> of a given IC, or could they go between VCC of the IC and some central
> 
> ground location, or even another IC's ground, for that matter?
> 

"Despiking" caps?  I'll assume you mean "decoupling" or "bypass" caps, rather
than something specifically intended to filter noise coming in from elsewhere.

In this case, the best method is for the cap to be placed between power 
and ground planes of a multi-plane PC board, with the shortest possible
leads (chip caps are great!).  This is to reduce the lead inductance, and
so keep the self-resonant frequency of the cap as high as possible.  If you
do not have separate power and ground planes available (such as in a wire-wrap
board or a simple one or two-layer PCB), the cap should be between Vcc and
ground such that the leads are again as short as possible.  This may mean
placing the cap between the Vcc pin of one IC and the ground of one in the
next row - the important thing is not that a cap be "associated with" a
particular IC, but that you have sufficient distributed capacitance across
the power and ground "grids."  Also, put a small electrolytic (a few 10s of
uF or so) across the power and ground near the point where power enters the
board, to help take care of low-frequency noise from outside the circuit.


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.