David C Brown wrote:
> On a board without power planes and using chips with GND and Vcc on
> opposite corners is it best to place the decoupling capacitor nearest the
> GND pin or nearest the Vcc pn?

Nearest the Vcc pin.

The primary purpose of the decoupling capacitor is to supply the transient
current needed when chip outputs switch from low to high. The path for this
current is from the Vcc pin of the driving chip, through the high-side
transistor, through the PCB trace to the load(s) and then to ground. So in
order to be most effective, the inductance (and trace length) between the
capacitor and the Vcc pin needs to be as small as possible.

If you don't have a ground plane, you might think you have a similar situat=
ion
ond the Gnd pin with respect to high-to-low transitions; the pull-down curr=
ent
needs a low-impedance path, too. But the situation is not symmetrical; the
return path for this current is through ground, too, so you're still at the
mercy of the overall impedance of the ground network between the chips in
question.

In other words, make your ground network as beefy as possible, then be libe=
ral
with the decoupling caps on the Vcc pins.

-- Dave Tweed
--=20
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