18 Jul
2007
18 Jul
'07
4:51 a.m.
Message: 30
Date: Tue, 17 Jul 2007 17:16:16 -0600
From: "Michael B. Brutman" <mbbrutman-cctalk at brutman.com>
Subject: Re: Help identifying a capacitor
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.
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?
From some years in pcb design shop.
Bulk decoupling where the power enters the board 10-100uF + 0.1uF.
Then a 0.01-0.1uF per Ic (for TTL that is).
Keep the traces from the power pins to caps as short and fat as possible.
Powers should ideally be planes or gridded traces.
Newer logic families / packaging (TQFP, PLCC Etc), usually have very
specific decoupling requirements set my the manufacturer