Is [one gate sourcing or sinking current into paralleled gates that
don't switch at
exactly the same time] a real problem? How far off can the prop
delay be for 2
gates on the same chip?
I seem to remember reading something saying it was OK to just wire them up in
parallel. It could have been an app note, or it could have been a rumor on
usenet.
Most manufacturers sanction direct paralleling of gates on the same
chip (and forbid any paralleling, ballasted or otherwise, of outputs
from different chips). However, there are other considerations
(short-circuit protection, maximum output current rating per gate,
maximum supply and/or ground current rating per package, maximum
power dissipation, etc.). Good design practice is to put series
resistors on each gate output that will prevent any of these ratings
from being exceeded into a short circuit to ground or to Vcc. For
HC, AC, NC7NZ, and NC7SZ, the per-gate output current rating is +/-
24 (or 25) mA, which suggests that each gate should have a 200 ohm
series resistor -- but you need to check all of the other ratings
mentioned above for the chip you use, to make sure they won't be violated.
Unfortunately, if the load is 50 ohms to ground, pulling it to even
4.5v from a 5v logic supply requires a source resistance of only 5.6
ohms, or 36 parallel gates each with 200 ohms in series (this assumes
that the gates can pull all the way to 5v while delivering rated
current, which they can't -- so the reality is even worse). You can
make things a little better by terminating the output into 50 ohms to
1/2 Vcc (i.e, the center of a voltage divider with 100 ohms to Vcc
and 100 ohms to ground), but (i) it only gets a little better, and
(ii) now it won't pull all the way to ground.
All that said, why do you want to generate a high-current square
wave, anyway? If you're distributing a frequency standard, it is
much better to distribute a sine wave (you don't have to worry about
the harmonics being skewed, and there is exponentially less of a
problem with it radiating and getting into every radio and other
sensitive electronic device in your house and the other houses on the block).
Best regards,
Charles
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