[time-nuts] 50 ohm Driver

Charles Steinmetz csteinmetz at yandex.com
Wed Mar 4 19:44:29 EST 2015

>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

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,


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