[time-nuts] Clock Driver Design

[Hal Murray]

I assume you are driving 50 ohm coax with a 50 ohm series termination at the 
driver and a 50 ohm terminator at the far end.  That's 100 ohms or 50 mA at 
5V.

The series terminator avoids junk if the far end isn't terminated and also 
provides short circuit protection.  It also cuts the voltage at the far end 
in half when it is terminated.


> SN74LVC1G125  - Single Bus Buffer Gate with 3-State Output

It's a fine chip, but it's running right at the limit.

If you are reading data sheets, there are several limits to consider.

The first current spec is Ioh or Iol, the current where it will meet Voh/Vol, 
the output voltage specs.  Those are guaranteed limits over Vcc and 
temperature.  It's OK to exceed this, but the voltage might not be what you 
want.  For a one off, check with a scope.

The next parameter is Io, the absolute max continuous output current.  For 
this chip, that's 50 mA.  In this context, I think "continuous" includes x% 
duty cycle when the pulse is more than a few ns.

The descriptive text doesn't say anything about time limits.
  http://www.ti.com/lit/an/szza036b/szza036b.pdf
> This is the maximum output source or sink current that can the device at
> voltages within the normal operating range.


The other parameter to keep an eye on is the max current through a 
power/ground pin.  On this chip, that's 100 mA so probably not a problem.

If you know the far end is not terminated, the max current limits are not a 
problem unless you have long cables.



-- 
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