[time-nuts] IC for 1 PPS Output

buehl buehl at superlink.net
Fri Apr 7 16:46:51 EDT 2006


HI everyone:

last year I did a design requiring a full 5V out (higher than TTL) into 50 
Ohms with real 50 Ohm source Z and 1 nS risetime.

For that I made an output driver using 15 V supply and high speed op amps, 
two in parallel.  But that would be an overkill for what you are doing.

See some comments inserted below.




At 04:17 PM 4/7/2006, Hal Murray wrote:


>You might find good chips intended for use as clock buffers.  They probably
>won't come in DIPs so wiring them up as a hack will be a pain.  They usually
>come with good data sheets and app-notes.

There are also many high speed "line drivers" which are multiple to a 
package and typically have good section-to-section matching.



>I'm pretty sure I've seen an app-note saying it was OK to parallel drivers in
>the same package.  I wonder what the technology was.

This may have been for one particular IC.

Do not directly connect multiple outputs, even in same package.  Phase 
difference in the risetime of sections can cause very high instantaneous 
currents between sections.  Even though the durration is short, and 
therefore no heating problems, the high current causes "glitches" within 
the IC.  This can mess up the timing of the rise, which is important for a 
clocking application.

I put a nominal R in each output path, 22 Ohms in my case, to limit current 
from section to section.  Then at the junction of all these output 
resistors, I put  a single R to the output connector to set the desired 
output Z.

Tom Buehl 




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