One Shot with Fast Rise/Fall Times

[drkidd22]

Hello,

I'm working on the circuit shown below. Basically there is a fiber input at J2 with a 20us PW. The output from J1 would ideally go out in sync with input from J2, but there is a 2uS delay which is ok, But I would like to know how could I get this better. Then the input to pin 1 of U1 should delay the output of U1 pin 6 for about 192us and thus far this is close enough. My issue with this circuit is the output on pin 6 of U1 has a pretty long rise time as you can see in the scope image. I need to achieve a faster rise time, 10ns would be great. Any help and ideas will be appreciated.



CH1 in the scope is the actual output of 192uS.
You can also see there is a delay of about 2uS on rising edges from CH2 to CH1.

CH2 is rise time of input to J2 which is about 30ns.

This rise time of output in pin 6 of U1 which I'm more concerned with, I was expecting atleast a 5ns rise time. but that's not the case. This unload output righ from the pin.

 

[ronv]

Here is a fast one>

https://www.ti.com/lit/ds/symlink/sn74lvc1g123.pdf

 

[drkidd22]

It seems to me that they both spec for about 1ns tr/tf whats the advantage of the ti one?

 

[moffy]

If you want to improve the risetimes of the receiver then reduce your resistor values. I would definitely put a cap between pin 4 and gnd for the LTC6993. The slow risetime could be due to excessive capacitance on the output, or poor power supply. You can always clean it up by using one or several 74HC14 schmitt inverters in parrallel. They have good drive capabilities.

 

[ronv]

To tell you the truth I didn't look at your part, only that the rise time was 30ns unloaded. Did you use a 10X probe?

 

[drkidd22]

To tell you the truth I didn't look at your part, only that the rise time was 30ns unloaded. Did you use a 10X probe?

Yes I used a 10 prove.

 

[crutschow]

Two main reasons for a slow rise/fall time are high capacitive output load or inadequate decoupling of the LTC6993. Is C1 connected directly from pin 5 to pin 3 with short leads?

 

[drkidd22]

It was a bad probe. So much for Tek probes
Thanks for your help guys.