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Limitations of LT1721

Hello,

I didn't know where to post this questions so please move my post if this is the wrong place.  I couldn't find any info on this part in engineer zone.  I'm trying to design a laser diode driver that can hit 1-2nS.  I'm using the TI LMG1020 GaN fet driver and an EPC2019 GaN FET.  I'm looking to use the LT1721 to generate the digital pulse IN TO the LMG1020 GaN FET driver.  It seems that these high speed comparators can handle this but what about the and gate at the end?  Can that actually produce a 1nS pulse?  I tried looking in the data sheet of the and gate ( 74AhC08 ) and could not find a spec that describes its pulse performance.  

Below is a link to the circuit I'm referring to:

http://www.analog.com/en/design-center/reference-designs/circuit-collections/lt1721-pulse-generator-has-0ns-to-10ns-width-520ps-transitions.html#cc-overview

TI has a dev kit (schematics attached) that claims to be able to produce a 1nS pulse but the and gate used at the very beginning to generate the digital pulse into the LMG1020 says it can only operate up to 100Mbps which leads me to believe it can only produce a 10nS pulse.  I have been testing with this dev kit and it is very hard to get < 5nS pulse out of this and gate.  I have to end up dropping the voltage down lower and lower until it just barely trips the FET driver (LMG1020).  In using this dev kit it seems that the and gate really doesn't operate well under 10nS and you really end up fudging an output signal to trigger the fet driver, but it isn't a very clean pulse.  I'm thinking of replacing TI's and gate with your high speed comparator circuit but I want to ensure I'll get better performance and am not wasting my time changing over to a new design that is still bottlenecked by an and gate.

http://www.ti.com/lit/ug/snou150/snou150.pdf



added link to 1020evm
[edited by: m3atwad at 6:09 PM (GMT -4) on 6 Sep 2018]
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  • In the first page of LT1721 show clear that delay is 4.5ns. So, it is not suitable for your application.

    If you only want to test the wave form (because you don't mention to rise/fall time & duty), you can you ADI higher speed comparator to generate pulse as ADCMP551/ADCMP552/ADCMP553 with lower delay and support 700 ps minimum pulse width.

    ADI also support other comparator with 20Ghz speed as:

    brg!

Reply
  • In the first page of LT1721 show clear that delay is 4.5ns. So, it is not suitable for your application.

    If you only want to test the wave form (because you don't mention to rise/fall time & duty), you can you ADI higher speed comparator to generate pulse as ADCMP551/ADCMP552/ADCMP553 with lower delay and support 700 ps minimum pulse width.

    ADI also support other comparator with 20Ghz speed as:

    brg!

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