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Cleaning up an RC delay circuit

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  • bill rowe
    I m using a 10K resistor and an 82pf cap to generate a delay of around 1us. It works fine in test but am I correct in thinking that I have to clean it up with
    Message 1 of 3 , Sep 17, 2013
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      I'm using a 10K resistor and an 82pf cap to generate a delay of around 1us.  It works fine in test but am I correct in thinking that I have to clean it up with something that has schmitt trigger inputs before it hits regular cmos or lsttl?
    • joshbensadon
      Hi Bill, Yes, ideally you want to put it into a gate with schmitt trigger input, but you don t have to. If not, then when the analog voltage is between the low
      Message 2 of 3 , Sep 17, 2013
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        Hi Bill,


        Yes, ideally you want to put it into a gate with schmitt trigger input, but you don't have to.  If not, then when the analog voltage is between the low and high thresholds of the gate, the output might oscillate and/or the chip can get warm (ie, when it's neither a 1 or a 0, then you might have both hi and low transistors ON at the same time).  If the target circuit is not tolerant of oscillations, then you better use a schmitt trigger.  The IMSAI front panel uses capacitors directly on the TTL lines to impose small time delays (<1uSec).  I'm sure there are many more examples.  I've done it before when I'm fixing a small timing glitch.  Recently I had to do it in a Sync Separator circuit, no ill effect.  If you choose not to use a schmitt trigger gate and the chips do get warm, then watch out for issues with extra spikes on the VCC lines.


        :)J




        --- In cosmacelf@yahoogroups.com, <cosmacelf@yahoogroups.com> wrote:

        I'm using a 10K resistor and an 82pf cap to generate a delay of around 1us.  It works fine in test but am I correct in thinking that I have to clean it up with something that has schmitt trigger inputs before it hits regular cmos or lsttl?
      • bill rowe
        so a schmitt trigger it is. To: cosmacelf@yahoogroups.com From: joshbensadon@yahoo.com Date: Tue, 17 Sep 2013 15:26:06 -0700 Subject: [cosmacelf] RE: Cleaning
        Message 3 of 3 , Sep 17, 2013
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          so a schmitt trigger it is.



          To: cosmacelf@yahoogroups.com
          From: joshbensadon@...
          Date: Tue, 17 Sep 2013 15:26:06 -0700
          Subject: [cosmacelf] RE: Cleaning up an RC delay circuit

           

          Hi Bill,


          Yes, ideally you want to put it into a gate with schmitt trigger input, but you don't have to.  If not, then when the analog voltage is between the low and high thresholds of the gate, the output might oscillate and/or the chip can get warm (ie, when it's neither a 1 or a 0, then you might have both hi and low transistors ON at the same time).  If the target circuit is not tolerant of oscillations, then you better use a schmitt trigger.  The IMSAI front panel uses capacitors directly on the TTL lines to impose small time delays (<1uSec).  I'm sure there are many more examples.  I've done it before when I'm fixing a small timing glitch.  Recently I had to do it in a Sync Separator circuit, no ill effect.  If you choose not to use a schmitt trigger gate and the chips do get warm, then watch out for issues with extra spikes on the VCC lines.


          :)J




          --- In cosmacelf@yahoogroups.com, <cosmacelf@yahoogroups.com> wrote:

          I'm using a 10K resistor and an 82pf cap to generate a delay of around 1us.  It works fine in test but am I correct in thinking that I have to clean it up with something that has schmitt trigger inputs before it hits regular cmos or lsttl?

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