4 Replies Latest reply on Jan 28, 2011 12:01 PM by yihong.yang

    slew time = raise/fall time?

    yihong.yang
    yihong.yang Jan 26, 2011 2:43 PM

      Hi guys,

       

      I am trying using the CMOS device in the .mod library. The device I choose has 300ps slew time. Is the slew time is the raise/fall time, right?

       

      I ran the interactive simulation by using the oscilloscope. From the plot, I can measure the raise/fall time is about 600ps. It is different from the slew time in the model. In the simulation, there is only one driver, no transmission line and no reciever.

       

      is there anyone who can tell me why?

       

      Thanks,

       

      Yihong

        • 1. Re: slew time = raise/fall time?
          jduquette
          jduquette Jan 27, 2011 7:19 AM (in response to yihong.yang)

          There are various techniques for defining rise time.  Typically the time from 10% to 90% of the final value is used.  Sometimes 20% to 80% is used.

           

          Slew rate is typically in units of volts/second.  Slew time is an odd term from my perspective.

          • 2. Re: slew time = raise/fall time?
            yihong.yang
            yihong.yang Jan 27, 2011 2:56 PM (in response to jduquette)

            But how can i understand the slew time defined in the .mod library? It is different from what I measured in simulation plot.

            • 3. Re: slew time = raise/fall time?
              Ed Bartlett
              Ed Bartlett Jan 27, 2011 5:19 PM (in response to yihong.yang)

              Hello Yihong,

                  Slew time specifies the 10%-90% switching time of the upper- or lower-stage transistor for the .MOD Model Parameters. The scope can measure it for you when you select the waveform and then press the 'fall slew rate' or 'rise slew rate' button on the lower part of the scope. I just ran some tests with unloaded .mode model and 300ps setting. I am seeing the 300ps, not 600ps. I am not sure what caused the 600ps result that you saw. If you chose a different model than you had expected you could have gotten a longer switching time, but I do not see one with 600ps slew time. If you contiinue to see this result with 300ps setting, please open a service request on supportnet.

              Thank you and best regards,

              Ed.

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              • 4. Re: slew time = raise/fall time?
                yihong.yang
                yihong.yang Jan 28, 2011 12:01 PM (in response to Ed Bartlett)

                I tried again. The measured raise/fall time still cannot match with the slew time. I will open a request on the supportnet.