[SI-LIST] Re: Lumped capacitance estimation

  • From: Sushil Kumar GUPTA <sushil.gupta@xxxxxx>
  • To: a.ingraham@xxxxxxxx
  • Date: Thu, 07 Apr 2005 16:55:08 +0530

Hi Andrew,
                       Perhaps I couldn't explain the problem in right way.
                Suppose you have a transmission line (cable) which is 
very long let's say 30ns delay. If we assume cable has no loss then 
whatever waveform you apply, you will get FAR-END wave after 30ns delay. 
I am assuming both end are perfectly matched.  
        Now if you cansider the NEAR-END rise/fall time (let's assume 
avg value 600ps) , it will not depend on the length of the cable. So my 
question is that waht will be the approximate value of capacitive load 
at the NEAR-END (this time no cable is used) which will provide the same 
rise/fall time achieved with cable.
           I can state the problem in a different way also. Let's assume 
that IO driver has 600ps rise/fall time with 10pf capacitive load.  If I 
increase the capacitive load obviously rise/fall time will increase. 
 The PCB trace has 2.72pf/inch capacitive load.  If now I connect cable( 
PCB trace) with IO driver, it will see 2.72pf/inch capacitve load.  The 
driver has 45Ohm dc impedance w.r.t ground.  So when driver starts 
charging PCB trace, every inch of trace will be charged, but only that 
part of PCB trace will impact the rise/fall time which is travelled 
during transition time.  So how will I know how many inches to be 
considered.
Andrew Ingraham wrote:

>>         I need to calculate equivalent lumped capacitance seen by an IO
>>driver which  is connected to PCB trace (approx 3 Inches) and then cable.
>>The average rise/fall time is 600ps. I have information about the PCB
>>trace capacitance/inches. Will my assumption  be correct if I take
>>PCB-trace length (for lumped-capacitance calculation) which provides 300ps
>>delay.
>>    
>>
>
>For lumped capacitance, just multiply capacitance/length times the length.
>
>The rise/fall time doesn't make any difference as far as equivalent lumped
>capacitance is concerned.  However, if the rise/fall time is fast enough,
>you shouldn't be using a lumped capacitance in simulations.
>
>Are you sure your PCB trace delay is only 100ps/inch?
>
>  
>
>>What would be the best way to simulate IO driver as far as rise/fall time
>>accuracy is concerned assuming lumped model for Tx line.
>>    
>>
>
>For accuracy, best way = abandon the lumped model for the line.
>
>What did you mean by "best way" anyway?  If you have an I/O driver model and
>a transmission line model, just simulate them and look at the results.
>
>Regards,
>Andy
>
>
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