FreeLists / si-list / [SI-LIST] Re: Lumped capacitance estimation

[Sushil Kumar GUPTA <sushil.gupta@xxxxxx>]

steve, I have found a document from agilent technology which says  that 
if rise time is greater than 6 times of cable propogation delay then It 
can be considered as lumped load.  Does this mean that I can consider 
1/6*Tr delay of cable as lumped load for my approximation. I have 
provided the link of the document also.  I am confused with different 
conclusions.  If you have any link which can provide detail of Tx line 
characteristic and delay approximation , it would be great.

http://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2005/Feb/High_Speed.pdf

Thanks
sushil


steve weir wrote:

> Sushil, that capacitance you are trying to model does not exist in a 
> vacuum w/o incremental inductance that oppose the changing currents 
> needed to charge the line.
>
> For purposes of argument, assume that the propagation velocity is 
> 6"/ns.  Every fs of that 600ps rising edge, the wave propagates 0.006 
> mils away from the driver, and does so through an incremental 
> inductance.equivalent to the partial inductance for that bit of the 
> transmission line length.  So, while you increase the capacitance of 
> the wire by lengthening it, each incremental bit of capacitance is 
> decoupled more and more from the driver by the inductance along the 
> way.  Let us assume the inductance is about 6.8nH / ".  Would it make 
> anymore sense for you to ask what the equivalent inductance of the 
> line should be to get the same loaded wave form as your present 
> question concerning the capacitance?  Both effects are definitely 
> present and interact with each other.  Surely if you break the line 
> into fs long LC stages you can get an accurate representation of the 
> behavior.  You might be unhappy with the solution time for a 500,000 
> stage LC network.  Perhaps you can approximate with fewer stages and 
> still get a decent answer.
>
> Steve
>
>
>
>
> At 04:55 PM 4/7/2005 +0530, Sushil Kumar GUPTA wrote:
>
>> 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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>

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