[SI-LIST] Re: How does the loading affects the rise time of a PAD ?

  • From: "Greim, Michael" <mgreim@xxxxxxxxxxxx>
  • To: "'cheard@xxxxxxxxxx'" <cheard@xxxxxxxxxx>,"'james.f.peterson@xxxxxxxxxxxxx'" <james.f.peterson@xxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
  • Date: Tue, 21 Aug 2001 14:02:15 -0400

Yes, 

it is that subtle difference between modelling your interconnect
as an ideal transmission line or a lossy, frequency dependent 
one.  Throw in crosstalk and power supply noise, and you might
actually be able to make a career out of this stuff.

Oh, that's right, we do........ Obligatory 8-)  ;-)

best regards,

Michael Greim

-----Original Message-----
From: Heard, Chris [mailto:cheard@xxxxxxxxxx]
Sent: Tuesday, August 21, 2001 1:02 PM
To: 'james.f.peterson@xxxxxxxxxxxxx'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How does the loading affects the rise time of a
PAD ?



This of course is true assuming absolutely no dielectric or skin effect
losses are present.  

If that very nice 15" long trace is 4 mils wide and if the risetime is
100ps, the risetime will certainly increase.

Chris

-----Original Message-----
From: Peterson, James F (FL51) [mailto:james.f.peterson@xxxxxxxxxxxxx]
Sent: Tuesday, August 21, 2001 12:02 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How does the loading affects the rise time of a
PAD ?



The 50 ohm 15 inch long transmission line is a thing of beauty. It will not
degrade your rise time. It will appear like a resistor (50ohms) to the
buffer, therefore you do not need to "lump" the capacitance (see the bottom
of page 143 of the "black magic" book by Johnson and Graham....).

best regards,
Jim

-----Original Message-----
From: C.Y. Cheng [mailto:cyc@xxxxxxxxxxxxx]
Sent: Monday, August 20, 2001 12:05 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] How does the loading affects the rise time of a PAD?


Hi folks,

I have got a basic problem to choose a PAD for GMII interface.

The system requires a PAD which could drive a 5 pF loading in 
1ns rise time.

If I can choose a PAD which could drive a 3-inches long, 50 ohms 
transmission line and meet spec. .
What if I use the same pad to drive a 15-inches long, 50 ohms 
transmission line? The lump capacitance may increase to be 5 times of 
previous case. Will it " slow down " the rise time of the PAD ?

Thanks,

C.Y. Cheng
ASIC designer
ADMtek Taiwan



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