[SI-LIST] Re: SV: Total load capacitance
- From: Bob Patel <whizplayer@xxxxxxxxx>
- To: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- Date: Thu, 10 Jan 2002 09:18:36 -0800 (PST)
Hi! Andrew, This load capacitance estimate is for determining the drive
strenght needed for an ASIC I/O. This capacitance is beiong calculated wiht the
fact that I need to maintain the characteristic impedance to 50ohms and this
way I ensure that the driver can source/sink the required amount of current in
order to maintain the rise and fall times of the signal.
Thanks
Bob
"Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx> wrote: > Hi! Anders, Even if
I take a transmission line
> approach the total capacitance faced by the driver
> will always be there i.e. even if the edge rates are
> faster and the driver is weak then the rise & fall
> time will not be maintained at the receiver and if the
> C is too large then the driver may not be able to
> drive it above Voh. SO, either way this lumped
> capacitance approach has to be taken into account to
> determine the drive needed.
That depends. Take the 1000 meter cable with Zo = 50 ohms, and put a 50
ohm load on the end of it. Driving it, you can't tell if it's 0 meters
or 1000 meters long. It looks the same to the driver. (Ignoring
discontinuities, non-ideal loads, and cable losses.)
If it didn't have the 50 ohm load, the situation is different. If the
cable is long enough, the initial edge rates would be determined by the
ability of the driver to drive the characteristic impedance of the
cable; but eventually the reflections would come back and modify the
waveforms in some way that you can't quantify by simply adding up the
cable capacitance. If the signal is periodic and the electrical length
is an exact multiple of half the repetition rate, then the amplitude
(and therefore edge rates) might be enhanced by the reflections. If it
is an odd multiple of a quarter of the repetition rate, the reflection
might kill the edge rates.
The answer to your question also depends on what you need the
capacitance estimate for, i.e., power estimation, vs. output levels or
edge rates.
Finally, I would think that Cout of the driver has already been
accounted for so you don't need to include it, but check with the ASIC
vendor to be sure.
Regards,
Andy
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