# [SI-LIST] Re: No need of termination within critical length??

• From: olaney@xxxxxxxx
• To: sonukurian@xxxxxxxxxxxxxxx
• Date: Thu, 17 Sep 2009 01:07:48 -0700

```Flip the conceptual machinery around; critical length is by definition
the boundary between where termination doesn't matter and where it does.
It has no direct relation to backmatching at the driver, which is a
popular way of terminating lines longer than critical length.   Below
critical length, the time delay on the line is short enough compared to
the risetime of the signal that the voltage delta between the ends is
small enough to ignore, i.e. the signal is 'local'.
It's a fuzzy boundary that depends on lots of things, but another way to
look at it is that if the transmission line is a small fraction of a
wavelength, the energy reflected from a mismatched load is so nearly in
phase with the driving end (or 180 degrees out, depending on the sign of
reflection) that the formula that transforms load impedance to what is
seen at the driving end never strays very far from unity.  Therefore the
transmission line is very nearly transparent and can be ignored (all else
being equal), even to the point of not requiring matching at either end.
A resistor might still be required for pull up, pull down, or voltage
divider purposes, but on a short enough line there is no need to tie the
value to trace impedance.

Orin Laney

On Thu, 17 Sep 2009 12:27:07 +0530 "Sonu Kurian N.D"
<sonukurian@xxxxxxxxxxxxxxx> writes:
> Hi All,
>
> I am puzzled about why  termination is not required if  trace length
> is
> less than critical length.
> I read like if a driver encounters a reflected signal when it is
> rising(driving) , its able to absorb reflection.Is it so?
> Or is the situation like ..the reflections will be there..but the
> Setup
> condition can be met..so we need not worry..??
> Can anyone please throw some light on this..
>
> Many thanks,
> Sonu
>
>
>
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