[SI-LIST] Re: Series termination value
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: <erwin-sp_seow@xxxxxx>
- Date: Wed, 20 Mar 2002 08:09:54 -0500
> We have calculated the Characteristic Impedence to be about 71 Ohm.
>
> With an output impedence of about 20 Ohm, the resistor value for =
series
> termination is 71 Ohm - 20 Ohm =3D 50 Ohm. My question is this value =
too big?
Depends on what you want to do. A 50 ohm resistor is just about
right for ideal series termination where you nominally match the
Zo of the line. (But check what happens when you tolerance the
driver, resistor, and Zo independently over their full ranges.)
Sometimes it is useful to slightly under terminate (aim for total Rs
somewhat less than Zo). This may give you cleaner edges that switch
fully, then a little overshoot which is usually clamped by the
receiving IC. Might be better than a plateau or glitch within the
switching region; plus the edge rates may be slightly faster.
> Will it attenuate the signal? We are running at about 90MHz.
If the trace lengths are short compared to the pulsewidth, and you
ignore line losses, and no parallel termination, then series
termination does not attenuate the signal; it still reaches Voh/Vol.
There's just less overshoot and ringing on its way to the final
values. If you treat that overshoot as part of the signal
amplitude, then you would say that series termination does
attenuate it.
If there is parallel termination too, then series termination
definitely attenuates the signal.
If the line length is comparable to the signal pulsewidth, then
series termination reduces the output amplitude you may see
within the pulsewidth time.
If you use far too much series termination, then the output
amplitude and edge rates will suffer.
Regards,
Andy
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