[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter?
- From: Jim Roberts <jgroberts@xxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx
- Date: Mon, 28 Oct 2002 18:31:02 +0100
Though there maybe arguments for wanting increase overshoot, etc. there is
also a very good reason to use the series resistor to reduce EMC.
The basis for this is to disipate the reflected signal of the unterminated
line. The capacitance at the receiving device can also accentuate this problem.
I would also like to highlight.
Series termination shoulkd NEVER be used wher the path is going to more than
one input i.e bus configuration.
I have seen this happen in too many designs and the result of double clocking
due to the non-monotonic edges of the clocks!!!
--
Regards, __________ James G Roberts
/___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
Jim __ / /___/ / jgroberts@xxxxxxxxxx
/ /_/ /---| | Room: BE436, Hilversum
\____/ /_/ Tel: +31 35 687 4308 Fax: 5976
Scott McMorrow wrote:
>
> I am in agreement with Ray.
>
> I have found that the best way to approach termination design is from
> the worst case overshoot perspective. Determine the worstcase overshoot
> that your bus can tolerate. Set up a simulation where multiple drivers
> in a package are driven in the even mode on a bus with parallel traces
> from the driver to the receiver. Adjust the termination resistance and
> the trace spacing to optimize the worst case fast process response at
> the receiver for overshoot that is within the maximum limit that you
> require, and crosstalk noise on a quiet line that is within your noise
> limits. (We include crosstalk and sso in the simulation experiment,
> since these will also cause some amount of overshoot at the receiver.)
> Optimize for the lowest series resistance in conjunction with the
> minimum allowable trace spacing. The optimization can be done through
> multiple sweeps, or through the Hspice optimize functions.
>
> Once you have found the minimum resistor tolerable for the design, you
> will need to increase it's value to compensate for % tolerance and
> select a value which meets your minimum tolerance requirement. At this
> point, you have designed the interconnect to be as fast as possible.
>
> best regards,
>
> scott
>
> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>
> Ray Anderson wrote:
>
> >All of what Jeff says is very true.
> >
> >One other thing about the series termination technique which has been
> >discussed on the si-list before, but which may bear repeating for new
> >members, is that by slightly "undersizing" the series resistor by a few (say
> >5 or less) ohms such that Zdrv + Rt < Znom of the transmission line you can
> >induce a small amount of overshoot at the receiver that has the net effect of
> >reducing the effective rise time of the signal and reducing the delay by a
> >small amount. You don't want to get carried away and make the series
> >termination resistor too small or you will induced too much overshoot and
> >ringing at the receiver.
> >
> >As an example, assume the output impedance of your driver is 25 ohms and you
> >are driving a 50 ohm line. A reasonable value for the series resistor might
> >be 20 or 22 ohms in lieu of the 25 ohms for a perfect match.
> >
> >Either set up a spice simulation and see what happens with a specific
> >value resistor or get out the scope and soldering iron in the lab
> >and try it there.
> >
> >-Ray Anderson
> >Sun Microsystems Inc.
> >
> >
> >
> >
> >>Typically, the series resistor is sized such that the transmitter
> >>impedance PLUS the series resistor match the trace impedance. This
> >>will be the reason why you are using a 33ohm resistor. If you check
> >>the imedance of your transmitter, you should find it is about 20ohms
> >>give or take a few ohms. This is typical in a LVCMOS type
> >>transmitter, I would believe. I'm sure folks will correct me if I am
> >>wrong.
> >>
> >>In this manner, you create a transmitter with the desired output
> >>impedance, so there is little or no impedance mismatch between the
> >>transmitter and the line trace impedance. For placement, it should
> >>be placed "as close as possible" to the transmitter. As a rule of
> >>thumb, it should be well within the rise time of the transmitter
> >>otherwise, you effectively have a transmission line between the
> >>transmitter and the series resistor which will give you two
> >>discontinunities rather than eliminating one.
> >>
> >>Hope this helps.
> >>
> >>Jeff Reynolds
> >>
> >>
> >>
> >
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--
Regards, __________ James G Roberts
/___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
Jim __ / /___/ / jgroberts@xxxxxxxxxx
/ /_/ /---| | Room: BE436, Hilversum
\____/ /_/ Tel: +31 35 687 4308 Fax: 5976
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