**From:** Shayle Hirschman (*shayle@mho.net*)

**Date:** Fri Feb 25 2000 - 08:49:25 PST

**Next message:**Vinu Arumugham: "Re: [SI-LIST] : Zener used to clamp Vcc?"**Previous message:**WAUGH,RAY (HP-SanJose,ex1): "RE: [SI-LIST] : Zener used to clamp Vcc?"**Maybe in reply to:**Shayle Hirschman: "[SI-LIST] : Fast edge termination choice"**Next in thread:**Peterson, James F (FL51): "RE: [SI-LIST] : Fast edge termination choice"

That's interesting, Andy. Can I infer, then, that EMI or perhaps coupling

is reduced more by series source termination since the voltage and current

are halved near the source?

Shayle

At 05:19 PM 2/25/00 -0500, you wrote:

*>>Does series source termination defeat the purpose of selecting the fast
*

*>>slew rate option? Will it slow the edge transition time down, or will the
*

*>>signal still transition and travel as fast as if there was no termination,
*

*>>but simply be divided (thereby depending on the reflection at the
*

*>>destination)?
*

*>
*

*>If you imagine driving ideal transmission lines, then there is no difference
*

*>on edge rates of the voltage at the far end of the line.
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*>
*

*>But with a small capacitive load, series termination would slow the edge
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*>rate down a little.
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*>
*

*>If the line length is small, you have an RC network with some time constant.
*

*>Adding series termination increases R. Adding parallel termination reduces
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*>R somewhat.
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*>
*

*>For a long line, it's more complicated. But if you looked just at the load
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*>end and small time intervals compared to the line delay, your time constant
*

*>at that node is Zo*C without parallel termination, Zo*C/2 with it.
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*>
*

*>> Could it be that it
*

*>>reduces EMI, not by slowing the edges down, but rather just simply because
*

*>>it terminates? And that, therefore, as far as EMI goes, parallel
*

*>>termination would have a similar EMI result?
*

*>
*

*>Either termination can help, but there is more to it than just that.
*

*>
*

*>Series termination reduces the size of both the voltage steps (delta-V) and
*

*>current steps (delta-I) of each pulse. The signal driven into the line is
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*>approximately a half-sized replica of what it would have been even with
*

*>parallel termination. dv/dt and di/dt are about half. At the end of the
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*>line, the voltage doubles, so the magnitude (and dv/dt) there are the same
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*>as they would be for parallel termination, except for capacitive loading
*

*>effects noted above. di/dt at the load end is nearly zero for series
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*>termination.
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*>
*

*>Regards,
*

*>Andy
*

*>
*

*>
*

*>
*

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**Next message:**Vinu Arumugham: "Re: [SI-LIST] : Zener used to clamp Vcc?"**Previous message:**WAUGH,RAY (HP-SanJose,ex1): "RE: [SI-LIST] : Zener used to clamp Vcc?"**Maybe in reply to:**Shayle Hirschman: "[SI-LIST] : Fast edge termination choice"**Next in thread:**Peterson, James F (FL51): "RE: [SI-LIST] : Fast edge termination choice"

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