[SI-LIST] Re: Source impedance?
- From: "D. C. Sessions" <si-list@xxxxxxxxxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 8 Jan 2002 23:11:01 -0700
On Tuesday 08 January 2002 15:38, David Kaiser wrote:
# Theory question. Assume perfect world with no parasitics.
#
# If a 50 Ohm trace on a board is terminated with a 50 Ohm resistor at the end
# of the trace, is there any advantage in terms of SI
# to driving the trace with a source that has 50 Ohm output impedance compared
# to a source with 10 or 20 Ohms of output impedance?
#
# It seems to me that the only thing the series source impedance would do to a
# 50 Ohm line that is terminated with a 50 Ohm resistive load would be to
# divide the amplitude in half without improving the signal integrity.
# Correct?
Half amplitude is pretty easy to live with, since most receivers are
happy with <200 mV (usually a LOT less). Matching the driver to
the line reduces the settling time for whatever reflections aren't
eaten by the far-end termination (e.g. media mismatches).
SLVS was scoped with exactly that in mind: start with 800 mV,
divide by two for source matching, divide by two again for losses
and margin, and you get 200 mV (the actual receiver spec is
160 mV). Meantime, garbage is damped at both ends.
--
| The race is not always to the swift, nor the battle to the strong. |
| Because the slow, feeble old codgers like me cheat. |
+--------------- D. C. Sessions <dcs@xxxxxxxxxxxxxxxx> --------------+
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