RE: [SI-LIST] : Return path for stripline, two ground planes or one power and one ground?

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From: Mark Nass (markn@serverworks.com)
Date: Mon Apr 03 2000 - 12:05:32 PDT


Assuming you had an 8 layer board with the drivers described below.
What
stackup would you recommend, 1 or 2 below? Stackup #1 would seem to be
better
for lower power plane impedance(assuming 4-4.5 mils between planes).
This
would also provide good board capacitance and allow the AC currents to
switch layers
as needed for the reasons you discuss below. The application I am
thinking
of, DDR, would also have DIMM connectors on this board and the signals
would need to
pass thru connectors. It seems like board capacitance of stackup #1
would help
AC return currents that may have to switch layers near the DIMM
connectors also.

............ Stackup 1 ..................
Top --
GND ------
PWR ------
S1 --
S2 --
GND ------
PWR ------
Bot --

............ Stackup 2 ..................
Top --
GND ------
S1 --
PWR ------
GND ------
S2 --
PWR ------
Bot --

............Text deleted ...............

>With a push-pull driver, you will get different currents through the
>driver's power and ground pins depending on whether it switches low-to-high
>or high-to-low, and depending on the load and the previous voltage.
>
>One special case is when the driver is symmetrical, and the load is
>terminated on the far end to Vdd/2. Let's say it's a 3.3V CMOS driver and
>it drives 50 ohm traces. A full rail-rail output would switch 66mA through
>the signal pin. If terminated to Vdd/2, half of this delta-I, or 33mA, is
>from the power pin and the other half is from the ground pin (one going from
>0mA to 33mA, the other going from -33mA to 0mA, or vice-versa).
>
>In this case, physically symmetrical stripline between power and ground
>planes would be best, because the return currents (which are equal in the
>two planes) would match the switching currents in the driver IC's power AND
>ground pins. In theory, no switching current would flow through bypass
>caps.
>
............Text deleted ...............

  
>
>Regards,
>Andy
>
>
>
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