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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Tue Apr 04 2000 - 12:10:55 PDT


"Ingraham, Andrew" wrote:

> 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.
>

Are you implying that systems using SSTL/HSTL signals with terminations to VDDQ/2 can operate without VDDQ bypass caps if the lines are implemented as symmetrical striplines between power and ground?

Vinu

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