[SI-LIST] : Split supply termination

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Sep 18 2000 - 14:21:02 PDT


I have a board, where I have two separate 3.3V Power supplies. Let's
call'em A and B.

I have a driver, powered by 3.3V_A. It will drive a long line, with
loads powered by 3.3V_B.

I need to terminate this line using split (Thevenin) termination on the
3.3V_B end (there's no supply A plane in that end of the board).

There is a common GND plane.

The questions are:

1. If I want to use Thevenin termination, the resistors will connect to
3.3V_B and GND, while the drive is powered by 3.3V_A to GND. How exactly
the return current will go (I understand, The current will have to jump
some caps). And how will it affect termination quality?

2. This is probably the only trace, crossing the boundary A/B. Do I
place some de-coup. caps at the boarder where the trace crosses and
again, will it affect termination badly?
This is a distributed capacitance line and bad enough as it is.

PS. So far I used slow rise time, made by use of RC at the driver's
output (the period is slow). But it is a revolting practice and I want
it real clean. Despite the signal being slow, the distortions because of
some problems gave me double-clocking in the past. So I research the
best termination possible :-))))

Mike.

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