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RE: XAUI AC coupling

Title: RE: XAUI AC coupling

Rich & all,

You may count my vote as mandatory SUPPORT for AC coupling.

WHAT THAT MEANS is that input circuits have a weak DC bias generator such that if you DO use AC coupling, then a SERDES-style terminator for a transmission line trace will put the DC bias point at the optimum point for the ASIC.

By SERDES-style terminator I mean that for a differential pair (assuming 50 Ohm lines) you have two 50 Ohm resistors with one attached to each input. The resistors are tied together at the other ends to give 100 Ohms differential impedance, and this center tap is AC grounded. This has evolved into the "standard" terminator for SERDES inputs at 1 and 2.5 Gb/s.

This _allows_ you to AC couple from the driving ASIC and lets the receiver input bias itself to its favorite DC common mode level.

Now I would expect that this would be used for either long trace runs (transmission line rules) or where you need to have plug-in interchangeability, like, for optical transceivers, like.

BUT, if you are looking at a short ASIC-to-ASIC interconnect, then you could let the driving ASIC's outputs overwhelm this weak bias generator and set the DC levels if they are appropriate (i.e., compatible chips). Thus, for a densely packed board you could do without AC caps, series terminators and everything else if you are within the trace lengths where lumped parameter rules apply.

You get the best of BOTH worlds. Optimal performance if and when you use AC coupling and also DC coupling if the chips mate up and you need minimal parts count.

What's so darned hard about this? The 1.25/2.5/3+ Gb SERDES technology chips and ASICS _already_ do it! It works COOL!