Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
The driver should act as a switched current source
with a common-mode voltage that is set by feedback
when you're talking LVDS. LVPECL assumes an external
termination network. The LVDS independent common
mode control makes termination simple (differential
only) and minimizes wasted power, which a resistor
divider external creating (say) VTT @ 50 ohms
from 5V is going to burn about 200mA per output-pair,
a sweet watt of power going nowhere useful. The CM
feedback (w/ reference) is what allows supply independent
(or, quite usefully, cross-supply-domain) operation specs.