we know the LVPECL output common voltage is 2V ,but according to this picture. Isn't the common mode voltage is 1.3V? I'm confused becase it is TI's document.(cannon find document in ADI) Thanks for anyone who can help me to solve this problem.
You are correct that the voltage divider will result in a 1.3V common mode voltage, but only if there is no power applied to the LVPECL driver.
LVPECL drivers are open emitter, and for normal operation require a DC current path to a voltage that is always lower than Vol of the LVPECL driver. In this case, it's 1.3V, and we choose this voltage because Vol of the LVPECL driver (which is normally about 1.6V and tracks Vcc) will never drop to lower than 1.3V even with Vcc and temperature variation.
When the LVPECL driver is powered on in normal operation, current will flow from the LVPECL driver though the pull-down resistor. Because of this additional current, the classic voltage divider relationship (in which equal current flows through both resistors) is violated, and the common mode voltage becomes ~2V.
Thanks for your help! Thanks ADI's engineers.
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