I want to use ADCMP551 as a high speed comparator . I am confused about output configuration. Is it possible to generate output level between 0 to 2.5v or close. Vcc is 5 volt.
No that's not possible. ADCMP551 has PECL/LVPECL differential output, the output swing is too low. Look at the principle schematic of a similar device. The output swing is set by the differential pair bias current and load resistors and can't be increased above the level given in the datasheet. It's however easy to reduce the swing or level shift the output to match it to LVDS receivers. You can use LVDS or PECL receiver with LVCMOS output to generate the intended output level.