convert the analog signal to digital signal using A/D 
converter and give the resultant to PLC data lines

You are right Hmt,but you are giving the digital signal only
read the question carefully

Digital Signal :- for this purpose use zero speed controller
to measure the 0-1400RPM.
in this controller there is one delay & one instant contact 
The proximity switch sense 0-200RPM speed gives signal to
Controller ,controller gives the instant relay contact to
PLC as Digital signal as Motor is Runing  if motor reach to
the statable speed with the time period the controller give
the delay contact to PLC as digital signal.

Analog Signal:- For the purpose use encoder inside the motor
shaft with give conitious pulse with every roation in the
form of (4-20mA & 0-10v)with respective to )-1400RPM or
whatever speed we configure to PLC as Analog Signal