pi/2 lag signal generation

Hi Manesh!

Anyway you need to define the period before making the shift. If you do not know the signal's frequency then the notion of "a shift of 90 degrees" is undefined. In my opinion in general your algorithm might be following:

Input Signal> ADC > Determining the period T of waveform (for example through the method of "the zero crossing") > Shift T/4 > DAC> Output Signal.

Regards,

Igor

Can achieve the phase shift just by storing 1/4 of the samples and use the oldest one. Say sampling rate is 200/ cycle, means at every 20ms/200 = 100us (10KHz) for a 50Hz case. 
Allocate a buffer of 50 size, keep on storing the values (pointer 0-49), use the oldest one before over writing it as the phase shifted signal. For best results vary the sampling time to get always 200 samples per cycle.
  
Hope it is clear