I want to sense ac current wave using Arduino and perform fft on it to obtain contents at diff frequency
You have my permission.
There is an FFT library (and an FHT library) for the Arduino. I've never used either one, but you can find lots of examples of people using FFT or FHT to make audio spectrum analyzer effects.
The ADC reads voltage so if you want to read current you'll need a Hall effect sensor or a current transformer, etc.
The ADC reads 0 to +5V, so if you want to read AC the input has to be biased.
If you are reading the power line, the Arduino will have to be electrically isolated from the power line, and of course you'll have to reduce the voltage.
…It that's not enough to get you started, we'll need some more details about your project.
My project is to make differential protection with harmonic restraint for a 2 kva transformer so for that I have to sense primary and secondary current , I have designed cT which 1100:1 ratio , hence my 10amp current reduces to 9 ma and by providing potentiometer at output I can convert is into nearly 2 volts ......
So my question is-
How can I bias two analog input pins for both the current ......?
And I need procedure to use fft
A resistor or two would work.
How to interface a CT sensor with Arduino
https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/interface-with-arduino