The input to the circuit is current source fed through coaxial cable , can anyone explain me how output is calculated, Thanks in advance

 

Where is the current source? Where is the coaxial cable?

 

Where is the current source? Where is the coaxial cable?

The FCT in is the input source, the fast transformer current output is carried through coaxial cable from field.

 

The current through the CT divided by the CT turns ratio multiplied by the burden resistor multiplied by the gain of the amplifier.
The burden resistor in your case is 2Ω.
The part number of the amplifier is cut off, so I don't know its gain.
By the way, the TLV333 is limited to 5mA output, so might not be the greatest to drive a cable screen that could be miles long.
I would suggest that you use an amplifier which has a Common mode input range that includes ground and connect the burden centre tap to proper ground.