Hi,

I would like you to help me understanding the working principle of "Precision AC/DC converter" circuit.
Also how to calculate the input and output.

Thank you,

 

Welcome to AAC!
Check the entry for "precision rectifier" in Wikipedia for a basic description.
As for the circuit shown, it has no power supply for the op-amps so won't work You have to specify, not calculate, the input to that circuit.

 

The basic principle is that the diode is in the feedback loop so the forward drop of the diode is reduced by the open loop gain of the op amp.
Since the open loop gain of most op amps is very high, the apparent drop of the diode is reduced to the microvolt level.