current transformer and its connections to the microcntroller(8051)
You may use a resistor + an OPA to measure the AC current. However, there are a few things that make this less desirable:


a resistor + an OPA is not galvanically isolated (the current transformer is),
the power lost on the resistor will limit how large signals you can get, and the offset voltage of the OPA will limit how small signals you can detect - these two together will limit the usable range of your measurements (the current transformer can amplify the signal level, and still represent only a very little load to the circuit to be measured).


If I were you, I'd use a current transformer, and some specialized IC (like the ADE7753 and its relatives). See this project: http://www.delorie.com/electronics/powermeter/