You mean that I should use a 5V zener diode to regulate the voltage?You don't need another 7805 to generate the VREF voltage. What you'll need is a 5V reference diode/module that is small and precise. You have to ask someone here for a particular part number. I only know of TL431 but there are more modern parts I don't know of that would do just the same function.
I am located in Sao Paulo - Brazil...The 741 Opamp will not function with just 5V total supply voltage. You'll need a rail-to-rail Opamp for this one. Or you can power the Opamp from the +12V supply. In this case, you are correct to add a 5V voltage clamp to the Opamp output.
It is difficult to recommend a part if you do not have your physical location filled in so we know which part of the world you're in. Also the transformer connection is missing a return path and your signal conditioner is "unusual" in that you have use the offset control pin to bias the Opamp output. People usually don't use that method but condition the Opamp inputs instead. Again you will need to ask for a suitable Opamp circuit here.
Yes it is 4V pk-pk for the 120V AC input.... What software did you use to draw this simulation that you made?Your goal is to produce a +/- voltage swing at the Opamp output upon a fixed the 2.5V reference level for input to the ADC.
What is your design opamp output voltage pk-pk at the output of the Opamp? 4V pk-pk for 120V AC input?