thank you very much for all the Reply before any

ince nothing to fear I will not make a practice just a theoretical study to measure AC voltage through a 16F877.

my point of view eg for a maximum voltage of 400V is equivalent to 5 v or there is a report 8O my principle is:
5Vmax ===> RMS 3.535
ie to determine the max value of real power will be multiplied by K = 80 * √ 2
I want to know what are the constraints to purpose to have a good accuracy as for example by taking 1Vmax image 160V max formula Trap for me was giving good results to small voltages such as 0.5 max I have an rms of 0.7071??
for sampling cobien I must take samples at least? to measure in a period of time knowing that the signal input 50HZ and I will work on adc = 10
I had to commit to measure with good accuracy as the RMS value of voltage between 0 and 5v?