signal uncorrelated with delta in the origin I have a stochastic process completely uncorrelated. Why the autocorrelation function has a delta of dirac in the origin?
Which is the reason of that?

$R_{XX}({\tau})=A{\delta(\tau)}$

where $R_{XX}$ is the autocorrelation function of $X$ process and $A$ is a constant dependent by process spectrum
It has a delta at the origin because you are multiplying your time series with itself (i.e. there is no time delay).  Any non-zero time series correlated with itself will give a positive value.