*I'm rewriting the question after I deleted the previous one because I wasn't clear enough. Hope it's fair
Given a system of the general form:
\begin{align} x_{[k+1]} &= A\,x_{[k]} + B\,u_{[k]} \\ y_{[k]} &= C\,x_{[k]} + D\,u_{[k]} \end{align}
I would like to know how I should place the poles for the closed-loop observer system $A−L\,C$. I know that the observer has to be faster than the real system poles so the poles of $A-LC$ should be more close to zero than $A+B\,K$. But I don't know about any other constraint of the position. If there isn't any other bound why we don't place the poles at 0 and make $A-L\,C$ converge in one step?