Comments on "On the structure of digital controllers with finite word length consideration"

In the above-mentioned paper by Li (ibid. vol.43 (1998)), a condition is presented as necessary and sufficient to give the optimal realization of state space controllers for minimal sensitivity of all closed-loop eigenvalues subject to finite word-length effects. The condition is shown here not to be sufficient. In addition, the condition for minimal sensitivity of a single closed-loop eigenvalue presented in the above-mentioned paper is not complete. The complete conditions are presented in this paper.