I. Introduction
In many control applications signals derivatives are required for the closure of feedback loops. When such derivatives can not be directly acquired through sensors, they are estimated by means of differentiators. In industrial ap-plications, the most widely used differentiators are based on backward difference approaches. The resulting signals are typically noisy, thus they are subsequently low-pass filtered. More advanced numerical differentiators are the ones proposed in [1] or [2]. The main advantages of such differentiators are represented by their low computational burden and easy implementation. Conversely, their main drawback is given by the undesired phase-lag affecting the estimates, which may worsen the performances of feedback control systems.