In this article, we propose a new observer design for systems whose output is sampled in an aperiodic and asynchronous way and for which a continuous-time homogeneous observer of negative degree is available. The proposed method consists in adapting the existing continuous-time observer in order to cope with the fact that only sampled measurements are available instead of continuous ones. The obtained observer error is shown to be globally uniformly ultimately bounded for any upper bound on the sampling periods. Furthermore, the ultimate bound decreases as the upper bound on the sampling periods decreases. The stability analysis is based on a Lyapunov approach and the performances of the proposed observer are illustrated with simulations.