This technical note is concerned with stability analysis of aperiodic sampled-data systems. The stability problem is tackled from a pure discrete-time point of view, where the at-sampling behavior of the system is modelled as the response of a nominal discrete-time LTI system in feedback interconnection with a structured uncertainty. Conditions under which the uncertainty is positive real (PR) are identified. Based on the PR property, a number of integral quadratic constraints (IQC) are derived and the IQC theory is applied to derive stability conditions. Numerical examples are given to illustrate the effectiveness of the proposed approach.