This paper describes a technique for the estimation of aperture uncertainty in A-D converters. In the proposed methodology the aperture uncertainty is estimated by measuring the total system jitter, the analog input signal jitter and the sampling clock jitter. The total system jitter measurement is based on the degradation in the signal-to-noise ratio performance of the converter. The timing jitter of the analog input signal and the sampling clock are estimated from their phase noise measurements. Also, the model accounts for the converter's quantization and differential non-linearity noise, which are critical to accurate aperture uncertainty estimation. The technique was applied to a 10-bit converter and the results are discussed.