Aliasing is a source of error most commonly thought of in reference to the analog to digital conversion process in data acquisition systems. Converting an analog input signal into its digital equivalent involves a "sampling" process in which a digital value is created to represent each sequential sample's "snap shot" of the input signal. The rate at which these samples are taken directly effect the maximum input frequency that can be converted. As the frequency of the input signal increases, the ability of the samples to reliably and accurately capture sequential portions of the input signal decreases. A theoretical upper limit of the input signal frequency is 1/2 that of the sample frequency (Nyquist), but in practice a ratio of 1/4 is typical and 1/10 considered ideal. The obvious approach of using high sampling ratios (often termed "oversampling") is limited by factors such as the A/D's conversion speed and the resultant high digital data rates (especially when serial data paths are required). Most often, analog pre-filtering (often called "Anti-Alias Filtering"), and digital post-filtering are applied to the conversion process to optimize cost and performance.