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Before discussing data conversions, it helps to understand the number systems used, why they're chosen, and how fast conversions must be made to be meaningful.
Definition: Analog quantities are those that are (or appear to be) continuously variable. Examples include electric potential, electric current (when the current is high enough that the granularity of individual electrons isn't important), concentration (when high enough that counting individual molecules isn't important), light intensity (when counting individual photons isn't practical), speed, and time.
Digital quantities are those that are represented by integers. Any counted quantity (number of photons, electrons, or ions) is inherently digital. Analog quantities can only be approximated by digital numbers. For example, time can be reported as an integer number of years or nanoseconds (different granularity), but if time is continuous any specific number of time units has forced granularity onto the continuous flow of time.