http://en.wikipedia.org/wiki/Successive_Approximation

I do not recognize the term "actual value conversion" foer A/D circuits. possibly you ment "direct conversion" (also known as Flash conversion A/Ds)?
This will give you an insight on the various technologies used in A/D conversion:

http://en.wikipedia.org/wiki/A/D#ADC_structures
Of course there are many ressources explaining A/D and D/A converters, with much more advanced information.

However, this does not apply to a digital input module, that only sees a 1 or a 0 on its terminals, not an analog value to be converted. The high speed for this module would be governed by the transistor circuitsused for each channel, probably.
Hope this helps,
Daniel Chartier