A question about the book writed by Allen

[wjxcom]

hello, all:

please see the book writed by Allen, in page 271, in the third paragraph, there has a sentence:"If the slew-rate specification is not given, then one can choose a value based on settling-time requirements. Determine a value that is roughly ten times faster than the settling-time specification, assuming that the output slews approximately one-half of the supply rail"

I do not understand why there have a ten times faster than the settling-time specifiation.

 

[Humungus]

Settling time is determined by the slewing phase plus the exponential settling. Normally settling is specified by a time for a given error percentage. An exponential signal needs about 5 time constants to settle to less than 0.01%. By making you slewing time much smaller than the exponential settling, it is the latter that dominates the total settling. Anyway, it is many times better to have longer slewing phase, compared to the settling specification, so that the exponentila settling starts from a closer to final value.