How to understanding min/max input values of Schmitt Trigger from datasheet

I am having some difficultly understanding the values described in this product sheet for the SN74AHCT14 Schmitt Trigger Inverter:

http://www.ti.com/lit/ds/symlink/sn74ahct14.pdf

Given a Vcc of 4.5V, I would like to know the following:

• What is the lowest input voltage that would cause the trigger output to switch from low to high output?


• What is the highest input voltage that would cause the trigger output to switch from high to low output?

And where can I find this in the product sheet?

Answer

The parameters you're looking for are V_T+, "Positive-going input threshold voltage", and V_T-, "Negative-going input threshold voltage". They are given in section 7.5, "Electrical Characteristics".

They have minimum and maximum values, and the appropriate boundary must be used for the given condition. The lowest voltage that would result in a guaranteed switch from low to high would be the maximum value of V_T+ at 4.5V, which would be 1.9V. Likewise, the guaranteed high to low transition voltage would be 0.5V. It is possible to characterize individual parts to get more accurate values, but those are the values you can depend upon across the board.