SN74AHCT1G08: Required VIH & VIL values for SN74AHCT1G08 at TA = –40°C to 125°C.

Hello Kedar,

V_IH and V_IL are recommended specifications for a good reason - there is no electrical specification for them.

Using V_IH as an example, if your input voltage is larger than or equal to V_IH, then you are guaranteed from TI that the device will recognize that signal as a logic 'HIGH.' If your signal is below that value, we cannot guarantee that the input will recognize the signal as a 'HIGH.'

Physically, the device will transition from a LOW input to a HIGH input somewhere around V_CC/2, which is called the 'threshold voltage,' or V_T in datasheets. You will note that we don't specify V_T for standard CMOS devices -- this value isn't particularly useful in operation since the input should be transitioning from LOW to HIGH in just a few nanoseconds, and the threshold value is inconsequential for the ouput timing when operating the device appropriately.

If you need a device that can support slow inputs, or one that has a well defined threshold voltage, those are two different devices -- for slow inputs, use a Schmitt-trigger input logic device.  For a specific threshold voltage, use a comparator.