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Original question:

SN74AHC08: With Schmitt trigger input or not

SN74AHC08: Oscillation occur

Zoe Ko
Expert 3930 points

Part Number: SN74AHC08

Hi team,

As the thread you reply in below, the SN74AHC08 is with Schmitt trigger input and the range is 200mV.

But still confuse that from your post, when VCC=3V, VIL and VIH are 1.5 and 1.7V.

In the SN74AHC08 datasheet, VIL and VIH are 0.9 and 2.1V and between 0.9~2.1V, they are unknown or they are Smitt trigger range since I cannot see the spec from the datasheet?

https://e2e.ti.com/support/logic/f/151/p/852977/3154580#3154580

Also, for the SN74AHC08 operated as below conditions and waveforms as below pictures.

You can see Y is oscillate.

Does that cause the SN74AHC08 damage if usually under the conditions even that's no over absolute max rating?

VCC=3.3V

A is yellow waveform (SR is less than the datasheet max value), B is 3.3V.

Y is blue waveform.

Thank you!

  • 0
    Guru 326790 points

    The recommended operating conditions limit Δt/Δv (the Input Transition rise or fall rate) to at most 100 ns/V. That curve is slower.

    To quote the previous quote:

    If signals with considerably longer rise and fall times are processed, the specially developed Schmitt triggers, such as the SN74AHC(T)14, should be used. These components have a considerably larger hysteresis of about 800 mV at VCC = 5 V and, therefore, allow processing of very slow edges without any problems.