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SN74AHC1G08: What is the schmitt trigger action?

Kazuya Nakai59
Guru 20760 points
Part Number: SN74AHC1G08
Other Parts Discussed in Thread: SN74AHCT1G08, , SN74LVC7001A, SN74HCS08, SN74LVC1G97, SN74LVC1G08

Tool/software:

Hello guys,

One of my customers is considering using SN74AHC1G08 or SN74AHCT1G08 for their new products.

At this moment, they have the following questions.
Could you please give me your reply?

Q1.
SN74AHC1G08 datasheet says on page 1 "Schmitt-Trigger Action at All Inputs Makes the Circuit Tolerant for Slower Input Rise and Fall Time".
What is this Schmitt Trigger action? I think this doesn't mean Schmitt trigger input which has hysteresis between rising edge and falling edge because the device doesn't have hysteresis spec.

Q2.
The sentence about Schmitt-Trigger Action is not described in SN74AHCT1G08(TTL level input version of SN74AHC1G08) datasheet.
Does SN74AHCT1G08 not have Schmitt-Trigger Action?

Q3.
Do you have any recommended device with Schmitt-Trigger input which has same function as SN74AHC1G08/SN74AHCT1G08?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    Guru 278840 points

    1. All modern logic families have a little bit of hysteresis at the inputs, but it's only 100 mV or so. Its only purpose is to work around the inductance of the pins and the bond wires, so there is no hysteresis that is guaranteed to be available for your circuit. And your edges are still required to be faster than the Δt/Δv limit. It is an implementation detail that has no effect on your circuit design.

    2. Both AHC and AHCT (and LVC/AUP/AUC etc.) have Schmitt-trigger action.

    3. The SN74HCS08 and SN74LVC7001A are four-channel devices. The SN74LVC1G97 has a different pinout. There is no Schmitt-trigger AND gate with TTL-compatible inputs.

  • 0 in reply to Clemens Ladisch
    Guru 20760 points

    Hello Clemens,

    Thank you very much for your reply.

    The customer measured shoot through current of SN74AHC1G08, SN74AHCT1G08 and SN74LVC1G08 with slow rising input signal.
    As the result, a shoot through current of SN74AHC1G08 and SN74AHCT1G08 were not observed.
    But a shoot through current of SN74LVC1G08 was observed.
    Is this difference related to Schmitt-trigger action?

    Also does SN74LVC1G08 have the schmitt-trigger action? Because the input transition rise or fall rate of SN74LVC1G08 is faster than SN74AHC1G08, and SN74AHCT1G08 one

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    Guru 278840 points

    All CMOS inputs have shoot-through current, even Schmitt-trigger ones. For LVC inputs, it's much higher, but it is also there for AHC(T) inputs.

    LVC inputs also have Schmitt-trigger action. Without it, the rise/fall rate limit would be even lower.