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SN74AHC1G08: Δt/Δv Input transition rise or fall rate

cafain
Mastermind 6655 points
Part Number: SN74AHC1G08

HI, staff

I have a question about SN74AHC1G08.

On page 1 of the data sheet (SCLS314P)
It says "Schmitt-Trigger Action at All Inputs Makes the Circuit Tolerant for Slower Input Rise and Fall Time".

It is considered that this is not affected by the rise and fall times(Δt/Δv) of the input signal.

However, the Δt/Δv Input transition rise or fall rate item of Recommended Operating Conditions on page 4 says 100ns/V (Vcc=3.3V±0.3V).

Please let me know if it is necessary to specify Δt/ΔV with a Schmidt device.

best regards
cafain

  • +1
    Guru 306790 points

    "Schmitt-trigger action" is not the same as a real Schmitt-trigger input.

    The AHC application report says:

    Input hysteresis of typically 150 mV is included in AHC devices, which ensures the devices are free from oscillations by increasing the noise margin around the threshold voltage during low-input transitions.

    The AHC Designer's Guide says:

    Hysteresis in the input circuit is intended only to process reliably signals that have a slew rate of <10 ns/V. With a signal swing of 5 V, this corresponds to rise and fall times of about 50 ns. 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.