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SN74AHC1G86-Q1: Schmitt-Trigger Action

Egon Hirsch
Expert 3420 points
Part Number: SN74AHC1G86-Q1
Other Parts Discussed in Thread: SN74HCS86-Q1

Hello all,

About the NOR gate SN74AHC1G86QDBVQ1, why in the features TI write Schmitt-Trigger Action at All Inputs Makes the Circuit Tolerant for Slower Input Rise and Fall Time,

But also have input transition rise or fall rate request in 7.3 recommended operating conditions?

Best regards

Olaf

  • 0
    Guru 268700 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.

  • 0
    TI__Mastermind 26553 points

    Olaf,

    Clemens does a good job of covering the 'Schmitt-trigger action' distinction here.
    Do you need an automotive XOR gate? The automotive XOR gate with real Schmitt-trigger inputs would be the SN74HCS86-Q1. It'll be a 2 channel 2-input device. It covers the same voltage range but drive strength would be slightly less than the AHC device.

    Thanks,
    Rami

  • 0 in reply to Rami Mooti1
    TI__Expert 3420 points

    Hallo all,

    thanks for your answers. This helps a lot.

    One more question from my customer I want to verify with you:

    I can understand that the Input hysteresis will help for the slew rate.

    But why there have so much gap of slew rate between 150mV and 800mV input hysteresis?

    The 150mV need slew rate of <10 ns/V, but the 800mV don’t have any request.

    Best regards

    Olaf

  • 0 in reply to Egon Hirsch
    Guru 268700 points

    The pins and bond wires have a certain inductance and capacitance, which can result in oscillations even for fast signal edges. The 150 mV hysteresis is needed to overcome that. However, very slow signals can result in oscillations larger than that.

    The 800 mV hysteresis is enough for arbitrarily slow signals.