SN74AHC123A: What is the threshold level and hysteresis of the Schmitt-Trigger logic inputs?

Harald Hellmann

Part Number: SN74AHC123A

The actual data sheet (October 2005) is talking about: "Schmitt-Trigger circuitry on /A, B and /CLR inputs for slow input transition rates". Later on standard logic input levels for high: VIH ≥ 3.85V @ Vcc = 5.5V and for low: VIL ≤ 1.65V @ Vcc = 5V is given. Do these levels correspond to the threshold level with hysteresis? Thank you in advance for any kind of helpful information.

over 6 years ago

0 Dylan Hubbard over 6 years ago

TI__Mastermind 44665 points

Hi Harald,

There should be specs for positive/negative threshold and hysteresis for parts with S.T inputs. Let me do a little digging to figure out why those aren't included and i'll get back to you. Thanks!

0 Harald Hellmann over 6 years ago in reply to Dylan Hubbard

Prodigy 80 points

The answer suggested to get some more information but even after eight more days I didn't get it.

0 Dylan Hubbard over 6 years ago in reply to Harald Hellmann

TI__Mastermind 44665 points

Hi Harald,

My apologies I didn't mean to hit the resolved button, it closed it on my end so I wasn't able to track it. I do have the answer now. So this device does have schmitt trigger inputs, but we just don't spec the hysteresis. The VIH/VIL will be your input levels for this device. However, you can still have a slow or noisy input signal without having any issues since this device does have schmitt trigger inputs.

0 Harald Hellmann over 6 years ago in reply to Dylan Hubbard

Prodigy 80 points

Hi Dylan,

thank you for your work but I think you understand that this answer is not very helpful. Even is we measure some parts we don't know if the next production will have the same levels.

0 Emrys Maier over 6 years ago in reply to Harald Hellmann

TI__Guru* 96842 points

Hi Harald,
If you have a need for a very precise input thresholds, it's best to add an analog comparator that you can control through external components before the logic input. Any logic device with Schmitt-trigger inputs will have a pretty broad range of input threshold values and we can't guarantee that they will be exactly the same from one component to the next (especially from one year to the next, or one manufacturing plant vs another).

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SN74AHC123A: What is the threshold level and hysteresis of the Schmitt-Trigger logic inputs?