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SN74ABT2244A: Hysteresis specification

Steve Widener
Genius 9765 points
Part Number: SN74ABT2244A
Other Parts Discussed in Thread: SN74LV8151, SN74HCT244

The SN74ABT2244A has a typical hysteresis specification of 100mV at 25C form the SCBS106E datasheet. This is specified along with the output specifications rather than the input specifications where it would normally be specified. Please confirm that the Vhys specification does in fact apply to the input Vih & Vil logic levels.

  • 0
    TI__Guru* 96621 points
    Hey Steve,
    I see you're still on the search for a buffer with hysteresis. Is the customer trying to find a fix without modifying their boards?

    Yes - the Vhys spec is applicable to the inputs, although it only provides a typical spec.

    It's important to note that the device also has a listed input transition rise/fall time of 5ns/V which indicates to me that the device doesn't have Schmitt-trigger structured inputs and thus that hysteresis is basically coincidental to the design (I can't say for sure if it was intentional since the device was design many years ago). Many CMOS inputs have this bit of 'wiggle room' on their input thresholds, but it won't replace a device with designed in hysteresis like the SN74LV8151.

    They could switch to something like an SN74HCT244 that has a much lower switching threshold -- avoiding the non-monotonic edge by shifting the threshold instead of adding hysteresis. Another option would be to try to impedance match the input to eliminate transmission line effects.
  • 0 in reply to Emrys Maier
    TI__Genius 9765 points
    Yes - thank you for the clarification. Yes, the customer is trying to fix this without PCB modification if possible. We have suggested a few options but I will pass along the idea of using the HCT244.