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Original question:

Schmitt-trigger inverter with V_T+ (rising) max of <= 1.8V with VCC = 5.5V?

SN74AHCT1G14: Vin = 1.0 V Square Wave is not resulting in a Vout = 5.0 V Square Wave for Vcc = 5.0 V

Tarunyaa Sivakumar
Prodigy 10 points
Part Number: SN74AHCT1G14
Other Parts Discussed in Thread: SN74AHC1G14, SN74LXC1T45
So I've been simulating the SN74AHCT1G14 on LTSpice. According to the data sheet, it is approximately Vt+(min) = 1.0 V and Vt-(min) = 0.55 V for Vcc = 5.0 V. For that reason, when Vin = 1.0 square wave, I expect the Vout to be a 5.0 V square wave. This is because the component is a Schmidt trigger inverter that will switch the edge of the output waveform every time the input waveform hits Vt+(b/w min and max) or Vt- (b/w min and max). However, when I simulate SN74AHCT1G14 on LTSpice, Vout is just a flat 5.0 V line. It's only when Vin is a 3.0 V square wave that the Vout is a 5.0 V square wave. Even when Vin is a 2.5 V square wave, Vout is still a flat 5.0 V line, even though the Vin is clearly hits Vt+(max) or Vt- (max). Why is this happening?

  • 0
    Guru 316400 points

    The SN74AHCT1G14 model uses the same switching thresholds as the SN74AHC1G14. This is an error in the model; the actual device behaves correctly.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Ah I see. Is there any way to get the correct simulation model? I really want to test it beforehand because 1.0 V is the minimum value for Vt+ when Vcc = 5 V and I want to ensure that the component can output a 5.0 V square wave from a 1.0 V square wave before I order it… unless you’re sure that this component would be able to perform the behaviour I want or if there is any other component with a lower Vt+ (I wasn’t able to find any).

  • 0 in reply to Tarunyaa Sivakumar
    Guru 316400 points

    The worst case you have to design for is the maximum threshold, i.e., the input signal must reach 2 V.

    For lower voltages, you need to use a level translator (e.g., SN74LXC1T45). If the input signal is not digital, consider a comparator.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Correct me if I’m wrong but shouldn’t the inverter be able to flip the output voltage for anything between the minimum and the maximum threshold? Or is it that the particular component might have a specific threshold between the minimum and maximum value in the data sheet?

    Just need clarification on what exactly the maximum and minimum value in the data sheet means effectively and operationally.

  • 0 in reply to Tarunyaa Sivakumar
    Guru 316400 points

    Manufacturing and temperate variation can shift the thresholds. The datasheet guarantees only that the actual threshold is somewhere between the minimum and the maximum.