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SN74HCT14: Output not as expected - No Hysteresis

john van strien
Prodigy 20 points
Part Number: SN74HCT14
Other Parts Discussed in Thread: CD74ACT14

Tool/software:

I have an oscillator circuit that consists of a 10MHz oscillator, an SN74HCT14 and three 74HC390 dividers. The osc. is 3.3V and everything else is 5V. I'm using the '14 to level shift and clean up the osc signal (yellow trace). The level shifting is happening and the hysteresis is not - at all. I've got the first '14 output fed into a second one. The output of the first looks identical to the output of the second (green trace).

From the datasheet, I would expect the '14 output to essentially be 0V or 5V, not a sine wave. Why does the output look identical to the input vs a square wave? Why is there no hysteresis here?

FWIW, I included three scope images because when I have both probes connected, the wave forms change somewhat (as you can see from the second image), which is a question for another forum.

Thanks.

  • 0
    Guru 287110 points

    Please show the schematic.

    How are you trying to measure the hysteresis?

    HC outputs have a very low drive strength. I do not know if your test circuit has the same capacitive load as your intended real application, but if it has, try the CD74ACT14 instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Thanks for responding.

    Clemens Ladisch said:
    How are you trying to measure the hysteresis?

    I'm just looking at the scope. I see from the scope image that the rise and fall times for the output are about 20ns. Is that what you would expect from this table from the datasheet? i.e. is the tt parameter the rise and fall times? 

    Clemens Ladisch said:
    HC outputs have a very low drive strength. I do not know if your test circuit has the same capacitive load as your intended real application, but if it has, try the CD74ACT14 instead.

    I'm using the HCT part and only driving one input of the 74HC390 divider. I'll look at the ACT part.

    Maybe I can just use the 74HCT390 (I didn't see a 74ACT390) and do away with the '14.

    Thanks.

  • +1 in reply to john van strien
    Guru 287110 points

    The rise/fall times are valid for the specified capacitive load. The actual load will be higher if you're using a breadboard or cables. (Are you?)

    The ACT device would output sharper edges, but it does not look as if your current waveforms are actually problematic.

    Connecting oscilloscope probes should not affect the circuit in this way. If the schematic is correct, then I'd guess that you should add decoupling capacitors.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Most of the circuit is on a PCB (including decoupling caps that I didn't show on the schematic). I had something else between the osc and the first '390. I removed that and inserted the '14 in its place, connecting it to the existing circuit with short (< 1") soldered WW wires.

  • 0
    TI__Mastermind 20080 points

    Hi John,

    These signals actually look pretty ok for a HCT device operating at 10MHz. Cleaner/sharper edges are dependent on capacitive load, drive strength, and signal integrity adjustments like Clemens has already said, but there is nothing unusual about the waveforms you've posted. I've seen similar performance at 10MHz from a majority of our buffers in lab.

    Best,

    Malcolm