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SN74LVC1G14: Threshold symmetry

Alfred Fuchs1
Intellectual 320 points
Part Number: SN74LVC1G14
Other Parts Discussed in Thread: SN74LVC2G14, SN74AUP2G14

Given that the positive and negative switching thresholds have a pretty high tolerance, how safe is the assumption that they are symmetric around the actual swithing threshold of the internal logic?

It is unknown how the hysteresis is implemented, but I'd guess that it is some resistive feedback network.

In that case I'd expect symmetry, but there are reports about anomalous negative thresholds at least above 3V.

Background: I want to so a simple RC-Oscillator, but the duty cycle should be near 50%. I.e. the thresholds ought to be roughly symmetric around Vcc/2.

Same question applies to SN74LVC2g14 (of course) and also to SN74AUP2G14.  

  • 0
    TI__Guru* 96591 points

    Hi Alfred,

    Unfortunately, I can't share the exact internal structure of our device, however I can say that the 'standard' way to build a Schmitt-trigger in CMOS is like this:

    Because the feedback network relies on both the N and P doping processes, there can be variations that result in unbalanced input thresholds. It's not common to see this - but I can only guarantee things within the datasheet specs.

    If you're looking to keep exactly 50% duty cycle (or very near it), I would recommend a different method of building your oscillator.  One that's time tested is the unbuffered inverter oscillator: