I current have a Schmitt trigger chip to clean up an output from one of my optical encoders and I am thinking about using an XOR gate to feed in both encoder outputs to double my encoder output frequency. (Its a quadrature encoder)

When I was looking up XOR gate ICs, I saw in the data sheet it literally saying

  • All inputs have a Schmitt-trigger action

So does that mean I can fully do away with the standalone Schmitt trigger IC and the XOR gate will take its place as well as being and XOR?

It'd just when it says "action" it doesn't sound as robust as if it were to say all inputs have Schmitt triggers.

The IC is 74AHC86


1 Answer 1


According to TI's "AHC/AHCT Designer's Guide February 2000", Section 2, "DC Characteristics", subsection 1, "Input Circuit", interfaces with lower-speed circuits with a lower slew rate can cause oscillation at 74AHC/T inputs. To prevent this, 74AHC/T input stages contain a feedback loop which causes the input to have hysteresis much as a Schmitt trigger input would without containing all the circuitry for an actual Schmitt trigger input.

AHC/T input stage

However, the guide goes on to say:

Hysteresis in the input circuit is intended only to process reliably signals that have a slew rate of <10 ns/V. With a signal swing of 5 V, this corresponds to rise and fall times of about 50 ns. If signals with considerably longer rise and fall times are processed, the specially developed Schmitt triggers, such as the SN74AHC(T)14, should be used. These components have a considerably larger hysteresis of about 800 mV at VCC = 5 V and, therefore, allow processing of very slow edges without any problems.

  • \$\begingroup\$ Intresting. So for my case, the 74AHC86 will do both, but not any of XOR just this one? Shouldn't there be information in the datasheet in regards to the hysteresis? \$\endgroup\$ Commented Mar 17, 2014 at 22:54
  • \$\begingroup\$ Generally for digital parts you're expected to look at the user's guide for the family to see the waveforms, since they are usually invariant across the family. \$\endgroup\$ Commented Mar 17, 2014 at 22:56

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