enter image description here

Above is the unshaped input pulse train. Red line is the set point for the comparator.

I see many pulse conditioning and sharpening circuits use Schmitt triggers instead of comparators.

Imagine the input pulse train is not sharp enough so we need to make it sharper. There are some applications in industry where you need sharp pulses to count frequencies. My question is:

Why not only to use a comparator? Is that because rising and falling edges are sharper in case of Schmitt trigger? Could one explain it in an illustrative manner?

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    \$\begingroup\$ very rarely will you find a comparator without positive feedback, thus you are using a schmitt trigger. And the advantage of a schmitt trigger over a non-positive feedback comparator is to eliminate multiple edges at point of threshold crossing. \$\endgroup\$ – JonRB Jan 25 '16 at 22:37
  • \$\begingroup\$ Noise rejection is the name of what @JonRB said.. \$\endgroup\$ – Eugene Sh. Jan 25 '16 at 22:42
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    \$\begingroup\$ Yo will never get a perfectly smooth signal, so you can't (usually) assume monotonically rising/falling line near the crossing points. \$\endgroup\$ – Eugene Sh. Jan 25 '16 at 23:03
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    \$\begingroup\$ Yes, that's the thing \$\endgroup\$ – Eugene Sh. Jan 25 '16 at 23:05
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    \$\begingroup\$ Take a look here. Slide 13 has the illustration. \$\endgroup\$ – Eugene Sh. Jan 25 '16 at 23:06

If you're feeding a digital input, a Schmidt trigger is preferable in most cases because it will clean up the signal and match your logic levels. It will also help avoid meta-stablity (ie when the voltage level is right in the middle of on and off, its bad). A comparator could possibly give you metastable values if the amplifier is fast enough and if your digital input is clocked really fast or an interrupt. It also depends because a comparator has only one voltage value and a schmidt trigger has two points when it transistionsenter image description here

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  • \$\begingroup\$ when would you prefer a comparator to a Schmitt trigger? can u give an application example? \$\endgroup\$ – user16307 Jan 26 '16 at 9:29
  • \$\begingroup\$ Depending on the input signal if it was beyond the rails of my digital vcc it may be advantageous to switch to a comparator. Comparators have a different input structure with generally higher impedance instead of gates on a schmitt trigger. It depends on how they are built, you can actually make a comparator behave like a schmitt trigger but you have to add external components. \$\endgroup\$ – Voltage Spike Jan 26 '16 at 18:50
  • \$\begingroup\$ but im already telling you the spesific input \$\endgroup\$ – user16307 Jan 26 '16 at 18:51
  • \$\begingroup\$ I don't see any specific voltage level on that graph, is it 50V is it 3V? What do you want the output to be? Do you have any timing requirements? \$\endgroup\$ – Voltage Spike Jan 26 '16 at 22:51

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