1
\$\begingroup\$

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?

\$\endgroup\$
11
  • 1
    \$\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\$
    – user16222
    Commented Jan 25, 2016 at 22:37
  • \$\begingroup\$ Noise rejection is the name of what @JonRB said.. \$\endgroup\$
    – Eugene Sh.
    Commented Jan 25, 2016 at 22:42
  • 1
    \$\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.
    Commented Jan 25, 2016 at 23:03
  • 1
    \$\begingroup\$ Yes, that's the thing \$\endgroup\$
    – Eugene Sh.
    Commented Jan 25, 2016 at 23:05
  • 1
    \$\begingroup\$ Take a look here. Slide 13 has the illustration. \$\endgroup\$
    – Eugene Sh.
    Commented Jan 25, 2016 at 23:06

1 Answer 1

3
\$\begingroup\$

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

\$\endgroup\$
4
  • \$\begingroup\$ when would you prefer a comparator to a Schmitt trigger? can u give an application example? \$\endgroup\$
    – user16307
    Commented Jan 26, 2016 at 9:29
  • 1
    \$\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
    Commented Jan 26, 2016 at 18:50
  • \$\begingroup\$ but im already telling you the spesific input \$\endgroup\$
    – user16307
    Commented Jan 26, 2016 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
    Commented Jan 26, 2016 at 22:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.