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enter image description here

What does the symbol mean?what is this comparator difference?what is its function?can anyone introduce it to me?

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    \$\begingroup\$ If you google "comparator symbol", this is your first hit which links to electronics.stackexchange.com/questions/39404/…, which in turn explain the phenomenon. \$\endgroup\$ – winny Apr 6 '18 at 14:52
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    \$\begingroup\$ Possible duplicate of comparator schematic symbol \$\endgroup\$ – LMS Apr 6 '18 at 15:18
  • \$\begingroup\$ Not a duplicate question, the answer might be the same, but the question is different. The posted question says nothing about the hystersis. \$\endgroup\$ – Voltage Spike Apr 6 '18 at 16:18
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Schematic Symbol for a comparator with hysterisis or Schmitt trigger.

schematic

simulate this circuit – Schematic created using CircuitLab

A shifted zero-crossing detector to deal with a noisy input.

Comparators have high gain, so a zero-crossing detector would transition between the rails for a signal oscillating near the reference.

Reason for Schmitt Trigger

You shift the reference point once it is crossed so the circuit is less susceptible to noise. Noise has to be greater that \$ V_{HYS} = 2 \times V_{UTP} \$ to cause a state change.

$$V_{UTP} = \frac {R_2} {R_1 + R_2} V_{CC} $$

Schmitt Trigger

Once Upper Trigger Point (Red) is reached, op-amp changes to \$V_{OUT}\$ shifting reference to Lower Trigger Point (Blue). Noise would have to be larger than Lower Trigger Point to transition.

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It is a comparator with hysteresis. Here is a diagram from this Maxim datasheet

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When the output is in the high state the voltage required to drive it low (B) is higher than the voltage required to drive it high again (F), so for input voltages between F & B the output will be in either one state or the other, depending on the history of input voltage.

By incorporating hysteresis, a noisy signal can be "cleaned up" as the output will not transition many times for small changes in input voltage.

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    \$\begingroup\$ Maybe writing it correctly, providing some infos on what it does, linking to some pages to deepen the topic would have been a nice addition... \$\endgroup\$ – frarugi87 Apr 6 '18 at 15:13
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    \$\begingroup\$ @frarugi87 Yeah, sometimes real work and money gets in the way. Updated. \$\endgroup\$ – Spehro Pefhany Apr 6 '18 at 15:54
  • \$\begingroup\$ The common name for this is "Schmitt Trigger". \$\endgroup\$ – Chromatix Apr 6 '18 at 17:59
  • \$\begingroup\$ @Chromatix It's a particular type of Schmitt Trigger. Most Schmitt triggers are like a 74HC14 with a single input. \$\endgroup\$ – Spehro Pefhany Apr 6 '18 at 18:01
  • \$\begingroup\$ The Schmitt triggers I'm familiar with were constructed from an op-amp using positive feedback. This is just an integrated version of that. The 74HC series version is a specialised version with a built-in reference voltage, which is simply not brought out to a pin. \$\endgroup\$ – Chromatix Apr 6 '18 at 18:06
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This is a Schmitt Trigger, which exhibits hysteresis.

Instead of switching the output as soon as the inputs cross, it requires a positive difference between them. It then requires a similar difference in the opposite direction to switch back.

The advantage is in cleaning up a noisy input signal, which might otherwise cause spurious glitches downstream. The disadvantage might be a reduction in sensitivity.

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