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When I was about to implement a circuit with a 24V supply voltage I noticed that the comparator's inverting input in the part below receiving 24V with respect to ground when the input signal goes high. I was investigating hysteresis and I also add 2Vpp noise to the input signal which makes the input voltage higher than supply voltage. So the non-inverting input goes at least up to 24V in the circuit below: enter image description here

As a remedy to limit the input voltage less than 24V just after the C1 I added to 15V zener diodes back to back as in the below schematic(I think I have to set R1 R2 again in this case):

enter image description here

Is my concern correct? What should be the max input voltage to an opamp comparing to the supply voltage Vcc? I used zeners to limit the voltage which works in simulation. Is that doable?

eidt:

I just learned opmap input could exceed Vcc depending on the opamp:

enter image description here

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  • \$\begingroup\$ Use the data sheet information specific to the device \$\endgroup\$ – Andy aka Oct 8 '16 at 15:28
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The LM290x survives differential input swings up to 36Vmax and functions with common modem to gnd, so Zener is not necessary.

Are you having switching delay issues? Hysteresis looks like 4V to me.

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  • \$\begingroup\$ I was having glitches I wanted to set hysteresis extremely high to be sure if the noise is coming from the input signal, But I still don't know setting the hysteresis that high would have disadvantages. I just need to count the each freq of clean output pulses. \$\endgroup\$ – user16307 Oct 8 '16 at 15:37
  • \$\begingroup\$ Schmitt Triggers in logic are usually 33% which affects delay but improves noise rejection. Best solution defines noise spectrum Amplitude, rise time, duration and filter blocks that from the signal to ensure hysteresis has no false triggers. Use shielded twisted pair and terminated lines. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 8 '16 at 15:41
  • \$\begingroup\$ just a question. you said differential input swings up to 36Vmax. i edited my question and added the max ratings of the data sheet. Does input voltage VI in the data sheet mean (Inverting input voltage - Non-inverting voltage)? Or wrt GND? \$\endgroup\$ – user16307 Oct 8 '16 at 15:44
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    \$\begingroup\$ either input wrt. gnd. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 8 '16 at 15:48

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