I am building a simple op-amp comparator with a reference voltage using a voltage divider. The input to the non-inverting terminal is an AC voltage while the reference is a DC voltage. I find that comparator outputs a voltage when the AC peak to peak voltage is significantly lower than the reference voltage. Any ideas why?

 

Use a comparator IC if you want a comparator circuit.

 

You don't mention the part number, nor resistor values, so it's a guessing game.


But some opamps behave strangely when the input voltage exceeds the legal common-mode voltage range, like reversing phase!

 

I am building a simple op-amp comparator with a reference voltage using a voltage divider. The input to the non-inverting terminal is an AC voltage while the reference is a DC voltage. I find that comparator outputs a voltage when the AC peak to peak voltage is significantly lower than the reference voltage. Any ideas why?
View attachment 123326

How much AC voltage? What is VCC?

 

For testing purposes I am using 12V Vcc and 2 10k resistors to set my reference at 6V. I dont recall exactly the AC voltage that would trigger an output but it was something like 2V pk-pk

 

For testing purposes I am using 12V Vcc and 2 10k resistors to set my reference at 6V. I dont recall exactly the AC voltage that would trigger an output but it was something like 2V pk-pk

If the AC input is referenced to ground, that means it's going BELOW ground on the negative cycle, most likely not within the legal range of input voltage.

 

So how would i get around this issue

 

So how would i get around this issue

AC couple the input.

It would be helpful if you posted pictures of the waveforms you're seeing.

 

AC couple the input.

If you do that, you need to add a couple resistors between +Vcc to ground to bias the capacitor output at 1/2 Vcc.

 

For testing purposes I am using 12V Vcc and 2 10k resistors to set my reference at 6V. I dont recall exactly the AC voltage that would trigger an output but it was something like 2V pk-pk

What do you expect the output to be when the non-inverting input is always less than the inverting input? Assuming the opamp doesn't respond poorly if the input is outside of the common mode input range...