Hi everyone, I tried low pass second order filter in isis, but it doesnt work. error is "no model specified for op amp" . can you help me? I attached the pis

 

Is the simulator confused? You have put in an LM339, which is a quad voltage comparator.

 

Emphasis on comparator. A comparator is a digital gate that resembles an op amp, but is not an op amp.

 

A comparator is a 1 bit ADC.

 

Are you SURE you have or downloaded the correct model for the OPAMP you want to use?

Are you SURE you put the model in the correct folder?

 

The circuit will not work with an opamp because the circuit has a single positive supply voltage and the input is biased at 0V instead of at half the supply voltage.