Electronic – Op-amp input voltage gets clipped off

comparatoroperational-amplifieroscilloscope

I'm trying to build a simple voltage comparator using LM324N. Here is the circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The strange thing that happens is that after apply the input voltage, a small part of it gets clipped off. Here are the pictures of the circuit and the resulted voltages:

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I've connected all of the grounds and negative terminals (i.e., CH1, CH2 and the negative terminals of the input voltage and power supply) together. Also using another LM324N and applying input voltage with a resistor didn't solve the problem. As soon as I disconnect the input voltage from LM324N, it gets back to its normal shape which is a sine wave with 3V amplitude.

How do I stop the voltage from clipping on the opamp input?

Best Answer

On many opamps, they have input protection diodes that prevent the input from exceeding -0.3V. The input goes to -3V so this is a problem and the diode is turning on to protect the input. Either make a bidirectional rail (so the v- terminal is -3V) or change the function generator to have a DC offset so the 3V sine wave has a DC offset of 1.5V and centered around that value.

enter image description here Source: LM124 Datashet