Difference amplifier show fixed voltage on its output


I'm using a difference amplifier as shown on this page:


I double checked my resistors values, I have RA=10k and RB=56k. I'm using a 5V reference and a 0V ground.

On the actual board, I have V1=0.1V to V1=0.4V (this is adjustable) and V2=0.5V. However the output is always Vout=3.85V, even when I change the V1 voltage.

I double checked the resistors and LM324 pins, and see nothing wrong.

I also measured V+=V-=0.684 when V1=0.4V and V+=V-=0.542 when V1=0.1V

How could I determine what is the cause of the problem? Do you have any idea at what the problem could be?

Best Answer

Hmm so the opamp supply pins are +5 and ground?
The 3.85 V output suggests the opamp is hitting the positive rail.

V(+) should always be 0.5V * 56k/66k. Check all the pins and such and make sure you haven't mixed up the inputs.
Then try a new opamp maybe you let the magic smoke out of this one and didn't see it. :^)