ATmega32 withRC filter and OpAmp
The OPAMP is totally unsuited to what you want with a 5V power supply. 


It has an input voltage range of 0V to about 3.5V
It has an output voltage range that might reach down to 1V and probably won't rise above 4V. It's guaranteed to swing only to 2V of either supply rail on a +/-15V rail.


Fix this then re-examine the problem you are having by disconnecting the output transistor (just to make sure this isn't causing a problem due to mis-wiring the device.

Try an AD8605 for the op-amp - it has rail-to-rail input and output capability but still remember that getting it's output down to 0V when the most negative rail is 0V isn't going to happen - you might get the output down to 50mV or a bit less.