how can i reduce noise when connecting a light to voltage converter to the ADC? The output bits fluctuate a lot even when output voltage of light sensor is constant.

This cannot be true. How did to find out that light sensor signal is constant? By assume? Because light is constant? Don't forget, that such light sensors can produce a big amount of noise and hum!!

Is output of light sensor low impedant, classical low ohmic voltage source? Then, noise must come from light sensor. If, on the other hand, input of ADC sees a very high impedance the same is happening as if input would float, like Steve told you.

As Abhishek stated, we can only solve your problem, when having a look at schematic...

Kai