How to collect a data and compare it to another two values

Hardware Arduino Zero
2

You shouldn't drive a led with the DAC directly. Could you share you schematic? I can't tell if the issue is noise or the schematic for example.

I see no issue with your code :

if voltage on pin A4 is between 2.45V and 2.55V, the DAC outputs 0.3V, else, it outputs 0V.