open-source-silicon.dev

Considering your reference voltage is the supply voltage, 1.8 V and D9 is your most significant bit, 0.5 input voltage it still outputs 1, it must not be working. You must test your circuit with other values and make a table with your results. Test your ADC as it were 3 bits, for example, so you would have 9 input voltages (0, 0.225, 0.45, ..., 1.8). Which are the outputs? What is the real resolution?

I don't know the circuit you're using, so I can't identify its possible bottlenecks. Share it with us.