Dear readers!

I am designing SAR ADC for the first time and have a little problem in designing binary weighted capacitive array for the DAC. I am using a unit capacitance of 8fF. My input signal voltage range varies from 0 to 1V but my capacitive array only covers signals upto 600mV. I have tried changing the ref voltage and the value of capacitors but to no avail. I have attached a figure which shows each bit setting to high value in an  8 bit ADC. When all bits are set the amplitude I am getting is 0.6V. What is that I am missing!