Thank you guys for your replys!
To tgootee, what I am doing is the a common-centroid layout, the unit capacitance are layed-out with a specific pattern. The reason why I cannot use parasitics as a part of the circuit is that the largest bit, for example 12, has at least 50% of the parasitic proportion. The parasitic capacitance are not in proportion with each bit.
To keith1200rs, I'm using the smallest unit capacitor as I can, because the size of the DAC is 700um x 700um, which is as big as the other circuit combined. The area that I was allowed for tape-out for my circuit is just 1.5mm x 1.5mm. I was also thinking increasing the size of the DAC, but this would make the circuit too big. Is there any other way?
Thanks again