Babar,

Note that the outputs on the 80c family are open drain outputs.
The integrated pull up resistors have a value of +/- 120K. Don't ask me the tolerance.... (? %)
You can always try to improve your dac by pulling up the 80C51 outputs to positive supply, using relatively small value 1% resistors.(1K)
Connect the output DAC to non inverted input off a opamp, like TL071/TL081 series or LM324 series. Just connect the supply +V to + supply and -V to GROUND. The output to the inverted input.

Filip.