Hi! For anyone that is interesetd, i've made some measurements of the on and off resistance of the altera chip. For reduce the dac error first i've tried to increase r-2r values from 499/1000 to 1000/2000, but someone here pointed out that the accuracy eventually will be reduced by this mod.
So i've measured the resistances, and i've found that when warmed up has an Ron of near 9,53ohm, and Roff of 7.86 ohm. Average near 8.7 ohm (rounded 9 ohm)
I've also glued (with apposite themal conducting glue) a big dissipator over the altera chip for mantain the temperature as stable as possible.
So, remaining around the same 500/1000 orginal range we need or 491/1000ohm, 500/1018ohm or similar values.
Now i need to try to find some hign precision resistors with this values...