Let's take a similar circuit. I've used a general purpose Op-Amp (LM318) here, but any general purpose Op-Amp that's fast enough will do.

The R2R resistors: The actual values aren't that important as the R2R is a proportional circuit. The current is much more important than the voltage in this case. If the value of R is too low you'll be pulling too much current from the TTL outputs and you'll damage the chip (in this case the ASIC!). If the values are too high the current at the Op-Amps input will be below the Op-Amps Input bias current and the Op-Amp won't output anything. I've choosen 10K/20K (so as not to clutter the schematic), but 100K/200K would also be fine as the LM318 has an Ib of just 120nA. (Just for info: with 10K/20K you'll pull about 125µA per pin at 100K/200K you'll pull about 12.5µA per pin - both high enough to bias the Op-Amp).

The only resistor value you really need to calculate is Rf, the Op-Amp feedback resistor. I would suggest calculating this roughly and using a suitable variable resistor to tune the circuit when it's built. The calculation will only be rough because it's ignoring certain losses in the circuit etc.

The maths: You can calculate the voltage at Vout with: Vout = 1.66 x (Rf/2R) x Dec where Dec is the decimal value on the bus.

So Vmax is approx. 25 x (Rf/2R)

Hope that helped.

Bryce.

Note: To those who are about to post "Aaargghh, that's all wild approximations and you've rounded off the figures" - Yes, This is a simple Video DAC with a variable resistor to tune it. I'm not trying to design the input stage of a Keithley precision DMM here, just some rough figures.