Best way to attenuate R2R DAC output?

Given an N-bit R2R DAC, with input voltages of e.g. 0-3.3 volts, what is the the best way (signal integrity and quality maintained) to attenuate its output voltage to a range e.g. 0-.7 volts for a VGA signal.

Is it as simple as using a voltage divider, or would something like an opamp be more appropriate?

• depends... how much drive strength do you need? what is the load impedance? – JonRB Jun 19 '18 at 17:41
• VGA would be 75 ohms, unless someone forgot to terminate it. Essentially, one either designs the ladder for the load impedance, or you need a suitable amplifier (video amplifier is a category) with less than unity gain or with an output divider including the load impedance. Also, given likely AGC it may not matter, as long as electrical maximums are not exceeded. – Chris Stratton Jun 19 '18 at 17:44
• @jonRB current draw would never exceed 40 mA, and load impedence would be 75 ohms. For the purpose of making this question/answer more useful for everyone, I would be interested in the answer for high vs low current draws and high vs low load impedances. – Connor Spangler Jun 19 '18 at 17:45
• @ChrisStratton even if designing for the load impedance, how would you factor in the specific output voltage? I understand how to design an R2R DAC with a max output equivalent to the input level, but not with a specific lower gain. – Connor Spangler Jun 19 '18 at 17:48
• @Answoquest - Conceptually one way you could do it would be to design your DAC with some additional high order bits that you only ever drive as zero - ie, go from 8 to 10 bits and now your maximum is 1/4 of the previous voltage. Then you could simplify the redundant components into a smaller number. I believe if you study the result you'll be able to see how you could then use different values to create a gain fraction which is not a power of two, but I haven't actually done it on the back of an envelope. – Chris Stratton Jun 19 '18 at 17:51