Well, the minimum frequency is zero or DC of course. Just output a constant value all the time. But your max frequency calculation is flawed somewhat. Your maximum frequency has nothing to do with the resolution of your DAC. In fact, I believe that they now use one bit DACs to reproduce CD audio. The max frequency as you stated earlier is one half the sampling frequency. If you can sample (update your DAC) at 1 us intervals then Nyquist says you can achieve a max frequency of 500 KHz. For example, at t=0 output the min voltage from the DAC, then at t=1 us output the max voltage from your DAC, then at t=2 us output the min again and so on. This will produce a square wave at 500 KHz. An analog low pass filter can then filter out the harmonics and leave the 500 KHz fundamental sinewave.