If you want to avoid monotonicity errors, resistor tolerance (actually matching) has to be within 2**(-n), which would be 0.1% for a 10-bit converter, and 1 ppm for 20 bits. This is achieved in integrated DACs through the nearly perfect matching obtained with lithography. Non-monotonic DACs are really interesting to watch on a spectrum analyzer as you vary the amplitude of a sine wave, and even more interesting if you add a bit of dc offset to the digital waveform.
Still, it¡¯s a fun and cheap way to build a DAC of modest resolution and you will learn a lot along the way!
Gary NA6O
