Hi Darren,

I've played with the numbers from the website you started with.
And either I got it wrong, or that's one "messed up" example.

To start with, it's not the way an R2R network is supposed to be laid out. Which accounts for the lower amplitude.
A description of a proper R2R network can be found here ...
http://en.wikipedia.org/wiki/Resistor_Ladder
But his way does have fewer parts.

Barring the fact that his binary numbers are all backwards ...
With the program from that web page, it doesn't make a Sine wave.
But it's not a bad "Triangular" wave.

Here's a spreadsheet of my results. (html)

http://www.pbpgroup.com/Calculators/...R_SineWave.htm

The top section calculates the voltage output for each of the digital possibilities.
These numbers seem quite different than what the web page author came up with.

The resistor values can be changed to test different conditions. If you change them to exact multiples (1000, 2000, 4000, 8000), you can see that Vout is very linear, although still low amplitude.

The section below the graphs calculates Vout for the "SIN" sequence.
These numbers can be changed, and might help test your calculated SIN values.

But I have to wonder if a 4-bit sine wave at 50hz is going to be what you want. It'll look more like a staircase than a sine wave. You could add a lot more capacitance to the filter, but at 300 hz it may be too much.

hth,