Raspberry PI Based DAC

[electro_pt]

I just got a new raspberry PI 3, and have been using it as a small TV based computer.  However as is often the case, once you have flexible hardware you start looking for other things to do with it.  I have been intrigued by the high speed one can switch the GPIO pins, and thought there might be potential for a DAC.

I know there is hardware already out for this, but I was interested in learning to do it myself for the experience.  Any suggestions or recommendations.  Perhaps a R-2R?  Maybe an RC circuit on the output of a pin...a bit like Raspberry's RCA audio out but perhaps with higher dynamic range?  Custom waveforms would be ideal, perhaps with 0-20MHz range.  Just starting in this sort of thing so thought it would be a good project to learn.

[NiHaoMike]

Beaglebone Black can do it. It has I/O coprocessors that can easily handle 100Mhz sample rate, input or output. There's even a logic analyzer based around that.