I've added the MIDI connector and optocoupler to the input of the board. This is taken from the MIDI electrical specification, I just used another optocoupler, a cheaper one.
I also added a 1 volt bias to the output of the DAC. I did this since I expect to use a VCO that doesn't go to 0 volt. By adding voltage I can use the whole range of the DAC, between 1 and 5 volts. To do this I first used a voltage divider to divide down the 4.1V reference to 1 volt. This volt is buffered using a unity gain opa amp circuit. The output of the op amp is fed into a non-inverting summing amplifier (also op amp), that sums the one volt and the DAC output. This gives me a range of just below 1 volt to just above 5 volt, giving me a full use of the whole range.
Since the board is a part of a bigger project, which is 9V powered, i added a linear regulator to power the 5 volt parts. The op amp circuit needs to go above five volts, so it's powered by the 9 volt rail.
I did a full Eagle design using parts I could find on https://www.elfa.se/. The schematics can be seen here: https://dl.dropboxusercontent.com/u/4404053/midi2vc_ver1.pdf. Be aware that I've missed the decoupling caps on the DAC.
I also made a small board that I ordered from http://oshpark.com/, preview below:
I couldn't find a full derivation of the non-inverting summing amplifier circuit in the books or on the net, so I'll do that in the next blog post.