Thursday, December 26, 2013

Finished first prototype of MIDI2VC(O)

A lot has been done since the last post. I changed the name of the project to MIDI2VC, since it actually converts MIDI to control voltage, VC.
 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 The schematics can be seen here: Be aware that I've missed the decoupling caps on the DAC.
 I also made a small board that I ordered from, preview below:

TOP side PCB preview
BOTTOM side PCB preview
The missing caps will be fixed when the other parts of the board are placed. Since this is the first board I've done in Eagle in a long time, I expect there to be some errors. The boards from Oshpark are so cheap (10$ for three of this one), that I can easily take a re spin to fix errors.

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.

1 comment: