Odp: [sdiy] DAC for MIDI2CV

[Seb Francis]

Ingo Debus wrote:

> If you're using a 16 bit DAC, all static errors (those that do not
> change with time) can be nulled out if you use a look-up table that
> contains the *real* (not the theoretical) DAC values for each MIDI
> note. This way perhaps even trimpots aren't required. The look-up
> table can be obtained connecting the MIDI2CV to a precision DVM with
> serial interface read by the MIDI2CV's processor. This procedure would
> surely take some time but hasn't to be done often.

I've read about some technology to do this automatically (can't find the webpage right now), but it all seemed a bit high end and over-complicated for what I need.  I'm hoping that by careful choice of components I can keep the max error <1mV, even when using purely software scaling.

I plan to have only 2 trimmers in total - I'm going to trim the output from the DAC (using precision opamp), and S&H this to non-trimmed precision opamp output stages for each CV.

The 2 trimmers will be set to give ideal 1V/Octave - I may also include software scale and offset adjustments for each pitch CV, but I'm thinking this won't be necessary (maybe for microtonal scales I'll include some preset scales, rather than a scale trim).

I still didn't decide whether to use a S&H array IC, or just caps and opamps.  I'm leaning more towards caps+opamps, since (a) S&H arrays seem quite expensive and in some cases a significant source of error, and (b) In any case I want an opamp buffer on the DAC output, and between S&H and CV outputs, so I'd only have to add a precision MUX and some low leakage caps to make a discrete S&H.

I've really no experience of trying to make a precision S&H, so (as always) any tips would be greatly appreciated.  In particular I'm unsure what the cause of Linearity Error is in an S&H - this seems significant in some S&H IC datasheets.  Obviously I need to use precision opamps (especially for an open-loop S&H), but I have no idea what the best caps are.  I'm guessing low leakage is of some importance, although I will be refreshing the S&H very often, so maybe there's some other attribute of the caps which is more important?

[I've seen in Tony Allgood's MIDI DAC that he uses 10n polyester caps with precision opamps (at least for the pitch), although I've read that polycarbonate and polypropylene caps have lower leakage]


> Seb Francis wrote:
> > Mono/Poly mode
> > => Mode1 = 3 independent MIDI channels
> >    (uses 3 Gate + 3 CV)
> > => Mode2 = 2 independent MIDI channels, first channel has gate on note off
> >    (uses 3 Gate + 2 CV)
> > => Mode3 = 1 MIDI channel, 3 note poly
> >    (uses 3 Gate + 3 CV)
>
> Only voltage outputs for pitch? What about velocity and polyphonic
> aftertouch?
>

I knew I'd forgotten something from the spec!  Velocity sensitivity and channel aftertouch will definitely be supported.  Perhaps also poly-aftertouch (although I'm not sure if I have a keyboard which supports this).

Seb