[sdiy] MIDI-CV DACs Part #2

[Paul Schreiber]

Now let's look at issues with the DAC itself.

When you think of a DAC, the first thing that pops into your head is "how many bits", but
*really* the first thing needs to be "What is the Integral Non-Linearity (INL) error?" Unless you
are an 'IT Professional', and the first thing you think of is "Who is buying the beer?"

INL is the overall ACCURACY of how well the DAC 'hits' the output you tell it to. You can have a
24-bit DAC, tell it to output 1.000V and if it outputs 1.0175V that's a problem. DACs are
designed to be used one of 2 ways: as an open-loop, absolute voltage generator, or in a "set
until happy" system (like say a volume control, where the user doesn't care what the digital code
is to generate the output, just the output matters). In a MIDI-CV converter, the WHOLE IDEA is to
translate a SPECIFIC code (the MIDI note) to a SPECIFIC voltage. That is why you really have to
slog through all of this stuff :)

Much DAC confusion is associated with MIDI being "only 7 bits of information". Heck, an 8-bit DAC
is one better, right?!?? Errr....nope. All that means is that there are 127 possible outputs, but
it says NOTHING AT ALL about how ACCURATE those 127 outputs have to be. I have designed systems
with 4 output voltages, but they had to be within 4uv at 2PPM ACCURATE. So, do not confuse "code
length" to "DAC bits".

So, just *how* do you select the DAC??!?

Part #3 shortly.

Paul S.