Slightly OT: DAC Questions

[Tristan Upton]

A 12 bit DAC will typically have 16 (2^4) times the resolution of an 8 bit DAC, the
12 bit having 4096 steps and the 8 bit 256. Most DACs will only be accurate to around
0.5 of the LSB.

An 8 bit DAC, used to control the pitch of a V/octave oscillator to a (nominal but
pretty bad) accuracy of 5 cents, will only have a range of ((2^8)*5)/1200 = 1280/1200
= 1.067 octaves. Converters such as the PAIA work around this by scaling the DAC
output for separate octaves.

On the other hand, a 12bit DAC can operate with an accuracy of 3 cents over a range
of ((2^12)*3)/1200 = 2288/1200 = 10.24 octaves. If greater resolution is required
then the range can be reduced or an even higher resolution converter used.

If it is desired to control a V/Hz oscillator then the output of the V/octave CV
converter can be used to feed an exponential converter, with suitable temperature
compensation. Alternatively, a much higher (16 bit) DAC can be used, and the
exponential conversion can be performed in software, as is done in the Frostwave
MIDI/CV converter.

To interface a DAC to an 8 bit processor there are many options, it very much depend
on the DAC you choose to use. You could use a DAC with a 4 or 8 bit microprocessor
interface to load in separate nibbles or bytes to the 12 bit DAC. Alternatively, you
could use a DAC which supports serial data input and run it off a port. Or you you
could add some external hardware (latched buffers) to hold the data to being fed to
the DAC. And if you only need a number of step (say 128 MIDI notes) then you could
just wire up the most significant 7 bits of the DAC and tie the rest low.

Hope this helps,

Tristan Upton

Gavin Muir wrote:

> Hi All,
>
>         Just pondering a MIDI to CV converter, and examining a few schems. Most I
> have seen use 8 bit processors with 8 bit DACs, and I have heard much talk
> about using higher quality DACs.
>
>         Now, as I am new to the Digital realm, exactly how do you benefit from
> hooking up a 12 bit DAC to an 8 bit processor, and how would you go about
> doing such a thing as there will only be 8 data bus outputs from the uP?
>
> Cheers and thanks,
> Gavin.