[sdiy] Choosing a DAC

From jerryge at cableone.net
Sent Sun, May 22nd 2022, 20:12

5 octaves * 12 semitones = 60 steps.

The 1 volt per octave is a liner input at 0.0833... Volts per semitone
(1Volt/12).
If you setup your LSB at this voltage an 8 bit DAC can range over 21 octaves
(255/12) however
that requires over a 21 volt range.

Assuming you are thinking of 0-5v for your output and using 4 steps per
semitone you can cover 5.3125 octaves using 5.3125 as the Vref. each step
would be 25cents or 0.02083... Volts.

If you wish to use a 5.00 Volt Vref you will need a higher resolution DAC,
for Good results you want to always be +- 3 Cents or less, preferably less
than +- 1 Cent.

roughly at 5 Volts Vref it runs
Bits
                  Steps/Octave        S/Semi        C/Step
8        255              51              4.25              23.52941176
9        511              102.2              8.516666667        11.74168297
10        1023              204.6              17.05              5.865102639
11        2047              409.4              34.11666667        2.93111871
12        4095               819              68.25              1.465201465
13        8191              1638.2        136.5166667        0.732511293
14        16383              3276.6        273.05        0.366233291
15        32767              6553.4        546.1166667        0.183111057
16        65535              13107              1092.25        0.091554131
17        131071        26214.2        2184.516667        0.045776716
18        262143        52428.6        4369.05        0.022888271
19        524287        104857.4        8738.116667        0.011444114
20        1048575        209715        17476.25        0.005722051
21        2097151        419430.2        34952.51667        0.002861024
22        4194303        838860.6        69905.05        0.001430512
23        8388607        1677721.4        139810.1167        0.000715256
24        16777215        3355443        279620.25        0.000357628

As you can see an 12 bit DAC looks very good at 1.46 Cents per step.
And a 16 Bit DAC is well into the overkill range at 9 Hundredths of a Cent
per step.

- Jerry



-----Original Message-----
From: xxxxx-xxx-xxxxxxx@xxxxxxx.xxxxxx.xx
[mailto:xxxxx-xxx-xxxxxxx@xxxxxxx.xxxxxx.xx]On Behalf Of Stewart Pye
Sent: Monday, July 06, 2009 4:57 PM
To: Synth DIY
Subject: [sdiy] Choosing a DAC


Hi,

I'm designing an "analogue" sequencer around an AVR micro and I need to
choose a DAC for the CV output. It will on only span 5 octaves (0-5V)
and output only the equally tempered scale. (there will be a separate
circuit for glide).

I guess my question really is - how many millivolts can you be off by in
a 1V/octave system before it becomes noticeable?

Regards,
Stewart.
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