[sdiy] Choosing a DAC

[Jerry Gray-Eskue]

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: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]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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