[sdiy] Choosing a DAC

[stew]

Thanks for all the replies.

I was more concerned about accurracy than resolution. Some 12 bit DAC's have 2 LSB intial error, whereas others have 0.5LSB.

My plan is to have 64 note values (a bit more than 5 octaves). I could possibly change it to have more but I don't think it would be very useful (others might) and the pot (user input) resolution starts to become a problem. For example, using a 12 bit DAC I would multiply each note value by 64, so there would be 64 steps between each value that is output from the DAC. 

I guess I'll have to use an adjustable reference. I was planning on scaling it afterwards but it would make sense to have the reference scale it.

I'll have to think more about this when I get home from work...

Regards,
Stewart


> Jerry Gray-Eskue <jerryge at cableone.net> wrote:
> 
> 
> 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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