DAC resolution

[Fraser, Colin J]

> -----Original Message-----
> From: Terry Michaels [mailto:104065.2340 at compuserve.com]
> Sent: 09 March 1999 16:37
> To: Fraser, Colin J; synth
> Subject: RE: DAC resolution
> 
> +/- 0.1% of full scale, which is 128 notes, is +/- one-eighth 
> of a note. 

Full scale is 256 notes - that would be +/- quarter of a semitone - that
probably would be noticable - I must have been lucky :-)
I have measured the output voltages of for the first 12 notes on an 8 bit
DAC, and the accuracy of each 0.083mv step was better than I could measure
with a 3.5 digit mm.

> The
> advantage of using 8 bits of a 12 bit DAC for this application is the
> maximum differential linearity, or note to note error, is 16 
> times smaller,
> because the 12 bit DAC has 16 times more resolution, and the maximum
> differential nonlinearity will be only 1/16th of a semitone. 

But surely you can only take advantage of this increased resolution if you
use the extra bits ?
Wouldn't you have to measure the input to output error of your DAC and store
the precise 12 bit value required to give each of the 256 output voltages
you were after for 8 bit resolution ?
 
> The maximum
> deviation of a straight line over 128 notes will also be 1/16th of the
> maximum deviation of an 8 bit DAC.

Ignoring the theory and specs for a moment - inside the DAC is a ladder of
resistors setting the voltage level for each bit.
In most DACs these will be laser trimmed to the correct values.
Why, assuming the manufacturer wants to make the most accurate part he can,
would the dividers in the 8 bit DAC be so far less accurate than the
dividers for the first 8 bits in a DAC with more bits ?


Colin f