DAC "error"

[Chris MacDonald]

> > 127 * 1/12V = 10.583 full scale DAC voltage
> >
> > 10.583 / 4096 = .002584V LSB step size
> >
> > .00258 / 4 = .000645V 1/4 LSB error
> >
> > So the error would be +/- 645uV.
> >

First off, this math was slightly off, sorry.

128 * 1/12V = 10.666V
10.666 / 2096 = .002604V LSB step size
.002604 / 4 = .000651V error
 
> No, the problem is that the "error" is even if the DAC is "perfect", you can
> hit in steps of 2.584mv. The error is 2.584/83.3 or 3,1%! Barf!

I believe what you are saying is that the desired output voltage might
fall between two steps and therefore the error is related to the step
size or resolution of the DAC.  This is certainly true if the
application requires accurate selection of arbitrary voltages. 

My point was that if instead the application only required selecting
83.3mV steps starting from 0V then one could tune the DAC output so that
every 32nd step was 83.3mV higher (this is where the .002604V LSB step
size comes from, as 32 * .002604 = .0833V).  In this way the error is
related to the relative accuracy (1/4 LSB or .000651V) of the DAC
instead.