DAC resolution

[Fraser, Colin J]

> -----Original Message-----
> From: Paul Schreiber [mailto:synth1 at airmail.net]
> Sent: 09 March 1999 12:40
> To: Fraser, Colin J; jamos at technotoys.com; synth-diy at mailhost.bpa.nl
> Subject: DAC resolution
> 
> 
> If an 8-bit DAC is spanning 1 volt, the dac has 1/2LSB accuracy.
> 
> So 1v/256 steps = 3.9mv and the DAC can only hit that +-1.95mv.
> 
> This is a ~2% error (1.95mv/83.3mv). This does NOT include 
> errors in the
> "octave switching" mux used or the Vref drift.

Reading this bit again, I can see where Paul gets his error from, but this
is not how to use an 8 bit DAC for CV generation.

To cover the full midi note range, you only need 7 bits (128 notes).
I have used either the lower 7 or higher 7 bits of 8 bits DACs without much
difference.

The output of the DAC must be scaled so that a one bit change in the input
gives 0.0833 mV change at the output (12*0.0833v=1v).
For 8 bits, scaled to give a full scale (theoretical) output of 21.333 v,
you get the required 0.0833 mv per bit.
The accuracy of 1/2 LSB is the quantisation error for a continuous signal.
If the steps in the output levels you require match the steps in the DAC
output, you don't have any problem with errors. 
The spec for the DAC-08 quotes non-linearity of +/-0.1% of FS - this is the
relevant error in this case.

Colin f