Vector Imagery

[joseph c lang]

On Sunday 02 July 2006 02:49, you wrote:
> On 7/2/06, Chuck Guzis <cclist at sydex.com> wrote:
> > > http://www.epanorama.net/circuits/dacs.html
>
> I've built that first simple ladder DAC (to hang a 1950s Triplett
> meter off of a parallel port for just random analog fun)... the
> problem is that it's difficult to tune the individual resistors to
> prevent the needle (or voltage) from going visibly *down* when
> transitioning the DAC from, say, 0x3F to 0x40 or 0x7F to 0x80.  The
> cumulative errors of the low order bits can get to the point that you
> don't get a constant output increment by incrementing the digital
> value.  The needle did deflect from 0 to full, but writing a ramping
> program showed very clearly that it wasn't a continuous progression.
>
> What I really would like to find is a formula for calculating R2R
> ladder values so that I could get a 0V-5V output from an 8-bit
> parallel port.  Constructing it is easy, once one knows what resistors
> to pick.
>
> -ethan

The problem is not the resistor values. You have the formula. R 2R ;^)  
It works for any value of R.  it's equal value that counts.
You are  more likely suffering from non-equal voltage on each driver.
The load is not the same for all bits,and any mismatch (high or low)
will directly affect linearity.

Buffering with a CMOS driver and using high value of R will help some.

joe lang