Vector Imagery

On 7/2/06, Chuck Guzis <cclist at sydex.com> wrote: 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