2 Jul
2006
2 Jul
'06
3:31 p.m.
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.
Eh? The resistors in the ladder are R and 2*R, for some suitable value of
R.
The (maximum) output voltage depends on 2 things. Firstly the output
voltage from the parallel port (and note, if those 8 output signals don't
switch between the same voltages, you will get non-monotonicity as you
observed). And secondly, the load you connect to the output.
I would also think that the output impedance of the parallel port needs
to be taken into account. And that may well be non-linear (which may make
things difficult).
Another cause of non-monotinicity is, of course, errors in the individual
resistor values.
Just out of curiousity, why do you want to use an R-2R ladder, rather
than a single-chip DAC? Commercial vector displays that I've come across
either used a DAC chip (Vectrex), a DAC module (DEC GT40), or a rather
complicated circuit with tweakers for the top 6 bits (HP 1350).
-tony