2 Jul
2006
2 Jul
'06
6:34 a.m.
On Sunday 02 July 2006 02:49, you wrote:
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
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