2 Jul
2006
2 Jul
'06
6:02 a.m.
Subject: Re: Vector Imagery
From: "Ethan Dicks" <ethan.dicks at gmail.com>
Date: Sun, 02 Jul 2006 18:49:18 +1200
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
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.
First, the 1950s triplett meter is likely 20,000ohms/volt at will load
any ladder using resistors larger than 1000ohms in an R2R ladder.
The 5v range looks like a 100K resistor across the probes.
Second, The output port is TTL? CMOS? IF TTL the output swing is
not to 5V to with even light loads. IF CMOS output voltage will sag
if the load is too great.
For 8bit R2R ladders I use R=10k and 74HC244(or other high current
output CMOS) as a buffer. The load is usually very high impedence
(VTVM, FETVM, DVM, OPamp voltage follower).
Allison
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