Vector Imagery

[Allison]

>
>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:
>> > 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.

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