Vector Imagery

[Don Y]

Jules Richardson wrote:
> Ethan Dicks wrote:
>> I was just hoping to upgrade my dodgy ladder with a more precise one
>> without resorting to additional cables and powered circuits.  I seemed
>> to have opened a can of worms.
> 
> :-)
> 
> I thought that pretty much all DACs are ladder-based internally anyway, 

Um, kind of.  :-/  Many are precision current sources that are
switched in and out.

There are also other DAC *techniques* like pulse width modulation, etc.

> just with far finer tolerances than the typical "bag 'o resistors" hack. 
> In other words, by fine-tuning a homebrew ladder and using suitable 
> buffering, you could probably get something pretty respectable running.

It's not just tweeking the R values.  You also have to ensure the
characteristics of the "switches" (in this example, the output
stages of that parallel port/latch) are well matched AND that
everything keeps behaving as temperatures change and power
supply voltages fluctuate.

There are also transient effects that may or may not be important
to consider (e.g., when switching from 0x7F to 0x80, does the
output momentarily assume the 0xFF level?  Or, the 0x00 level??)

There's a *reason* you buy these things when you need them  :>

> I stumbled across a few pages on DACs and ADCs a while ago:
> 
>   http://www.allaboutcircuits.com/vol_4/chpt_13/1.html
> 
> thinking about it, I wonder if the resistor packs often found in old 
> equipment are any finer tolerance (or at least more closely matched) 
> than using seperate resistors? If you have any scrap ones laying around 
> it might be worth investigating.