[sdiy] discrete DAC accuracy

[Czech Martin]

exactly.

Fredrik, before you spend a lot of time (and money)
please check what accuracy of resistors you can get
(matching, not absolute). The little shop arround
the corner today has 1% or even 0.1% absolute 
tolerance resistors. They cost a few cents.

No need to fumble arround with 2% or worse.

You should also check the tempco. 50ppm is good,
sometime you get 100 ppm or worse. 10 deg makes 0.1% shift,
then. This may or may not match in your set of resistors.

If you can get a belt, 1% absolute resistors have
often a matching of 0.1%, this is so good that
I have problems to measure it at home.

Then calculate the error in your R2R network.
This will be the lower limit of error, but you''l get
more , as I said.

The you can think if your goal is realistic.


m.c.

> -----Original Message-----
> From: René Schmitz [mailto:uzs159 at uni-bonn.de]
> Sent: Dienstag, 13. Januar 2004 12:05
> To: Fredrik Carlqvist
> Cc: Czech Martin; synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] discrete DAC accuracy
> 
> 
> Hi again,
> 
> Fredrik Carlqvist wrote:
> > My point is that I don't need absolute accuracy. I am just 
> after a monoton
> > output function when looping through the codes. If any one 
> code is 10% off,
> > I don't care, as long as it is about the right distance from it's
> > neighbours.
> 
> That won't be the case.
> 
> > I will use it to control noncritical things like the VCA, 
> LFO rate etc. The
> > important thing here is to remove the 'stepping' with an 
> 8-bit DAC. Even
> > though I don't get more precision with more bits, I get 
> improved smoothness.
> > At least, this is what I hope to achieve.
> 
> No you don't. When you add bits past the accuracy of the MSB, the DAC 
> becomes nonmonotoneous, the added bits are useless.
> 
> Cheers,
>   René
> 
> 
>