[sdiy] discrete DAC accuracy

[Fredrik Carlqvist]

I appologize if I seem daft, but please try the following: 

The relative error between two consecutive codes will be largest at
roll-over pairs like 0x8000-0x7FFF, 0x1000-0x0FFF etc. A pair like
0x8000-0x8001 will have a very small relative error.

Worstcase absoulte error for the code 0x8000 is 2% (1.02/2.00) with 2%
resistors. The absolute error for the code 0x7FFF is also 2%. The maximum
relative error between these two particular codes is then around 4%. There
is no principal difference between any other two consecutive codes. Ergo,
worstcase relative error for two consecutive codes is 4%.

I do realize that only 6 or even just 5 of the 16 bits has anything to do
with the absolute precision. But if any two consecutive binary codes are
always within +/- 4% error relative to the other, then the output function
must be monotonous!

Or am I missing something vital here?

Fredrik C


PS I have already built it, and it *seems* monotonous!



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Czech Martin
Sent: Tuesday, January 13, 2004 12:16
To: René Schmitz; Fredrik Carlqvist
Cc: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] discrete DAC accuracy


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
> 
> 
>