[sdiy] discrete DAC accuracy

[Fredrik Carlqvist]

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.

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.

Fredrik C


-----Original Message-----
From: Czech Martin [mailto:Martin.Czech at Micronas.com] 
Sent: Tuesday, January 13, 2004 11:23
To: Fredrik Carlqvist; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] discrete DAC accuracy


What yoiu try is very hard to achieve.

Without special techniques like laser trimming etc,

you can hope for 8-9 effective bits from a discrete DAC.
If you try to match resistors to 0.002% perhaps a bit better. Over
temperature and voltage range. There are many sources of error: resistor
mismatch, switch mismatch, switch leakage, parasitic capacitance, Add some
error for the s&h. Without s&h you will have enormous glitches at the output
due to nonideal switch timing.

You can build 16 bit lines, but the resolution will never
be that good.

That is the beauty of integrated solutions:
the absolute tolerance is awfull, but the matching
is cool. And the timing is also better controlled.
And you can have SiO2 capacitors.
Without special process and trimming it will be hard to get better then
10bit , even for integrated solutions.

m.c.


> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Fredrik 
> Carlqvist
> Sent: Dienstag, 13. Januar 2004 11:02
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] discrete DAC accuracy
> 
> 
> 
> Hello list!
> 
> I'm trying to calculate the absolute error of a discrete R-2R
> 16-bit DAC. If
> the resistor tolerance is 2%, is then the overall accuracy 2% 
> for any binary
> code? At least it seems so in the 3-bit case. I don't have 
> the tools for the
> 16-bit case.
> 
> This should also imply that the maximum relative error between two 
> consecutive binary codes is 4% since each code has a maximum absolute 
> error of 2% and two consecutive codes can have opposing errors (as
> 0x8000 and
> 0x7fff). Am I right? 
> 
> This is of course not accurate enough for a VCO control
> voltage, but should
> be ok for a VCA, don't you think?
> 
> 
> Fredrik C
> 
> 
>