[sdiy] discrete DAC accuracy

[Czech Martin]

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
> 
> 
>