Re: 24 bit ADC testing

From: Bob Masta (NoSpam_at_daqarta.com)
Date: 07/18/04 

Date: Sun, 18 Jul 2004 13:52:06 GMT

On Sat, 17 Jul 2004 20:05:42 GMT, "colin"
<no.spam.for.me@ntlworld.com> wrote:

>
>sure there is limits like you mention, but wouldnt the same errors aply just
>as much to a sinewave generator too if not more so ?
>
>to push the limits a bit further you could i gues use vaccum dielectric
>capacitors to get best linearity and you could bootstrap the transistors to
>reduce their voltage dependant capacitance efect, thermal drift would be not
>be too much of a problem over one cycle, and over many cycles could be seen
>in the output as drift rather than non linearity, or you could even ovenise
>it. im sure something else would then get in the way, but theres usualy a
>way of getting round them otherwise no one would of built an adc that could
>be acurate to 24 bits in the first place.
>

The virtue of a sinewave is that you can generate a very good one
quite easily using a Wien bridge, or a state-variable approach.
Then you can apply as many stages of filtering as needed to get
the desired purity. The last stage(s) could be purely passive to
remove any residual active distortion products. There is a temptation
to do everything in as few stages as possible with really high-Q
stages, but this can backfire since any component drift will
detune things so your oscialltor and filter stages are no longer
at the same frequency... and once you are off-peak of the
high-Q filter, you don't have much of a filter any more.

For a good starting sine wave, consider a 16-bit sound card
and a signal generator program like my (free) DaqGen.

Bob Masta
dqatechATdaqartaDOTcom
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
           www.daqarta.com

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