Re: 24 bit ADC testing

From: colin (no.spam.for.me_at_ntlworld.com)
Date: 07/18/04 

Date: Sun, 18 Jul 2004 19:36:47 GMT

"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:40fa7e96.3426384@news.itd.umich.edu...
> On Sat, 17 Jul 2004 20:05:42 GMT, "colin"
> <no.spam.for.me@ntlworld.com> wrote:
>
>
> 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.
>
>

yes i did think of using several stages of filter to clean up the sinewave
as you sugest, but as was pointed most dielectrics cuase capacitors to
change value with voltage, not to forget nonlinearity of ferrits in
inductors etc, so these filters wld tend to introduce their own non linear
errors wich would be hard to calculate or compensate for, you could use
pregresivly beter components but ultimatly the quality of the last stage
would have the most effect.

a ramp generator is pretty simple and hence i thought simpler to work out
the overal error and even compensate for, also the results are a doddle to
interpret.

i just worked out 24 bits has just under 60nv per step for 1 volt range., or
0.000006%

wich ever signal you generate you could always set the amplitude to cover
the botom half of the range then the middle bit then the top bit, then any
aparent change in the signal other than the offset is purly down to error in
the ADC and any such error should show up. adding an offset shld be posible
without introducing any error at all, infact the adc probably can do it

Colin =^.^=

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