Re: 24 bit ADC testing

From: colin (no.spam.for.me_at_ntlworld.com)
Date: 07/17/04 

Date: Sat, 17 Jul 2004 20:05:42 GMT

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:nqpif0dq55tcgf3alnb2bd9dkr94fmri3e@4ax.com...
> On Fri, 16 Jul 2004 17:27:13 GMT, "colin"
> <no.spam.for.me@ntlworld.com> wrote:
>
> >
> >"Bob Masta" <NoSpam@daqarta.com> wrote in message
> >news:40f67c54.4837317@news.itd.umich.edu...
> >> On 14 Jul 2004 06:48:50 -0700, sagarbirkar@gmail.com (sagar) wrote:
> >>
> >> >Hi,
> >> > I need to check the performance of a 24 bit analog to digital
> >> >converter.
> >> >as in its main properties such as Integral Non Linearity,Differential
> >> >Non Linearity etc without using an expensive setup.
> >> >
> >> > With
> >> >regards,
> >> > Sagar
> >>
> >> If the converter is part of a circuit that can send
> >> data to a computer, you might be able to do this
> >> by driving it with a very pure sine wave, taking
> >> a large FFT (to get a low noise floor) and then
> >> inferring the specs from the distortion spectrum.
> >> I don't know off-hand how to convert a spectrum
> >> into a discrete spec, but you can certainly use
> >> this setup to grade parts or otherwise compare
> >> units.
> >>
> >> Note that at this high bit-resolution, the surrounding
> >> circuits, power supply, clocks, and what you ate for
> >> breakfast are probably going to affect the results.
> >>
> >> Good luck with this, and please write back and let
> >> us know how things work out!
> >>
> >>
> >> Bob Masta
> >> dqatechATdaqartaDOTcom
> >>
> >> D A Q A R T A
> >> Data AcQuisition And Real-Time Analysis
> >> www.daqarta.com
> >
> >
> >depends how predictable the driving signal is, id probably try and use a
> >ramp generator as id probably be more sure of being able to design and
build
> >a linear ramp generator easier than i wld a precise sinewave genarator.
> >
>
> It would be hard to build a ramp generator that's linear to 14 bits,
> much less 24. Capacitors have dielectric absorption, c-vs-v effects,
> and temperature drifts, and every semiconductor you use is full of
> nonlinear capacitances. At 24 bits, every solder joint on the board is
> a thermocouple, and every drift of air flails everything.
>
> A ramp could give you some useful data on dnl, but delta-sigma ADCs
> have inherently good dnl. Integral linearity is the test killer.
>
> You can do some fun things by flipping over resistive voltage
> dividers.
>
> John
>
>

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.

Colin =^.^=