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24 bit ADC testing

Discussion in 'Electronic Basics' started by sagar, Jul 14, 2004.

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  1. sagar

    sagar Guest

    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
     
  2. John Larkin

    John Larkin Guest

    Sounds impossible to me.

    John
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: 24 bit ADC testing
    O.K. What do you consider to be a reasonable amount to spend?

    Good luck
    Chris
     
  4. Bob Masta

    Bob Masta Guest

    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
     
  5. colin

    colin Guest


    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 wld just depend on how good a current source u cld make and how low
    curent leakage into the buffer, and of course how linear capacitor is. duno
    if theres anything else ive mised.
    but yes id be watch what youd had for breakfast too as 1 part in 2^24 is
    rather small. (wonders how many bits you would need to theoreticaly detect a
    single electrons)

    Colin =^.^=
     
  6. Fred Bartoli

    Fred Bartoli Guest

    Yep, don't forget that those 24 bits converters are slow : you've missed DA
    which will be a very big factor at 24 bits accuracy : even a _low_ 0.001% DA
    makes for 16.6 bits accuracy.

    Thanks,
    Fred.
     
  7. John Larkin

    John Larkin Guest

    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
     
  8. colin

    colin Guest

    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 =^.^=
     
  9. Bob Masta

    Bob Masta Guest

    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
     
  10. John Larkin

    John Larkin Guest

    The OP specified "without using an expensive setup." The people who
    design and test 24-bit ADCs don't have that restriction.

    Actually, I don't think m/any of the 24-bit ADCs actually deliver even
    18 bits of integral linearity, so you could test one with a good
    Kelvin-Varley voltage divider and/or a good system-level DVM.

    John
     
  11. colin

    colin Guest

    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 =^.^=
     
  12. Bob Masta

    Bob Masta Guest

    On Sun, 18 Jul 2004 19:36:47 GMT, "colin"

    This seems like an excellent idea! The only problem would
    be with ADCs that don't respond down to DC (which I think
    includes sigma-delta types) or overall systems like sound cards
    that always block DC at the inputs. You may not even need to
    measure the DC level all that well if you are mostly looking
    for small discontinuities. However, this method may not show
    up small nonlinearities. Imagine a response that has a slight
    curve instead of being a straight line. In any small portion of it,
    it will look pretty straight to the limits you can resolve. It's
    only over a long range that you notice how far you have
    deviated from a straight line. To do this piecewise with a DC
    bias, you would need to know the DC level to the same accuracy
    you are trying to resolve. That might be harder than creating
    a straight ramp in the first place!



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  13. colin

    colin Guest

    yes i asumed it wld manage dc but as you say sound cards etc dont, you could
    however super impose it ontop of a sinewave at the lowest frequency, in fact
    using this method you just need two signals in fact this is how they measure
    intermodulation distortion in audio amplifiers anyway, you dont need
    particularly pure signals either as the errors introduced will be diferent
    to the distortion products from the signals. just need to do a fourier
    anaylsis and look for the signals that are a sum or diference of the two you
    start with and ignore any that are just harmonics of the two signals.

    ie if there is a slight curve as you say then when the large slow signal is
    at its lowest and say the curve is steepest, then the the super imposed
    smaler signal will apear larger, than when at the other end of the curve and
    hence efectivly modulate it to some degree, and so this will show up as
    intermodulation frequencies just as in a frequency mixer where you end up
    with products such as fsig1xfsig2, fsig1-fsig2 etc ..

    the amplitude of these frequencies wil tell you purly the distortion of the
    adc. im sure you can easily find the maths to do this somehere.

    i beleive this is just the cheap but acurate setup that was wanted. :)

    Colin =^.^=
     
  14. sagar

    sagar Guest

    Thanks for the suggestions.
    I used a simple voltage divider and a 5 1/2 digit Tek
    multimeter
    giving me a resolution of 100uV.However,the divider arrangement was
    not
    able to provide good resolution of voltage .

    my problem is that the codes are not at all steady
    showing
    a continues flicker of the last 10 bits.The whole testing was
    dependent
    on the stability of the reference source and the analog and the
    digital
    supplies.But i am unable to get the high resolution .I would need help
    on improving the resolution.

    A dynamic test set-up employing either sine wave or ramp
    would require too much time ,probably mathematical manipulations and
    money.No time
    for that.

    With
    regards
    Sagar
     
  15. John Larkin

    John Larkin Guest


    Try this:

    get a thinfilm 8 or 10-resistor network of nominally equal resistors.
    Thus will be a few bucks from Digikey maybe, or get a sample from
    somebody. Wire all 10 resistors in series and bring out the taps to a
    rotary switch. Apply the same reference voltage to the string as you
    do to the ADC.

    Fire it up and digitize all voltages at all taps, and note the
    readings. Now flip the divider over and make a second set of
    measurements.

    If tap 3 were 0.300001 of the supply, flipped over it should be
    0.699999. How well the ADC confirms this is a good indication of
    integral linearity.

    Of course connection resistances must be small, the flipping has to be
    done carefully, and everything has to be seriously isothermal (to
    avoid thermocouple effects) and temp stable. Not easy, but I'd guess
    that 1 PPM is achievable with a little care.


    John
     
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