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24-bit A-D converter - meaningless?

Discussion in 'Electronic Design' started by Peter, Aug 4, 2013.

  1. Peter

    Peter Guest

    I've been doing analog stuff since the 1970s, used to design HV power
    supplies stable to a few ppm, but it is still amazing to see what can
    be achieved e.g.

    http://www.analog.com/en/press-release/12_04_12_ADI_24bit_Sigma_Delta_AD_Converter/press.html

    How how can this possibly work? 24 bits of noise-free performance,
    never mind linearity, never mind any sort of absolute accuracy, seems
    completely unachievable in the analog world.

    I find that a 12-bit ADC inside a microcontroller might just about
    give you 12 bits, though usually you get 10 useful bits and to get the
    other 2 you have to take say 30 readings and average them.

    Once away from a microcontroller but still on the same PCB, with
    careful use of grounding and ground planes, and perhaps even a shield
    over the relevant bits (and a ground plane on the back of the PCB) one
    can use a 16-bit ADC and end up with only 1 or 2 bits of noise.

    I would imagine that to get better than 16 bits one would need to put
    the ADC in a shielded box, well away from any logic etc, but even the
    clock+data lines are going to radiate noise inside. Maybe one is
    supposed to bring them in over fibre. Maybe the timing is such that no
    transitions on the control signals are necessary during the actual
    conversion cycle?

    At work we make a product which uses a 12-bit ADC (ADS7828) which we
    calibrate to < 0.05% by using a precision voltage source, 0.1% 15ppm
    0805 resistors and storing calibration coefficients in an EEPROM (I
    saw the other thread here on 0.01% resistors) and we get pretty well
    the full 12 bits out of that. I'd like to go to a 16-bit ADC one day
    but I am very sure it won't give us more than maybe 2 extra bits that
    mean anything...
     
  2. Guest

    Where does that article talk about 24 noise free bits ?
    Low sampling rates and hence long integration times and low noise
    measurement bandwidths seem to give figures the marketing people
    like:)

    Audio codecs are usually specified as 24 bit (apparently to be SPDIF
    "compatible") at sampling rates of 48, 96 and even 192 kHz are
    specified with 3 Hz - 20 kHz noise bandwidth, i.e. at higher sampling
    rates, much of the quantization noise moves above this frequency band,
    giving nicer looking figures. In addition the low frequency limit
    nicely cuts out any 1/f noise issues at low frequencies.

    Still the SNR at 20 kHz bandwidth is about 120 dB, corresponding to
    about 20 correct bits.

    Putting a 16 channel analog multiplexer in front of the AD chip, each
    channel sampled at 15 kHz and you get less than 16 clean bits, not so
    much better than a 12 bit A/D converters used in industry for decades.

    At 5 SPS (typically used in scales) 20 - 24 bit claims has been made
    for a long time.

    You really have to be careful when reading data sheets written by
    marketing people :)
     
  3. A HP 3458A can do this. (Well, absolute accuracy would depend on the
    calibration).
    The delta-sigma ADCs seem a lot easier to use for slow signals, due to
    the way they average the signal noise I think (rather than sampling
    it). The number of "noise free" bits goes as the square root of the
    measurement time or number of samples averaged. I.e., each extra bit
    takes 4x the time. You can even get to 24 noise free bits if you don't
    mind the measurement taking a few seconds. The linearity can then be
    that of the device, typically 1-2ppm for a good one.

    TI claim a "32 bit" delta-sigma! :)

    <http://www.ti.com/product/ads1282>
     
  4. hamilton

    hamilton Guest

    Where did you read that !?!?

    AD7176-2 24-bit Sigma-Delta A/D Converter Key Features:

    5 SPS to 250 kSPS output rate for fast and flexible updates

    Up to 90 dB 50 Hz and 60 Hz line frequency rejection using enhanced
    50 Hz and 60 Hz rejection filters

    7.8 mA total current consumption
     
  5. Peter

    Peter Guest

    Interesting.

    I have the 3568 here. It is c. 20-25 years old and checking it against
    fresh stuff it seems less than 0.01% out.

    But these don't have to read fast. A 1 sec conversion time is fine.

    Great engineers in those days... I have the service manual for it too.

    And same for the 3314A, though the reed switches keep packing up.
    Fantastic circuit design, and the TMS9900 micro :)
     
  6. Guest

    If anyway there is going to be a multiplexor at the ADC input, why not
    reserve one channel for 0 V and an other for Vmax ? This way you can
    calibrate the ADC during each scan.
     
  7. Yep. In fact I am starting to suspect they can no longer do it. The 25
    year old 3458A is still Agilents premier DMM. Their only 8.5 digit one,
    and still pretty much the state of the art.
    The older 3458A are actually better than the new ones, since the
    references are aged, and the drift tends to go as sqrt(t). They still go
    for $4000 on ebay, not bad for a 1/4 century old meter.
     
  8. Peter

    Peter Guest

    There are NO analog engineers coming out of anywhere these days.

    Engineering/electronics education is crap in the UK and presumably
    also crap in the USA. My son was halfway through an electronics course
    (2yrs) and they just reached bridge rectifiers, but without a
    functional explanation!

    All the good analog engineers I know of are in their 50s, plus.

    When they retire, it will be fun :) It will be like the 1970s ... if
    you are good you can make a good living.
     
  9. miso

    miso Guest

    There are applications where you don't need the result good to the last
    bit in terms of a DVM, i.e. a little scaling error and offset are OK.
    Rather you want to capture the shape of the signal.

    Also note in some applications if you don't have a high dynamic range
    ADC, you end up with some sort of programable gain amplifier, which is
    probably worse than having the high dynamic range ADC.

    They use wide dynamic range ADC is seismic applications. If you are
    looking for the time information on the echo return, a little scaling
    error is tolerable.

    You have heard the expression that it is an analog world. If you take
    that one step further, it is an AC world in the sense that time varying
    signals are more important generally than DC. Of course there are cases
    where DC matters a lot, like in a scale.

    When you start to take contact potentials into account, DC accuracy at
    high bit levels is really tough too.
     
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