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ADC with successive approximation VS Digital ramp ADC

  1. Jan 16, 2012 #1
    I've asked this question before. But still I got some unanswered ones.
    I am really tired, but I cannot sleep if I got something laying there, tingling me.

    http://pokit.org/get/cfb750b79f49cfc12dc51a74a37f576e.jpg [Broken]

    This is digital ramp.

    In attachments I added a full circuit, from allaboutcircuits.

    Can somebody explain to me, how does the counter-DAC part works. I though I understood it, but i am still not comfortable with the idea. What kind of DAC gives even steps, how come it increments so evenly, that ramp? How does this counting works? What is that counter essentially?

    It is confusing me, because in succesive approximation, we have a "normal" DAC, like DAC with R2R.

    http://pokit.org/get/b75abb641b183f5128aa50a8eacd0770.jpg [Broken]

    Those jumps are uneven, and I understand that. But how come out of the digital ramp ADC, the DAC gives even steps.

    It doesn't matter whether you go from LSB to MSB, or the other around, those steps cannot be even!!
     

    Attached Files:

    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 16, 2012 #2

    jim hardy

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    ""It doesn't matter whether you go from LSB to MSB, or the other around, those steps cannot be even!!""

    i think in pictures.

    if you're incrementing LSB the steps have to be same size,
    0000=0
    0001=1
    0010=2
    0011=3
    0100=4
    0101=5

    one per step

    but start from opposite end, MSB, and what happens ?

    0000=0
    1000=8
    0100=4
    1100=12


    did i get the right question?
     
  4. Jan 16, 2012 #3
    We think the same then :)

    I though the increment was like this:
    0000
    0001
    0011
    0111
    1111

    or better yet,

    this crossed my mind

    0000
    0001
    0010
    0100
    1000

    and accordingly for from MSB

    0000
    1000
    1100
    1110
    1111


    There was my problem. So the increment goes like you said? Really didn't know that.
    Mr. Jim you are my savior.

    How do you know so much about electronics, if you were a energetic-trained?(Power lines etc). Every question I asked, you knew the answer about electronics...
     
    Last edited: Jan 16, 2012
  5. Jan 16, 2012 #4

    jim hardy

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    Gold Member
    2016 Award

    ahhh , logical enough.
    in high school electronics class(ca 1963) teacher had us boys all build simple two transistor flip flops. We hooked them in cascade, to make a binary counter of about eleven or twelve bits and connected Simpson multimeters(analog) to show status of each stage. (LED's were only a dream then.) Drove LSB with a low frequency oscillator.

    We watched the pattern on the meters as lsb ticked regularly and each successive stage of course ticked half speed. Took an hour to count up to all ones and we cheered as all eleven meters swung together back to zero...

    so i guess that visual demonstration imprinted the progression of a binary count and the concept of carry .

    i take it you're studying various ADC's. i like dual slope integrator the best.
     
  6. Jan 17, 2012 #5
    You know what, I will to my college demand an access to laboratory, and do just that! Thank you mr. Jim.
     
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