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Trying to answer a successive approximation question

  1. Apr 8, 2012 #1

    Femme_physics

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    The following component is connected to a reference voltage of Vref = 6V. The component is given the value (2C)16 to convert. Calculate Vout.

    http://img69.imageshack.us/img69/7396/figuringout.jpg [Broken]

    Basically I treated this supposedly successive approximation converter like any other converter is this calculation..hope it's valid?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 8, 2012 #2

    I like Serena

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    Looks good to me. :smile:
     
  4. Apr 11, 2012 #3

    Femme_physics

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    Thank you!
     
  5. Apr 11, 2012 #4

    NascentOxygen

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    One tiny error... :wink:
     
  6. Apr 11, 2012 #5

    Femme_physics

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    What? :uhh: What could that be?
     
  7. Apr 12, 2012 #6

    NascentOxygen

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    What we're discussing in your other thread. :uhh:
     
  8. Apr 12, 2012 #7

    Femme_physics

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    Which? I made way too many heh...:wink:
     
  9. Apr 12, 2012 #8

    NascentOxygen

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    I've suggested they create another sub-forum, Femmes_Forum.
     
  10. Apr 12, 2012 #9

    Femme_physics

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  11. Apr 13, 2012 #10

    Femme_physics

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    Did I actually made a tiny error? Can you please point it out, I can't see it.
     
  12. Apr 13, 2012 #11

    Femme_physics

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    I need to provide an explanation of how this A/D converter with 8 bits work based on successive approximation...but I'm still not sure I understand how successive approximation works. According to my notebook,
    "
    In this type of converter there is a logic system that's comprised of a shift register. The logic system provides a logic '1' to DAC from the MSB till the LSB, although if the comparator outputs '0' the logic system reboots the bit and this way you get binary digit that gets closer to the final number in each period. And Vout gets closer to Vin.
    "

    ...


    But if the logic system REBOOTS the bit how come we get a binary number closer to the final number?

    If I reboot my computer it just reboots my computer, it doesn't get me closer to any value.
     
  13. Apr 13, 2012 #12

    I like Serena

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    For starters, it's a DAC and not an A/D converter.

    "Rebooting" the bit only means that it sets the bit to zero.
    It's similar to rebooting your computer, which sets ALL bits to zero. ;)
     
  14. Apr 13, 2012 #13

    Femme_physics

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    Well they've made a mistake in the question...don't look at me.

    Yes, but Successive Approximation graph is like this:

    http://img85.imageshack.us/img85/7590/sasan.jpg [Broken]

    How...does...it do it?

    The explanation doesn't let me know that
     
    Last edited by a moderator: May 5, 2017
  15. Apr 13, 2012 #14

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    I am looking at you. :)
    Your problem statement shows a DAC, but a successive approximation converter is an ADC...



    So we are talking about an ADC now.

    It starts with the most significant bit, which corresponds to the number 128.
    In your case the input has a higher voltage, so this bit is set, and we move on the the second bit.

    The second bit corresponds to the value 64, for a total of 128+64=172.
    In your case the input has a lower voltage, so this bit is reset (or "rebooted" if you will).
    So we remain at 128 and try the 3rd bit next (corresponding to 32, for a total of 128+32=160).

    And so on...
     
    Last edited by a moderator: May 5, 2017
  16. Apr 13, 2012 #15

    Femme_physics

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    So it's basically doing guesswork based on constant numbers?

    I don't see how could it be necessarily faster than the continuous method?

    Ah..you're right, the question is not about the pic I posted
     
  17. Apr 13, 2012 #16

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    Not guess work, but successive measurements which determine each bit.

    In 8 steps the analog input signal is converted into the 8-bit number 175.


    The alternative method that counts from 0 up, would need 175 steps to count up to 175.


    Now I'd say that 8 steps is a marked improvement over 175 steps.
    (It's a gem in electronic engineering. ;)
     
  18. Apr 13, 2012 #17

    Femme_physics

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    Why do we even use the continuous one if it's so crappy? A matter of cost?
     
  19. Apr 13, 2012 #18

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    I think it's a matter of exercise and education.

    If they'd give you the schematic of a direct conversion ADC, that'd be pretty overwhelming.
    But the principle given by a count-from-zero circuit makes the material understandable.
    And ultimately it does the same thing, just slower.
     
  20. Apr 14, 2012 #19

    Femme_physics

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    Ah...understood. :) Perfectly.... I like it. Thanks. Do you know what made Nascent Oxygen say my calculation was wrong?
     
  21. Apr 14, 2012 #20

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    I believe NascentO2 meant there should be (28-1) in the denominator instead of 28.
    As far as I'm concerned both would be okay.

    Since your teacher gave you 28 in the denominator, I'd say there was no error.
     
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