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8 Bit ADC question

  1. Oct 30, 2007 #1
    Missed some class notes due to being sick. Anyway have a tutorial question I
    need some help with as I missed out on the subject.
    For an 8 bit ADC, how many clock pulses are required for an exact mid-signed
    I think 4, appreciate if someone can advise.

  2. jcsd
  3. Oct 30, 2007 #2
    8 bit = 2^8 = 256
    i don't know what clock pulses. but if its 8 bit unsigned it is 256. if its signed then it becomes 128
    ie: 1 111 1111 -- > +
    0 111 1111 -- > -
    typically last bit/MSB to indicate polarity. I may be wrong
  4. Oct 30, 2007 #3

    thanks for that
  5. Oct 30, 2007 #4


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    Oh I don't think i did something like this but.

    If its Signed :
    and Minus ..then the MSB bit 7 is a 1. Indicating - is on right?
    Then again it could be a Two's Compliment or a Sign and Magnitude.
    +128 64 32 16 8 4 2 1
    0 0 0 0 0 0 0 0
    2s compliment
    -128 64 32 16 8 4 2 1
    0 0 0 0 0 0 0 0
    Sign and Mag
    -/+ 64 32 16 8 4 2 1
    0 0 0 0 0 0 0 0
    I don't think edmongd is right. My opinion.
    Although the 8 bit indicating 256 location is correct.

    Clock pulses are orginating from the System clock diagrammatically speaking on the outside of the cpu into the Control Unit of the CPU or Inside the Control Unit.
    They send clock cycles of 1-0-1 into the cpu something like that..
  6. Oct 30, 2007 #5


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    What is an ADC? Clock pulses come in cycles of 1 0 1 if I'm not mistaken but probably am.
    mid-signed range??? do you mean sign and magnitude ? boq.

    Isn't a clock pulse 1/0 ?
    Which then becomes a clock pulse cycle of 101 or 010 ermm ******

    Wheres the Expert whos doing a Degree in Computing...lol
    I've only done it till advanced!
  7. Oct 30, 2007 #6
    It depends on what kind of ADC.

    Pretty much all ADC's today are the flash type, which means it is instantaneous (asynchronous)--zero clock cycles. However, the fact that the question implies there are clock cycles suggests its either a counter (nobody makes those--they're more for teaching first-years the basic idea of ADC) or a successive approximation. So from the context it looks like we are to assume its a successive approximation ADC. A SA ADC always takes as many clock cycles as the number of bits. So it takes eight clock cycles no matter what. But then, since we're having to make presumptions because the question is ambiguous, I'm wondering why the question would imply the clocks are related to the input voltage--which, as I said, is not the case with a successive approximation ADC.

    There are also hybrid ADCs, which might, for example, make every pair of bits a flash-type, and successively approximate through each pair--essentially its a base-four ADC instead of a base-two ADC.
    Last edited: Oct 30, 2007
  8. Oct 30, 2007 #7


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    certainly all 8-bit A/D converters today are flash. but i might think that most A/D converters today are what they put on sound cards and are almost certainly of the "sigma-delta" type. sometimes called a "1-bit converter".
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