Flash A/D error

Discussion in 'Feedback and Suggestions' started by Ron H, Mar 8, 2009.

  1. Ron H

    Thread Starter AAC Fanatic!

    Apr 14, 2005
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    This thread points out an error that needs to be corrected.
     
  2. Wendy

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    Mar 24, 2008
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    [​IMG]

    I don't see it, it looks OK to me.

    The top comparitor is to measure the overvoltage condition, which is mentioned in the text.

    Or are you refering to somewhere else in the text? To correct the text we need to pin it down.
     
  3. Ron H

    Thread Starter AAC Fanatic!

    Apr 14, 2005
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    Bill, I must be blind. Please point me to the text that explains the function of the top comparator.
    Below the schematic that you posted, there is a 7-comparator version, but the text implies that 2^n comparators are required.
     
  4. Wendy

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    They don't mention the top comparator specifically, but it does the same job. If the input voltage exceeds Vref then the 8th digit is on, the maximum number. Binary 000 - 111, which is eight permutations. Matches 8 comparators nicely.
     
  5. Ron H

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    Apr 14, 2005
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    But only 7 comparators are required to get all 8 codes. I agree that the 8th comparator will give some information about overvoltage, but if that's the intent, then it should be stated explicitly in the e-book, because it's a fact that only 2^n-1 comparators are required for an n-bit converter.
     
  6. Wendy

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    So how are you going to get 111 if you eliminate the top comparator? 8 possible numbers, 000-111, require 8 inputs. You can not have 000-111 with 7 inputs, it becomes either 001-111 or 000-110.

    Is the assumption if they are all zero then you have 000? I'm thinking of a mirror to the CD4512.

    **************************

    OK, I can see it. The author was making the same assumption I was, 8 inputs required for a mod3 output. This section is going to require a complete rewrite, I think.

    What he is showing is valid, but you can remove one comparator with the assumption of all zeros in equal all zeros out. Since removal of a component is more or less industry standard, I can see the confusion.
     
    Last edited: Mar 8, 2009
  7. Wendy

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    Another thing, look at the first 2 pictures, compared to the 3rd. It is correct on the 3rd illustration. Good catch.
     
  8. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
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    Thanks Ron and Bill for sorting this out.

    The corrections have been made to the master copy at ibiblio.org

    Ron H@allaboutcircuits.com is credited in the contributors page appendix-A-2 at ibiblio
    If you want to be credited under your real name, send me a private message.

    D Crunkilton
     
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