Absolute value of difference circuit

Discussion in 'The Projects Forum' started by kender, Aug 15, 2011.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    Colleagues,

    I’m trying to come up with an analog circuit that would let me detect when the difference (error) between 2 signals becomes greater than a preset threshold. Both signals are DC between 0V and +2V. Either one can be more positive. A negative supply rail is available in the system. Some inaccuracy can be tolerated.

    There are number of approaches to the problem. For example:
    1. Take the difference of the signals with a differential OpAmp circuit (or InAmp), then take absolute value of that with another OpAmp circuit, than compare it to V_{max,err} = const with a threshold comparator. That would require 3x building blocks: 2x OpAmp, 1x comparator.
    2. Alternatively, take the difference of the signals with a differential OpAmp circuit (or InAmp). Then do a window comparison -V_{max,err} < V < + V_{max,err}. That would also require 3x building blocks: 1x OpAmp, 2x comparator.
    Could there be more elegant ways of doing this (or am I asking for too much)?

    Any suggestion, insight or reference is really appreciated!

    - Nick
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Use a microcontroller with A/D converter builtin.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    That's the approach I would choose. However for a precision rectifier/absolute value circuit you'd need 2 Opamps, then the comparator.

    4 Opamps fit into one package. There is nothing inelegant to it.
     
    Last edited: Aug 16, 2011
  4. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    According to the figure on p.87 here (TI OpAmp handbook), it may be possible to make an absolute value circuit with a single OpAmp.
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Oops, looks strange, I wonder how it can measure the absolute value if for example the input voltage is negative and is limited by the lower diode's forward voltage drop....

    I don't get it, maybe because it's almost midnight.?

    The circuit on page 89 looks more like what I meant.
     
    Last edited: Aug 15, 2011
  6. SgtWookie

    Expert

    Jul 17, 2007
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    That "Absolute Value" circuit on page 87 absolutely doesn't work. I wonder how they snuck that one past the proofreaders?
     
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Actually I think your second approach is far simpler and most likely to work first time out of the box.
     
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