Help to increase the accuracy of op-amp[lm324] as possible as

Discussion in 'General Electronics Chat' started by booboo, Apr 27, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    Hi guys
    I'm working with LM324 to learn how to use Op-Amp and all aspects of it. currently my circuit is this(a simple non-inverting Amp):

    [​IMG]

    And here is a table of inputs and outputs and the gain of each input:

    [​IMG]

    The gain of each input is odd to me. I'm trying to increase the accuracy of this circuit as possible as. I have applied 9.6mv in the input and got 4.75v in the output. or for 11.2mv I got 5.39v. What's the problem? IIRC the max input offset voltage is 7mv. Am I right? please show me by mathematical calculation that why the output is this? the gain isn't correct. should I bias the input offset current or offset voltage? How?
     
  2. #12

    Expert

    Nov 30, 2010
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    Have you connected the other 3 amplifiers in that chip so they will stay at a stable DC voltage?
    Do you have the circuit shielded with metal so it can't pick up local noise?
    Do you have an oscilloscope to check for oscillation?
    Why don't you try for something like a gain of 10?
    How are you making the input voltage?
    Is your input voltage so purely DC that you can't be amplifying a ripple voltage by 392?
     
  3. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Are you sure about your measurement at 3.1v? Other than that measurement, the values you get are very close to expected. They can be estimated by:
    output=385.83*input + 1065.11​
    See the graph below...
    Capture.PNG
    Of course, this is the purely mathematical view, per your request. #12 has a much more realistic view of your post.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Other than a suspect data point, it seems clear what's happening.
    y = 385.49x + 937.46

    Screen Shot 2015-04-27 at 1.42.51 PM.png

    Oh wow, was I slow today.
     
  5. #12

    Expert

    Nov 30, 2010
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    I welcome your assistance.
    My experience with op-amps is that they are exceedingly predictable if you know how to arrange the circuit.
    Please label your graphs. That would make it easier for me to see that the gain seems to be 327 (Y= MX + B) and a few millivolts of input offset voltage can explain the B term.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Plot a graph of the values in post #1. Extrapolate it back to zero input. What do you deduce the actual input offset voltage to be?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Picky picky!
    Screen Shot 2015-04-27 at 2.05.53 PM.png

    Note that the 937.46mV intercept ÷ 385.49 slope gain = 2.43mV of offset. Well within specs.
     
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    You were slow??? I forgot to take the suspect data point out of the regression... D'oh
     
  9. wayneh

    Expert

    Sep 9, 2010
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    "Fast is fine, but accuracy is final. In a gun fight... You need to take your time in a hurry."
    - Wyatt Earp
     
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    ...although when I run it through Excel omitting the oddball value, I get somewhat different values.
    Capture.PNG Capture2.PNG
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Where'd yo get the last 2 data points? (Not the omitted one.) I see only values up to 7mv input.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    wayneh likes this.
  13. wayneh

    Expert

    Sep 9, 2010
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    Indeed they are! I checked - that explains the difference between my plot and djfantasi's.

    Another gunfight lost.
     
  14. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Bertus identified it. The TS mentioned those two points in the text.

     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    LM324 has a horrible +-7mV input offset. TS is running this (rather poor) opamp at much too high a gain...
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The OP484 would be a better choice.
    That opamp has a worst case offset of 450 μV.

    See the attached datasheet for more info.

    Bertus
     
  17. crutschow

    Expert

    Mar 14, 2008
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    I believe you missed the fact that the offset is amplified by the closed-loop gain, the same as the signal, so for the small signals you are using, the offset can give an output greater than the signal.
     
    Last edited: Apr 27, 2015
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Given the OP's aim ("I'm working with LM324 to learn how to use Op-Amp and all aspects of it.") I see no problem with using an LM324 in this exercise. He has now learned something about offset voltage.
     
    ErnieM likes this.
  19. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Anyone daring to suggest the "old lousy" 741 for the same educative purposes? :)

    Seriously speaking, not bad to start with them and then going to something better.
     
  20. #12

    Expert

    Nov 30, 2010
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    The common mode input voltage is 3 volts above ground to 3 volts less than the Vcc. Not best for a beginner that is making DC amplifiers with a single supply.
     
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