how to offset voltage in a instrumentation differential amplifier

Discussion in 'The Projects Forum' started by rhodes, Mar 24, 2010.

  1. rhodes

    Thread Starter New Member

    Mar 10, 2010
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    hi everybody,

    i would like to know how to offset a 28mV voltage to zero, that is present at the output of an instrumentation differential amplifier built with LM324 IC .I was suggested to use the output of 28mV as an input to a another amplifier to obtain an offest ,,, I really dont know how to proceed with that idea ?any suggestions would be appreciated. thnks in advance!!
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    post the shematic
     
  3. rhodes

    Thread Starter New Member

    Mar 10, 2010
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    the circuit is similar to my circuit with lm324 chip used instead of lm107IC and 100k resistors are used with 5k pot to obtain a gain of 500.
     
  4. rhodes

    Thread Starter New Member

    Mar 10, 2010
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    its the
    FIGURE 1. Differential-Input Instrumentation Amplifier used for my project
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    The image did not come across. Try the "Go Advanced" button, and look for "manage attachments". Posting in PNG format is very clear.

    Sight unseen, though, the 28 mv offset is probably due to unmatched resistors and the rather elderly LM324. Using a modern instrument amplifier might eliminate that problem.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    An LM324 has very poor spec's.
    An instrumentation amplifier has very good spec's and matched resistors. Use one instead.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Must be an echo in here....
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    :D hee hee hee
    So I've noticed
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    How many times do I need to tell school kids that a 741 opamp and LM358/LM324 opamps have very poor spec's? Echo, echo!
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    THAT... definitely was not the echo [​IMG]
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Then maybe the school kids should learn (it is simple Engrish) how to understand the spec's in datasheets?).
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Funny post.
    I just looked at the data sheet.
    Guess what? OP is trying to replace the original with a low quality one.
    Any one can see clearly the IC no. used.

    Some People try the impossible. Tsk. Tsk.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    I think it was the stupid old teacher who selected the lousy old LM324 opamp.
     
  14. rhodes

    Thread Starter New Member

    Mar 10, 2010
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    Could I atleast try to get some positive feedback from you people to proceed with my project ( which consists of building sensor circuits to measure output power, speed and torque of a testing rig generator) ..have you got suggestions on any other chipor circuit other than lm324 and lm741 for amplifying the data from the load cell and reduce offset in that measurement ,instead of making fun... so i could get some knowledge from this forum

    thanks
     
  15. rhodes

    Thread Starter New Member

    Mar 10, 2010
    8
    0
    Could I atleast try to get some positive feedback from you people to proceed with my project ( which consists of building sensor circuits to measure output power, speed and torque of a testing rig generator) ..have you got suggestions on any other chipor circuit other than lm324 and lm741 for amplifying the data from the load cell and have no offset in that circuit ,instead of making fun... so i could get some knowledge from this forum

    thanks
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Certainly.

    Please read at least the first few pages of this Application Note:
    http://www.intersilsemi.com/data/an/an1298.pdf
    It explains why it's so futile to try to build one yourself.

    Have a look at the INA122 and INA128 instrumentation amplifiers by Texas Instruments/Burr Brown.

    Note that the INA128 requires a dual supply, as it is not rail-to-rail. However, it's priced quite competitively.

    You really get what you pay for with instrumentation amplifiers. You really can't hope to "roll your own" instrumentation amplifier that's anywhere near as good. It's OK if you're just doing it as a learning exercise. But once you've learned how difficult it really is to get good operating characteristics, you'll appreciate how good the purpose-built instrumentation amplifiers really are.
     
  17. rhodes

    Thread Starter New Member

    Mar 10, 2010
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    thanks for the help guys.
     
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