Instrumentation amplifiers - what's the difference between models?

Discussion in 'The Projects Forum' started by summersab, May 24, 2010.

  1. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    I'm thinking of getting an instrumentation amplifier for a project of mine involving load cells. My question is thus: what is the difference between the multiple amplifiers? There's a slough of INAxxx models by TI, many more by other manufacturers, etc. Some are low power, and I take that to mean supply power, but does that make a difference? What's a general, single-amplifier IC package that is pretty cheap and most frequently used?

    My background is mechanical engineering, so feel free to talk stupid to me :)
     
  2. bertus

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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    This post seems to be related to your other thread - http://forum.allaboutcircuits.com/showthread.php?t=38198&page=6

    One way to see some of the differences is to take a look at the parametric data. Of course there is a slew of them - the differences are often pretty significant.

    I would suggest that you may need to find out some more detail about your strain gauges. Crushing with vice grips is not a successful mode of operation.
     
  4. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    @beenthere, not intending to be attacking, but you misunderstand what I was doing with the strain gauges because you didn't read the rest of my post (though I didn't expect you to since it IS 6 pages long). I mistyped - I wasn't crushing the strain gauges. I have load cells from a scale and was applying force to the load cells with vice grips. No harm there. Sorry to branch a thread like that. I was hoping to get a quick answer without the confusion of the previous 6 pages of my post.

    Since I don't get what "minimum offset current" and other lingo means, really, any suggestions on a general-purpose amplifier? My electronics coursework ended with Circuits 2, those datasheets are really way over my head. How are the INA114, INA126, and INA129 different, for example (other than some are more expensive)?

    Thanks for your patience. Seriously.

    EDIT: Being a bit more specific, I'm powering with a 9VDC "wall-wart" power supply for my project. The input to the amplifier will be coming from a Wheatstone bridge composed of 3 750 ohm resistors and one 750 ohm strain gauge/load cell. The change in voltage is VERY small when a load is placed on the load cell, so I need a high gain/sensitive amplifier.

    Here is a comparison of the three amplifiers that I see to be most common:

    http://focus.ti.com/paramsearch/doc...lResultsFlag=yes&totalCount=35&#COMPARE_TABLE

    Do I want lower input bias, lower input offset, lower noise, lower drift, etc to make a "more sensitive/accurate" circuit? In that case, the INA114 seems best. What's dual/single supply? Wow, I'm clueless.
     
    Last edited: May 24, 2010
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Unless you have some intent to create a precision weighing setup, the requirement for ultimate performance is unnecessary. I think you need something with flexibility for experimentation.

    This link puts up a table - http://focus.ti.com/paramsearch/doc...og&familyId=1612&uiTemplateId=NODE_STRY_PGE_T

    Things that are significant are, first of all, the package. For ease of use, a PDIP package is quite important. You can use a breadboard (with somewhat degraded results) and a socket on a PCB. Since you have a single supply, the table is limited to IA's that can operate with only one voltage.

    You also want a range of gain, as the sensitivity of the strain gauge is not known. The INA118 and 122 look to be good choices for wide ranges of gain. Consider using a trim pot for the gain setting resistor. Part of the fun is arranging the fixture with the gauges and the electrical output so the voltage translates to engineering units. That means the voltage signal can be displayed on a panel meter so the numbers show weight directly as grams or pounds or whatever.

    A bit of literature from Omega Engineering about strain gauges (they call them "gages") helps understand how they work.
     
  6. summersab

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    Apr 8, 2010
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  7. bertus

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    Hello,

    The specs of a INA126P are better than the INA126PA.

    Comparison chart:

    [​IMG]

    There are also ssome specs set for the INA126P wich are not named for the INA126PA..

    Bertus
     
  8. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    Yeah, I found that . . . but what does it mean, really? I mean, the column for the PA models is pretty much empty. Does that mean the specs are the same? that's practically different about these circuits?

    Like I said . . . EE background is sparse for me.
     
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