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Is Differential Amplifier badly inaccurate?

Discussion in 'General Electronics Discussion' started by eem2am, Oct 31, 2012.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,

    I need the following differential amplifier to provide a gain of 18 on the voltage across the series current sense resistor.
    (the diff amp resistors are 1K’s and 18K’s)

    In steady state, the voltage across the current sense resistor will be 0.1V.

    Schematic with differential amplifier….
    http://i50.tinypic.com/15ciz4i.jpg

    The opamp’s supply voltage is 10V, and the voltage at the diff amp’s non-inverting input is 3.6V….(with battery internal resistance of 0.06R)


    Do you think this will be badly inaccurate?

    Will I suffer from common mode effects

    Do I need to take measures to fight the input bias current of the differential ampifier

    The Opamp in the schematic is LT1006, but in the real circuit I will uise the cheap LM358.

    LM358 DATASHEET
    http://www.ti.com/lit/ds/symlink/lm358.pdf
     
  2. eem2am

    eem2am

    414
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    Aug 3, 2009
    sorry,

    the voltage at the opamp's non inverting input is not 3.6V.

    ....The voltage source at the left hand side of the schematic is 3.6V DC. The series sense resistor is 0.0876R.
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Admittedly my old eyes can't read the component values on that schematic but have you considered moving Rsens to the ground return of the source? You won't need differential input if you relocate it there.

    Chris
     
  4. eem2am

    eem2am

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    Aug 3, 2009
    Thanks CDrive, unfortunatley i cannot disturb the ground track as it is relied on to be the same for some other different devices in the system.
     
  5. eem2am

    eem2am

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    Aug 3, 2009
    Thanks CDrive.

    Sorry to go off on one, (its of course none of your fault) but I am amazed that in 2012, an opamp with a supply of 10V cannot accurately amplify (x18) a sense resistor voltage thats between 3.6V and 3.5V (i.e. about 100mV across the sense resistor total)

    ....there must surely be a way of doing this with a cheap differential opamp?.....otherwise, what on earth is the point of having a differential opamp?

    It seems to me that the wonderous "differential opamp" is a total waste of time?

    ........is the instrumentation opamp, costing 400% more, the only way round this?
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I suspect what's happening here is OpAmp overdrive but I can't be sure because I can't read the component values on your print.

    Please do me a favor and repost your schematic with larger font for the component values. Currently, the only things I'm able to read are your notations.

    Chris
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I just simulated your amplifier with an LM358. I used a 100mΩ for Rsens and gave the OpAmp a gain of 10 using 1KΩ input for Rin and 10KΩ for Rfb. I found it to track fairly well except where the OpAmp output is near zero but I would expect that. The closer the output is to the rails the poorer linearity is going to be. Even an OpAmp billed as "Rail to Rail" doesn't actually equate to exactly that.

    Chris
     
  8. eem2am

    eem2am

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    Aug 3, 2009
    Thanks CDrive,

    the pdf schematic of the above schem is attached.
     

    Attached Files:

  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Thanks for posting the pdf. You haven't given specifics of your inaccuracies so I don't know what they are. I also don't know why you chose an oddball value for Rs. Try simulating this. The Diode (D1) is used to greatly improve linearity when the output is near ground (0V).

    Chris
     

    Attached Files:

  10. eem2am

    eem2am

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    Aug 3, 2009
    Thanks CDrive, your graph shows no problems, which makes me think that all this "common mode stuff" regarding op amps is a hoax.

    I thought the whole entire point of a differential amplifier was that it amplified a voltage that was *not* with respect to ground......and yet all i hear from others (not yourself CDrive) is how bad they are.....

    ...well if theyre that bad , then why have them at all?......why not just skip the differential amplifier and go straight for the instrumentation amplifier?

    As CDrive you have found the same as me, that these mysterious problems of differential amplifiers dont actually show up on simulators......thats if these problems arent a hoax in the first place.
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Who are "they" and what "hoax" are you talking about? I don't believe that my circuit will behave much differently in the real world.

    It may interest you to know that an OpAmp, whether it's configured differentially or not, is referenced to circuit ground. Take my circuit for instance. The (+) OpAmp input is measuring the voltage between ground and the input side of Rs. The (-) OpAmp input is measuring the voltage between ground and the output end of Rs. The output will be the difference between the two inputs times the gain.

    Chris
     
  12. BobK

    BobK

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    Jan 5, 2010
    Each op amp has a specification of the allowed range of common mode. If you stay within this, they work as advertised. I don't see any problem with your circuit, as long as the voltage across the sensing resitor is between 0 and 0.5V, the output should be that difference * 18. Have you tried it and found that it does not work? What results are you getting that are wrong?

    Bob
     
  13. eem2am

    eem2am

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    Aug 3, 2009
    CDrive........I have read in many articles that say that plain, simple differential amplifiers with single opamps like i showed, are simply wildly inaccurate.

    They cannot handle the common mode voltage, and things like input offset voltage mean its just too inaccurate.

    ....These articles often say that an instrumentation amplifier is needed instead(comprising several opamps)

    I have only tried it on the simulator, and it works perfectly on that.....but i do not know if its "safe" for me to implement this in my PCB which i am preparing.
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It should be breadboarded first.

    Chris
     
  15. BobK

    BobK

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    An instrumentation amp can allow for common-mode voltage well outside the power supplies to the op amps. But you do not need that. Once again, if you stay within the common-mode specification from the datasheet it will work. I don't know why you are being so paranoid.

    Bob
     
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