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instrumentation amplifier

Discussion in 'General Electronics Discussion' started by PRIYADHARSHINI, Sep 23, 2014.

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  1. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    Hi
    i want to design the instrumentation amplifier..my input is 0.5v
    and my output should be with 5v..so i can vary the gain using Rgain resistor .
    i have designed the circuit...
    for 0.5v if i want to take 5v
    the formula given is (1+2(1k)/1.33k)(1k/1k)(2.5V-0.5V)=5V
    But in my circuit it always produce 3.88v at the output.
    if i remove the R gain resistor the same value will appear at the output
    .what is the reason for this...and i have used the LM324 opamp for this circuit
     

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    An LM324 cannot produce 5V at the output when operated from 5V supplies.Look in the datasheet, the output voltage is typically 2C less than V+ (or 2V more than V-), in the worst case up to 4V away from the rails.
    Your circuit will for the same reason never reach 0V at the output. Min. output voltage is +2V (since V- = ground in your circuit).

    Power your circuit from e.g. from +-9V (2*9V block battterie) and it should work. Or use so called rail-to-rail amplifiers (which still will not reach all the way to Vcc, a few Millivolts wil always miss due to drop across the internal output transistors).
     
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  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That circuit is wrong. Check where you have connected the inverting input of the bottom op-amp.

    Also as Harald says, you will need a higher VCC rail if you want to support 0~5V at your input.
     
  4. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    Thank u so much kris and Harald..i will try this.i have changed that bottom opamp connection....i have one doubt that i chose that ref voltage is 2.5v...input is 0.5v....is there any other issues will create wrong output?
    or we can set the ref voltage as per our wish?
    what happen if the ref voltage is less than the input voltage?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  6. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    thank u ..2.5V in voltage divider
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    There is no reference voltage marked in the diagram. Please add a marking for it, and update your diagram with the correction you made for the inverting input of the bottom op-amp.

    As long as you use a higher positive supply voltage, your amplifier will accept voltages in the range 0~5V at both inputs.
     
  8. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    How the min output will be +2v?
     
  9. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    here i have enclosed my updated schematic
     

    Attached Files:

  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    look in the datasheet. It tells you that the output voltage cannot be lower than V- +2V (or V- +4V in the worst case). This is due to the voltage drop across the output transistors(s).

    An instrumentation amplifier will amplify the difference between the voltages at the two inputs. In your case this is (2.5V - 0.5V) = 2V ! At a set gain of 10 (you said that you wanted to amplify 0.5V to 5V) this would lead to an output voltage of +20V (provided the amplifier can drive such a high voltage - which at 5V supply it can't).
     
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  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You haven't corrected the circuit error on the inverting input of the bottom op-amp.

    You haven't identified VREF; you have just marked some voltages on the diagram.
    Look at the LM324 data sheet. In the specification table there's a line called VOH. It gives the guaranteed minimum high output voltage for the device when it's powered from +30V.

    The worst case, with a 2 kΩ load, is only 26V. That's 4V lower than the supply voltage. Other values are 27V and 28V depending on the load conditions.

    So if your LM324 is powered from +5V the maximum output voltage is going to be somewhere arond 2~3V.

    So you need to power your LM324 from at least 8V. Preferably at least 9V. Which you have done. Good!
     
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  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Look at the bottom connection of R6. It needs to go to U1:3 pin 9, not pin 8, see the diagram in the article I linked.
     
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  13. PRIYADHARSHINI

    PRIYADHARSHINI

    137
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    Feb 6, 2014
    Thank u...is it correct?
    sorry that is not V ref...its V2(2.5V)
    V1(0.5V)
     

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  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yay! You've got the circuit right. Now does it do what you expect?
     
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  15. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    thank u so much kris and harald:):):)
     
  16. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    it gives -8.20v instead 3.3v at the output when i change Rgain as 3k
     
  17. LvW

    LvW

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    145
    Apr 12, 2014
    It would be much easier to help you if you could us all relevant information:
    * input signal
    * general gain formula used and required/expected gain value
    * output signal
    * Which kind of analysis?
    * mentioning of observed discrepancies
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Oops I spoke too soon. You have the inputs on the rightmost op-amp reversed.
     
  19. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    V1=0.5V
    V2=2.5V
    R gain=3k
    formula: (1+2(1k)/3k)(1k/1k)(2.5-0.5)=3.33v
     
  20. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    are u telling about U1:2?
     
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