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Problem with osciloscope and signal

Discussion in 'General Electronics Chat' started by spintinas, Feb 28, 2018.

  1. spintinas

    spintinas New Member

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    Hello, I have circuit with this amplifier, and with this circuit I amplify my pressure transducer signal from 30mV to 700mV. Amplifier works correctly, when applied pressure is increasing, multimeter shows higher voltage (range 700mV-1.3V). Both, transducer and amplifier are powered from Arduino 5V. Problem is that, when I connect scope cables to circuit output, voltage is decreasing to 500mV from 700mV (700 mV in multimeter) and when i applied more pressure to transducer, signal in scope still is about 500mV and there is no changes in signal amplitude, while there should be some increases in signal amplitude. (i attach photo with scope screen and my schematics).
     

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  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    What is the value of R1?
    What is the input impedancw of your 'scope?
    What is the voltage at pins 2 and 3?
     
  3. spintinas

    spintinas New Member

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    R1 is 10k'ohms, scope input impedance is 100k'ohms, voltage at pins 2-3 in range of 30-60mV.
     
  4. dave miyares

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    You would add offset to that poor excuse for a differential amplifier even if the scope input impedance was 10megOhms
     
  6. spintinas

    spintinas New Member

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    What do you mean to add offset to excuse? I connect all circuit on breadboard, and when I put osciloscope probe on my amplifier output, at the moment voltage decreases from 700mV to 300mV in multimeter.
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    What is the part number of the opamp?

    What exactly are the values of R1 to R4 as measured with an Ohmmeter?

    What are the values of C1 and C2?

    Where are you connecting the scope ground clip?

    Capture the scope trace again, this time with the scope horizontal sweep set to 0.5s.

    Why did you start a new forum thread with the same question?
     
  8. dave miyares

    Dave New Member

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  9. spintinas

    spintinas New Member

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    LM-324, C1 and C2 at the moment i didnt use, just resistors and amplifier, o connect the ground clip to the wire, which connected to blue line of breadboard, where also connect grounding from resistor and amplifier gnd pin.
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Many op-amps are not built to run from 5 volts.
    Many op-amps are not built to have a output that is very close to the supplies. In this case an output of 5V and 0V may be hard to get. You probably need a R-R output amp. (rail to rail)

    Many op-amps have a input common mode range that you might be violating. If the two outputs of the transducer are near 2.5V then that is OK.
     
  11. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    LM324 is not suitable to be used for this circuit. It is very marginal when operated from 5V. It is also a very noisy opamp.

    You never answered the questions about the actual measured resistor values.
     
  12. unclejed613

    unclejed613 Well-Known Member

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    you need an op amp that can operate from a 5V supply. the latest TI version of this op amp can run from a 3V supply, but earlier versions of it required 5V or more (and running it at the bare minimum of 5V was a dicey proposition). you should also have R1=R3, R2=R4 and Rb2= Rb3. Rb1 should equal the midrange value of the sensor.
     
  13. spintinas

    spintinas New Member

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    Op amp is supplied from 5V in my circuit, R1=R3 and R2=R4. Signal drops when i connect ground of the probe to the ground of the breadboard, when only probe tip is connected, signal dont drop.
     
  14. JimB

    JimB Super Moderator Most Helpful Member

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    Well, that indicates a problem right there.

    There has to be something odd about the power supply.
    Why are you powering your test circuit from an arduino?
    Power it from a proper power supply or a battery and see what happens.

    JimB
     
  15. spintinas

    spintinas New Member

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    Because I only have Arduino now for power supply.

    This is the circuit that I connect.

    [​IMG]
     
  16. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Please do a test: With out the scope! Ohm meter where the green lines are. Are the "grounds" connected together? On my boards they are not.
    Next add a capacitor from +5 to ground at the IC.
    upload_2018-2-28_16-45-41.png
     
    • Agree Agree x 2
  17. alec_t

    alec_t Well-Known Member Most Helpful Member

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    You have IC input pins left floating. They could be part of the problem.
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    See the ground pin 11 not connected to anything? On another website another kid in his class said it fixed it when it was connected.
     
    • Informative Informative x 1

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