I can't get my non-inverting op amp configuration to give me the right Vout?

Discussion in 'The Projects Forum' started by Bambi1091, Sep 10, 2015.

  1. Bambi1091

    Thread Starter New Member

    Sep 9, 2015
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    I have to build a non-inverting amplifier which has a gain of approximately 250 using an LM358. 220px-Operational_amplifier_noninverting.svg.png IMG_3022.jpg IMG_3023.jpg

    The values for my resistors are:
    Rf = 50.4 kΩ
    Rg = 201 Ω

    Therefore, I have a gain of approximately 251.7. I am sending a 5V output from an arduino to a voltage divider which makes the new Vout = 0.005V. I am then sending the 0.005V to the non-inverting input of my LM358. The power supply for my chip is a 8.75V battery. Whenever I check the Vout from my LM358 I am getting .95V when I should be getting approximately 1.25-1.26V. Any suggestions on what I have done wrong?
     
  2. RichardO

    Well-Known Member

    May 4, 2013
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    Some suggestions:
    First, make your pictures much smaller -- 100 Kb is probably big enough. Images this large take too long to load and will discourage replies.
    Second, give the schematic of the circuit that you actually built so we can see the details.
    Also, I don't see the LM358 in your pic's. There may be a wiring error that we can spot.

    By the way, welcome to AAC!
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The input offset voltage is, according to the datasheet, 3mV. Your signal is in that range. Since the offset is constant (at constant temp), you should now be able to correct for that offset in software and get predicted output for all other values.
     
    Last edited: Sep 10, 2015
  4. eetech00

    Active Member

    Jun 8, 2013
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    LM358:
    Looks like the input is fed to pin 3 (correct).
    Should be a 0.1uf cap from pin 4 (GND) to pin 8 (VCC).
    But I don't see a meter connected to the output pin (pin 1).
     
  5. Bambi1091

    Thread Starter New Member

    Sep 9, 2015
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    Ok, totally new to this so I will resize my pics in the future. I can't find an option to edit the post I have already made or else I would go ahead and resize them. Thanks of the heads up! I would give a schematic of the circuit I built but I don't have any software to do that. Do you know of any? Also the LM358 is the 8pin chip in both of the pictures I provided.

    Also I checked the output from my arduino and it is 4.9V. After I put that voltage through my voltage divider, I checked the voltage with my voltmeter and it is 0.0049V.
     
  6. Bambi1091

    Thread Starter New Member

    Sep 9, 2015
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    Yes, I took the wires that I was using to connect to my voltmeter out of pin 1 and ground in order to make the circuit look less messy in the pic.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    So, according to the datasheet, the "balanced" status of the two inputs is 0 to 0.003V. Since your input is 0.005V, the actual difference between ground and your signal could be anywhere between 0.002V and 0.008V. See "input offset voltage" on the datasheet. You will not get an exact value with a ground referenced DC circuit.

    The amplification is the amplification of the difference of the two input pins. You don't have to have one at ground. Just make them 5mV different to get your target voltage.
     
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  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Single supply, it seems.
     
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  9. absf

    Senior Member

    Dec 29, 2010
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    Here's a reproduction of your BB in proteus. The output in simulator seems better than you've expected but that is only in a simulator. In real life the input voltage offset of LM358 will affect the output voltage as others has explained.

    OP amp 1091.PNG
    Allen
     
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  10. RichardO

    Well-Known Member

    May 4, 2013
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    No need to be fancy. Just hand draw your schematic and post a picture of your sketch. For the future, you can download LTspice from Linear Technology. LTspice is a circuit simulation program. It is free and can be used to draw circuits which is useful even if you don't simulate them. At some point you will likely do some simulation and a number of members here use LTspice which makes it simpler to exchange design ideas.
     
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  11. crutschow

    Expert

    Mar 14, 2008
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    If you want to minimize problems with offset in a high gain DC circuit then you need to use a single-supply op amp with a low input offset voltage (as stated in the data sheet).
     
  12. #12

    Expert

    Nov 30, 2010
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    Here are some examples of free schematic software:
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I've been looking everywhere for a drawing package that includes such a comprehensive component library. I can't believe your library includes the LS7210!
     
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  14. eetech00

    Active Member

    Jun 8, 2013
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    Except those are schematics created with hardware :D
     
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  15. Bambi1091

    Thread Starter New Member

    Sep 9, 2015
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    Thank you everyone for your advice and help!
     
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