Strange output from Non-inverting amplifier

Discussion in 'Analog & Mixed-Signal Design' started by odm4286, Aug 7, 2016.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    I've wired up a simple non-inverting amplifier, using LM358N, and I'm getting some strange output on the scope. Thanks ahead of time, I know it can be a pain troubleshoot simply by looking at pictures of a circuit.
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    Here is a picture of the circuit
    [​IMG] [​IMG]

    Rf = 1M Rin = 100k so I'm looking at a gain of 11. Vcc is 12V and my signal to the non-inverting input is 0-500mV.

    So, am I missing something? Bad op amp? Thanks!
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    What you are looking at is 60Hz noise.

    As far as I can tell, you are making two common mistakes with your GROUND connections.

    1) LM358 can run on single supply or dual supply. You have chosen to use single supply (PSU) but your supply is "floating". It has no reference connection to EARTH GROUND. Jumper the WHITE (-ve) banana jack on the PSU to the black (GND) jack.

    2) Your input signal is not referenced to GROUND. Do the same. Jumper the WHITE terminal to GND.
     
    odm4286 likes this.
  3. WBahn

    Moderator

    Mar 31, 2012
    17,788
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    What is the waveform shape of your input signal?

    What is the frequency of the input signal?

    What is the op amp you are using? Is it rail-to-rail?

    How are you dealing with your ground references?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,808
    I see it is a LM358N
     
  5. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    1. It is a DC signal
    2. 0hz at the moment, I'm changing with a dial on my dual power supply
    3. its a LM358N
    4. I suspect that might be where my issue is but I'm not sure how to fix it. It is a dual mode power supply so maybe I have a ground problem? The digital ground in my circuit represents the negative of PS1 and the ground is the negative of PS2
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,452
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    It is not clear from the photos what is your input signal. I thought it was PS1.

    Ok, if you are using a dual power supply, here is how to connect them.

    Assuming we are using PS1 as negative supply and PS1 as positive supply
    PS1
    - GND +

    PS2
    - GND +

    Connect PS1 GND to + (i.e. BLACK to RED on PS1)
    Connect PS2 GND to - (i.e. WHITE to BLACK on PS2)
    Any of these is your GND reference. Connect this to the GND of R2 and the GND of your signal source.

    Connect PS1 - to LM358 pin-4
    Connect PS2 + to LM358 pin-8
     
  7. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Thank you, that solved my problem!
     
  8. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Actually PS2 is my Vcc and PS1 is my signal. Tying both negatives to earth ground fixed the problem
     
  9. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Woke up with a question. Lets say this circuit was battery powered and my signal came from some sort of transducer; Lets say a microphone. Would I have the same issue? Would I have to common up the battery negative and microphone negative? Hopefully I'm making sense, still learning op-amps and my brain is still waking up o_O
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    Yes.
     
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