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OP amp scaling voltage

Discussion in 'General Electronics Discussion' started by MangoKid, Jun 18, 2013.

  1. MangoKid

    MangoKid

    7
    0
    Dec 17, 2011
    Hey all,

    I have been trying to create a circuit to scale a 0 - 3.3v sensor to a 0 -5v signal. Any suggestions on how to do this. I have been trying some Op amp circuits but haven't really gotten anywhere yet.

    Thanks!
     
  2. GreenGiant

    GreenGiant

    830
    5
    Feb 9, 2012
    All you need is either a non-inverting op amp with a gain of 1.5 (gives max output of 4.95V with 3.3V in) or an inverting with the same gain and another inverter after that.

    (5V/3.3V = 1.5151515151 ~ gain = 1.5)
     
  3. MangoKid

    MangoKid

    7
    0
    Dec 17, 2011
    I have found this out through trying to solve this problem. I can achieve this using an Ideal OP amp in Spice. But when i have to build the ciruit using a real OP amp i am unable to get the ciruit working.

    Gain of 1.5 +5v (vcc) 0v (gnd)
     
  4. GreenGiant

    GreenGiant

    830
    5
    Feb 9, 2012
    are you making sure you have the supplies hooked up right? (+5V and -5V)
    Do you have the correct size resistors?
    Are you measuring input and output voltages to see what the gain actually is?
     
  5. BobK

    BobK

    7,607
    1,648
    Jan 5, 2010
    What op amp are you using and what power supplies? If the power supply is 0 and 5V, you will need an opamp capable of rail-to-rail outputs. If you are using and ordinary opamp you will need more like + and - 7V or more.

    Bob
     
  6. MangoKid

    MangoKid

    7
    0
    Dec 17, 2011
    I am using an AD8515 and yes i am using 0 an 5v not 5v and -5. I would like to avoid putting in a second supply for the -5v
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    It looks like that will allow an input voltage down to ground, a 5V supply, and will swing almost all the way to the supply rails as long as you don't try to load the output more than a couple of hundred uA.

    Show us the circuit you're using.

    Presumably the 5V supply and the 3v3 you're amplifying share a common ground?
     
  8. BobK

    BobK

    7,607
    1,648
    Jan 5, 2010
    Yep, the AD8515 looks capable of doing what you want. I await the circuit.

    Bob
     
  9. MangoKid

    MangoKid

    7
    0
    Dec 17, 2011
    The circuit attached i have doing 0-1v into 0-5v. The reason i am doing this at the moment is to make any problems apparent. Currently with this circuit i get an output of 1v no matter what the input is.
     

    Attached Files:

  10. MangoKid

    MangoKid

    7
    0
    Dec 17, 2011
    Great news! Solved it.

    The problem was in the net-list for the spice model i was using. So for anyone who is trying to accomplish sensor voltage level shifting the circuit in my last post works great. If you want 0-3.3v instead of 1-5v then use the resistors 15K and 10K for the divider and 22k and 33k for the feedback loop.

    Thanks for all the help guys
     
  11. BobK

    BobK

    7,607
    1,648
    Jan 5, 2010
    I doubt it. That circuit is an inverting amplifier with an offset. If you are getting the right results, something is wrong.

    Bob
     
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