DC Amplifier op-amp

Discussion in 'Electronic Design' started by google@bwgames.net, Sep 12, 2006.

  1. Guest

    Hiya,

    I have an input voltage that ranges between 0-1V, which can be at a
    high frequency.
    This output requires connection to a high-impedance load.

    I need to scale this to 0-4V or 5V so I can input it into an ADC.

    I'm currently looking at an Op-Amp but having difficulty getting it to
    work - currently have an LM358, Vee is GND, Vcc is 5V.

    I copied the design at
    http://img183.imageshack.us/img183/2161/opampvd8.jpg, but subsituting
    10k for R1 and 30k for R2.

    The input signal is connected directly to the + input however the
    output I get isn't as expected.

    Can anyone see anything wrong with it?

    Thanks,

    Ben
    , Sep 12, 2006
    #1
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  2. Guest

    wrote:
    > Hiya,
    >
    > I have an input voltage that ranges between 0-1V, which can be at a
    > high frequency.
    > This output requires connection to a high-impedance load.
    >
    > I need to scale this to 0-4V or 5V so I can input it into an ADC.
    >
    > I'm currently looking at an Op-Amp but having difficulty getting it to
    > work - currently have an LM358, Vee is GND, Vcc is 5V.
    >
    > I copied the design at
    > http://img183.imageshack.us/img183/2161/opampvd8.jpg, but subsituting
    > 10k for R1 and 30k for R2.
    >
    > The input signal is connected directly to the + input however the
    > output I get isn't as expected.
    >
    > Can anyone see anything wrong with it?


    Check out the data sheet for the LM358

    http://cache.national.com/ds/LM/LM158.pdf

    and find the specification for output voltage swing. The is only
    specified for a supply voltages of 30V, but note that the output -
    driving a 10k load - isn't guaranteed to get within three volts of the
    positive rail, and typically can only get within two volts.

    You want an amplifier with a rail-to-rail output - the National
    Semiconductor LM10 was the first one on the market, and Farnell still
    sell the LM10CLN for about $5 in small quantities. The Motorola
    MC33201P dual rail-to-rail op amp is a lot cheaper at about $1.50.

    http://www.onsemi.com/pub/Collateral/MC33201-D.PDF

    It has a marvellous low-output impedance output stage, but in all other
    respects is about as cheap and nasty as the LM358.

    --
    Bill Sloman, Nijmegen
    , Sep 12, 2006
    #2
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  3. Tim Auton Guest

    <> wrote:

    > I have an input voltage that ranges between 0-1V, which can be at a
    > high frequency.


    How high is "high"?

    > This output requires connection to a high-impedance load.
    >
    > I need to scale this to 0-4V or 5V so I can input it into an ADC.
    >
    > I'm currently looking at an Op-Amp but having difficulty getting it to
    > work - currently have an LM358, Vee is GND, Vcc is 5V.
    >
    > I copied the design at
    > http://img183.imageshack.us/img183/2161/opampvd8.jpg, but subsituting
    > 10k for R1 and 30k for R2.
    >
    > The input signal is connected directly to the + input however the
    > output I get isn't as expected.


    As you've kindly not said what you *did* see I'll assume the tops of the
    waveform are clipped and you never get less than a couple of ADC counts.

    You should check out the maximum and minimum output levels the LM358 is
    capable of. It's in the datasheet. It won't quite reach its -ve supply
    rail and it won't get within a couple of volts of the positive supply
    rail. A wider supply range for the op-amp ( Vee < 0V, Vcc > 7V ) or an
    op-amp with rail-to-rail outputs are possible solutions.


    Tim
    Tim Auton, Sep 12, 2006
    #3
  4. Phil Allison Guest

    <> wrote in message
    news:...


    ** Groper alert !!


    > I have an input voltage that ranges between 0-1V, which can be at a
    > high frequency.



    ** Sounds like video.

    Lotsa luck with a crummy LM358.







    ........ Phil
    Phil Allison, Sep 12, 2006
    #4
  5. Fred Bloggs Guest


    >
    > I have an input voltage that ranges between 0-1V, which can be at a
    > high frequency.


    No one can work with that kind of description, "high frequency." You
    either get specific or get lost. You know so little about things, that
    in your mind there is little to know.
    Fred Bloggs, Sep 14, 2006
    #5
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