Voltage amp solution needed

Discussion in 'Electronic Basics' started by Pete, Aug 9, 2003.

  1. Pete

    Pete Guest

    Hi Group

    Ive recently purchased a 10-bit resolution data acquisition card with a
    range of -1V to 1V sampling at upto 20MHz.
    Upon running a few tests ive realised that the raw voltage signal that i am
    measuring varies between around -20mV and 20mV. My problem is that the
    resolution in that range is not all that great and i fear it is too late to
    exchange for a 16-bit card (which would do the job at a much higher cost).
    Is there a basic solution where i could construct or purchase an amplifier
    off the shelf that would condition the raw signal to use the entire span of
    the card(-1V to 1V) and is also capable of performing at frequencies upto
    20MHz?

    Thanks

    Peter
     
    Pete, Aug 9, 2003
    #1
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  2. Pete wrote:
    >
    > Hi Group
    >
    > Ive recently purchased a 10-bit resolution data acquisition card with a
    > range of -1V to 1V sampling at upto 20MHz.
    > Upon running a few tests ive realised that the raw voltage signal that i am
    > measuring varies between around -20mV and 20mV. My problem is that the
    > resolution in that range is not all that great and i fear it is too late to
    > exchange for a 16-bit card (which would do the job at a much higher cost).
    > Is there a basic solution where i could construct or purchase an amplifier
    > off the shelf that would condition the raw signal to use the entire span of
    > the card(-1V to 1V) and is also capable of performing at frequencies upto
    > 20MHz?
    >
    > Thanks
    >
    > Peter


    You can build a fairly simple 5X ampifier that is pretty flat up to 20
    MHz with something like an LM7171 ($2.80 from digikey) if you can come
    up with a positive and negative supply. The offset specs are not
    perfect, but you can measure that with your A/D converter board and
    subtract it from the other measured results.

    --
    John Popelish
     
    John Popelish, Aug 9, 2003
    #2
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  3. Pete wrote:
    >
    > Thanks John
    >
    > I have had no prior experience with OP-amps so this will be a pretty steep
    > learning curve of which i am keen on pursuing if it will do the trick. Any
    > chance that you would have a circuit schematic to use in conjunction with
    > the LM7171 that would suit this application?


    The video line driver on page 16 of the data sheet:
    http://cache.national.com/ds/LM/LM7171.pdf

    gives the general idea, except that the two resistors in the voltage
    divider to the - input have to have a ratio of 4 to 1, instead of
    being equal, to give an overall gain of 5 (say 510 to ground and 2k
    feedback, with, perhaps a 100 ohm trim pot between them (with the
    wiper to the - input) as a gain trim. And I am assuming that you
    would put this right next ot your A/D input, so no series terminating
    resistor would be needed on the output.

    Read the application notes section for how to bypass the supply lines
    for such a high frequency opamp.

    --
    John Popelish
     
    John Popelish, Aug 9, 2003
    #3
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