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OpAmp Input Stage with different voltage ranges

Discussion in 'Electronic Design' started by Marco T., Jul 27, 2006.

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  1. Marco T.

    Marco T. Guest

    I must develop a system that should accept at the input the following
    voltage ranges:

    0 5v
    0 10v
    0 15v

    I thought two chances:

    1) Using a voltage divider where I could choose the couple of resistor using
    2 multiplexer.
    I connect every resistor to input of a mux and I configure the two mux using
    a couple of jumpers.
    Then I connect to oupt of voltage divider a opamp voltage follower or

    2) Using a programmable gain amplifier.

    There are orther chances?

    Any comments is very appreciated.

    Many Thanks
  2. Marco T.

    Marco T. Guest

    I forgotten to say that output stage must be in range 0 5v.
  3. mmm

    mmm Guest

    If you want to use jumpers to configure the mux just remove the mux and
    use jumpers to configure input topology ( or even place only needed
    resistors )

    o---R----o---o------ to OPAmp
    | |
    R R
    | |
    GND GND to ground trough jumpers

    leave jumpers open for 0-5 range put one jumper for 0-10 two jumpers for
    0-15 range

    input impedance change on different range

    max134 from maxim show an interesting topology for input range selector
  4. If you're using an ADC you could consider use more resolution than you
    need and throw away an LSB or two for the lower voltage inputs.

    If you don't mind using jumper blocks, this is pretty easy/cheap:

    In Jumper position
    o 1: 0~15V
    | 2: 0~10V
    .-. 3: 0~5V (storage position for jumper block)
    20K0 | |
    | |
    1 | 2 3
    .--|--------. Out
    | o--o--o----------------o
    | |
    | o o o |
    | || |
    | || |
    10K0 '-''-'20K0
    | |
    | |

    You can buffer the input and/or output if you like.

    Best regards,
    Spehro Pefhany
  5. If you have a high input impedance and minimum settling time is
    important then you might be better off having a seperate resistor
    divider and buffer for each input and then have the mux on the low
    impedance output of the buffers.

    One fixed divider plus a programmable gain amp is probably the simplest
    solution though.

    Or simply a fixed 15V range and a higher res ADC as someone else

    I think Maxim make ADCs with built-in selectable ranges like this.

    Dave :)
  6. jasen

    jasen Guest

    The third position isn't needed the jumper can be stored horizontaly

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