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Op-amp circuit....

Discussion in 'Electronic Design' started by FrozenOne, Jan 7, 2005.

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  1. FrozenOne

    FrozenOne Guest

    I am looking to map voltages from one point to another, in a specific
    maner. For example:

    11.7 to 3.7
    12. to 5
    12.5 to 12

    Can this be done with an op-amp circuit???
     
  2. Tim Wescott

    Tim Wescott Guest

    Yes. This is not a simple y = ax + b -- is that intentional? If so,
    what relationship do you really want?
     
  3. FrozenOne

    FrozenOne Guest

    Yes it is intentionally a non-linear relationship. If I can get
    something close to the above it would be great since it does not have
    to be a perfect match as long as I am close.
     
  4. Jim Thompson

    Jim Thompson Guest

    Yes, provided the input/output relationship is monotonic.

    Surf for piece-wise linear curve-fitting.

    ...Jim Thompson
     
  5. Tim Wescott

    Tim Wescott Guest

    -- or implement y = a2 * x^2 + a1 * x + a0. It all depends on what
    you're after, really. If you only care about those three points and the
    rest of your input range simply doesn't matter at all then you can do it
    with a few comparators and resistors, and you won't have to mess around
    with diodes or multipliers or any stuff like that.

    You may also want to consider doing this with a microprocessor, DAC and
    ADC. Depending on the precision you need and the complexity of the
    curve you may come to a more economical solution that way. Without
    knowing your exact requirements between these points I'd consider all
    four possible solutions.
     
  6. lasnake

    lasnake Guest

    if the relationship is monotonic, could you use a ADC, a lookup table
    implemented by memory and a DAC for it?
     
  7. gwhite

    gwhite Guest

    yeah, but you only gave spot points, a complete piecewise function would be
    nicer.
     
  8. Jim Thompson

    Jim Thompson Guest

    See.....

    Newsgroups: alt.binaries.schematics.electronic
    Subject: Piece-Wise Linear Question (from S.E.D) - PWL-Example.pdf
    Message-ID: <>

    ...Jim Thompson
     
  9. Bob Eldred

    Bob Eldred Guest

    Are there only three points or is it a continuum between 11.7 to 12.5?
    Second, what is the precision? I see three significant figures, two
    significant figures and one significant figure, which is it? Three
    significant figures implies an accuracy and precision of 0.1% while one
    significant figure can vary by 10%. This makes a big difference in how you
    approach the problem and the precision of the resistors and non-linear
    network required. There are also temperature and drift issues as well.

    One poster mentioned using a microprocessor with ADC's and DAC's. It could
    be a simple look up table to develop the relationship. Beyond that, can the
    whole problem be done digitally without ever being in the analog, voltage
    domain? That may be the best and simplest. In other words, where do the
    voltages come from and how are the results used? It might be easier if those
    were digital numbers and not voltages from the get go.
     
  10. John Larkin

    John Larkin Guest

    What about using r-r opamps and letting some of them saturate to
    define inflection points? Of course, that will have the same glitch
    problems of most of the ideal-diode circuits.

    I've done breakpoints with non-ideal (real) diodes to bend the curves,
    with some other diodes for tempco. The knees aren't as sharp, but
    there are no transient spikes.

    John
     
  11. Ken Smith

    Ken Smith Guest

    Many "single supply" op-amps will swing very close to the (-) rail. If you
    run the op-amp from +15 to +24 and ground, you can make an op-amp's output
    stay at ground until you hit the knee in the transfer curve. A second
    op-amp can combine the output of this op-amp with your original signal to
    make a transfer function with an abrupt change in gains.

    Op-amps like the LM324 come 4 to a package. You can make 3 knees in the
    transfer function this way.
     
  12. budgie

    budgie Guest

    Is this an expanded scale voltmeter for auto/bike use?
     
  13. Fred Bloggs

    Fred Bloggs Guest

    Very nice idea.
     
  14. Some op-amps have sharper "knees" close to the negative rails than
    others. I used this in a product about 20 years ago for thermocouple
    linearization-- the CMOS op-amp I used was nice and sharp. The LMV824
    is not so good. WIth the LM324 you may have to keep the current
    through the output to a few tens of uA to keep it sharp (there's a
    nominal 50uA pulldown internally).


    Best regards,
    Spehro Pefhany
     
  15. Rich Grise

    Rich Grise Guest

    Speaking of PICs, how would one go about selecting one from, say, this
    search:
    http://www.microchip.com/ParamChartSearch/params.aspx?branchID=1091&mid=10&lang=en&pageId=74

    Thanks,
    Rich
     
  16. FrozenOne

    FrozenOne Guest

    Okay so to answer some of the questions:

    1) The voltage has to be continuous between the points; I am not just
    looking for those points
    2) Precision has to be close, but not exact
    3) Do not want to implement digital method
    4) I am making the circuit to control the response of an RF attenuator
    that I am using to improve on an older system, and as you can see
    Analog stuff is not my strong point. The Existing system can change
    the output by 6dB by varying the control from 11.7-12.5 volts. These
    numbers cannot change. I am attempting to implement a COTS attenuator
    to do the same function, but I need to map the existing control voltage
    to the COTS attenuator voltages. I hope this makes sense.

    I am thinking the PWL approach might be a good way to solve my problem
     
  17. Ken Smith

    Ken Smith Guest

    Between the points given, what do you have to be close to?

    As I suggested elsewhere, a quad op-amp can give you 3 knees in the curve.
    That would be a 5 point PWL curve. This was based on the idea that the
    sense of all of the knees were the same way. If the slope does not
    monotonically increase, you can get extra knees by using rail to rail
    op-amps and having them hit each rail.

    You can also fit to curves using X^N sort of functions implemented with
    analog multipliers. There are several analog multipliers on the market
    and some parts like the LT1228 and some AGCed IF stages that will serve as
    multipliers. If you can't stand PWL methods erros, maybe combining PWL
    with some non-linear function such as X^2 will do it for you.
     
  18. Andy

    Andy Guest

    The slope looks increasing. If you have only 3 reference points, maybe a
    an exponential amplifier could suffice?

    (Cf. http://www.national.com/an/AN/AN-311.pdf , figure 2.)
     
  19. Guest

    12.08X² -282X +1649.5
     
  20. Jim Thompson

    Jim Thompson Guest

    Did everyone miss my piece-wise linear post....

    Newsgroups: alt.binaries.schematics.electronic
    Subject: Piece-Wise Linear Question (from S.E.D) - PWL-Example.pdf
    Message-ID: <>

    ...Jim Thompson
     
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