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Multiplying and Dividing voltages

Discussion in 'Electronic Basics' started by [email protected], Dec 15, 2004.

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  1. Guest

    Hi mates.

    Albert's here. I'm new here and i'm seeking your helps regarding the
    following.

    I'm working on a project which involves the arithmetic operations (+,
    -, X, /) on voltages. Using ADC conversion is not applicable in my
    case. The project is to capture 4 voltages from 4 testpoints from a
    PCBA, ie A, B, C and D. After capturing the 4 voltages, we need to do
    the following calculations:

    Result = ((A+B)-(C+D))/(A+B+C+D)

    For the addition and subtraction parts, i've got the idea of using
    summing amplifier and inverting amplifier to perform the functions. If
    you have other suggestion, kindly share with me.

    For the multiplying & division's parts, please give suggestion for the
    circuit.
    Thanks and hope to get your response soon.

    Cheers,
    Albert Leng
     
  2. Ah... There is an interesting circuit that will directly compute:

    differential_I = Ik.(I1 - I2)/(I1 + I2)

    So, let I1=A+B and I2=C+D

    http://www.anasoft.co.uk/EE/circuits/calculate.gif

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  3. Ban

    Ban Guest

    Albert,
    As you wrote, do first the sums with 2 opamps and then you can use a
    multiplier/divider to do the division. A very good unit is the AD538 and
    also AD539 or MPY100, but these are expensive. For a more economical unit
    have a look at the AD633, MLT04 and MPY634. I found the SSM2164 useful and
    use it in many of my designs, even if it requires some tricks to do a
    temperature and DC-stable division.
     
  4. Here is a link to Analog Devices multiplier/divider page:
    http://www.analog.com/en/subCat/0,2879,773%5F862%5F0%5F%5F0%5F,00.html
     
  5. Bob Eldred

    Bob Eldred Guest

    What is the accuracy required and what is the bandwidth of the signals? I
    know you have said that you don't want to go the ADC route, however than may
    give you the best performance for the money. I would explore using a
    microprocessor with built in ADC's such as the PIC 16F818 or something
    similar and couple the eight bit output to an inexpensive eight bit DAC to
    regenerate an analog result. That processor can run at 20MHz and can accept
    up to eight analog inputs, you need four. Once converted, the resulting
    digital signals would be added and subtracted in software, a trivial
    process. An 8 or 10 bit division algorithm in software would be used to
    develop the quotient and the result outputed as an 8 bit signal to a digital
    port on the processor. That would feed a DAC chip if required. Without
    actually writing the code, I'm sure it could be done in a couple of hundred
    steps and could easily run it a PIC at a 25KHz rate or so. A processor can
    also do other magic tricks like signal averaging or filtering, add or
    subract offsets and other things like auto zeroing to improve signal
    quality.
    Bob
     
  6. Couldn't you use op amps for log and anti-log curcuits to accomplish
    the MULT and DIV?
     
  7. Bob Eldred

    Bob Eldred Guest

    Yes you could. These circuits are tricky and need to be temperature
    compensated, but would work. Another method is pulse height pulse width.
    Bob
     
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