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Summing analog signals

Discussion in 'Electronic Design' started by Richard Holmes, Mar 11, 2010.

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  1. Can anyone give me some pointers for mixing DC-20KHz analog signals
    from different sources? For example, generated noise, sinewaves, etc.

    I have read about it and tried various circuits, resistor networks, a
    summing op amp, but it does not appear so straightforward. Adding a
    signal causes a voltage drop in the sum, etc.

    As a novice, there is obviously something I am overlooking. Any
    adfvice would be appreciated.

    Is there a sure-fire cirucit I can use for everything in this type of
    application?

    Richard Holmes
     
  2. Phil Allison

    Phil Allison Guest

    "Richard Holmes"

    ** Adding a signal source affects a resistor summing network - just adding
    a signal does not.

    A "summing" op-amp is not affected by either.

    Eg: http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm



    ...... Phil
     
  3. S1--R1---+
    S2--R2---+----Vo
    S3--R3---+

    Where + is all connected is the basic idea. The problem is that you cannot
    draw any current from Vo without distorting the results. This is called
    loading the output. You can, for example, connect a scope probe to Vo and
    expect to see S1 + S2 + S3.

    Now because we don't want to "load the output" we have to one of any number
    of methods to allow us to effectively load the output(i.e., still gives us
    Vo but let us draw a current from it without changing Vo).

    To do this you must use some type of buffering/amplification technique. A
    simple op amp buffer will work but will not provide amplification. You'll
    essentially get Vo out of the buffer but be able to drive larger loads than
    the resistor network will.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar.html

    The op amp buffer is at:
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html#c1

    So you connect Vo to Vin of the buffer and then Vo of the buffer is your Vo
    but you can now drive larger loads(up to the op amps spec).

    You can also implement buffers and amplifiers with bjt's and fets with
    effectively the same result(op amps are more precise).

    Here is what you are trying to accomplish:
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html#c1


    Make sure you get the resistor values right. If you have


    S1--R1---+
    S2--R2---+----Vo
    S3--R3---+

    Then the sum is

    (R1*S1 + R2*S2 + S3*R3)/(R1 + R2 + R3)

    which is known as a weighted sum. Generally you'll want R1=R2=R3. Also, The
    R's should not be too large. Something in the 10k's should work. Nothing
    below 1k in general.
     
  4. Guest

    It should be noted that the output signal will be the sum of the INn (where
    'n' is the number of the input) times the ratio of R/Rn. It's a very powerful
    circuit.
     
  5. So, assuming two input signals of 1Vpp and 2Vpp, and a single supply
    op amp, the following circuit will work?

    CA 3130

    +12V
    +-----|----+
    | | |
    | | [Trim pot] gain
    . | |
    [R2] |
    |___|
    |
    GND
    Not shown:

    1. R1 between +12V and non-invert IP.
    2. 120pf cap pins 1 + 7

    Sorry, I don't have the ASCII circuit program.

    Many thanks,

    Richard Holmes
     
  6. Phil Allison

    Phil Allison Guest

    "Richard Holmes"
    ** I see your pain.

    A summing amp is inherently an * inverting stage* - ie positive inputs
    generate negative outputs.

    If the input signals are DC coupled, ground referenced and swing positive or
    both ways - then you MUST use a suitable op-amp with dual supplies.

    The schem you posted will only work with AC coupled input signals - ie there
    is a cap between the sources and each input and the ouput will have a +6
    volt fixed bias.



    ..... Phil
     
  7. Rich Webb

    Rich Webb Guest

    One of the most often used is over at
    http://www.tech-chat.de/aacircuit.html

    Hasn't been updated in a while but it's quite useful.
     
  8. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    using bipolar input signals with a single supply op amp is not a good
    idea, especially if you want bipolar output signals too.

    Bye
     
  9. How about DC offsetting the input signals?

    Richard Holmes
     
  10. Phil Allison

    Phil Allison Guest

    "Richard Holmes"

    ** You will have to offset them in the negative direction.

    Do read what has been posted.



    ...... Phil
     
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