Help with 0 to 5v in 5,G,-5 out signal

Discussion in 'The Projects Forum' started by fxmech, Oct 21, 2011.

  1. fxmech

    Thread Starter New Member

    Oct 21, 2011
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    Hi I am trying to drive a galvonometer that takes +,G,- 5v input signal (3 wire).
    I was told it could be done using a DAC chip output to an opamp chip.
    The Opamp I was recomended to use is a TL082CN which is a dual JFET chip.
    I think I have the DAC part of the circuit down, but I need help taking the DAC output and running it through the OPAMP
    Could someone help me figure out how to do this?

    I am driving it off of an arduino using SPI output. I was able to get a bit of info on the net for the DAC part of the circuit but not the opamp.
    The setup is ARDUINO out ----> DAC -----> OPAMP ----> galvodriver board input so that it will connect to the +5v and -5v input signal pins on the galvo driver board.

    Here is a schematic of the DAC circuit:

    [​IMG]
    The DAC is a TLV5618 and the OPAMP is a TL082CN which is a dual JFET chip.

    BTW the galvonometer comes with a power supply and a set of driver boards so that part is already set... all I need is to get the signal that it needs as an input.
    Here is a link to the unit I bought and the user manual.

    http://www.ebay.com/itm/20Kpps-Hight...item3a5efda62b

    It sounds like a very simple circuit and maybe my problem is simply that I don't know the correct name to google.
    Any help would be very appreciated...
     
  2. #12

    Expert

    Nov 30, 2010
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    Here are some examples of op-amp circuits.
    Please examine these and reply with further questions if necessary.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Try it like the attached.

    Note that the +10v supply needs to be accurate and stable.
     
  4. fxmech

    Thread Starter New Member

    Oct 21, 2011
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    I'll try it. THNX for the help!
     
  5. #12

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    Nov 30, 2010
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    @wookie
    How did you figure out that it needs a non-inverting amplifier with a gain of 2, an offset of -5V and power it from +/- 10 volts that he doesn't have?

    Even after seeing the answer, I can't figure out how you did that!
     
    Last edited: Oct 22, 2011
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I cheated. :)

    [​IMG]

    Basically, what our OP needed was to offset the input by -2.5v, and then multiply by 2. That's what the circuit I posted does.

    To keep the supply requirements simple, instead of 10k to a 5v source, I used a voltage divider of 20k resistors. That works out the same as using a 5v supply and a single 10k resistor.

    I spun my wheels for a bit trying to sum the input with a negative on the input side - that just didn't go well. My algebra stinks, and this little problem was basically all algebra - or simply recognizing what needed to be done and toss an appropriate circuit at it. I'm pretty good at the latter.

    He'll need to use his ±24v supplies as an input to a couple of regulators to get the ±10 supplies. The negative doesn't have to be very accurate; as long as it's -8.5v to -15v it's good. The TL07x, TL08x and LF35x families can't "see" or output within 3v of the negative rail. Since the output swings to -5v, that means -8v is a requirement, but going to -8.5 or less ensures that it'll work.

    A fixed-voltage 7909,7910,7912 or 7915 regulator could be used for the negative regulator (0.33uF cap on the input to gnd and 0.1uF on the output to gnd are required), but the positive supply needs better regulation. An LM317 could be used; 120 Ohms for R1, and use a 750 Ohm and a 200 Ohm pot in series for R2. The pot will provide about a ±1v adjustment range from 10v on the LM317 output.
     
    Last edited: Oct 22, 2011
  7. #12

    Expert

    Nov 30, 2010
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    Dear Wookie, please indulge me while I clarify my question.

    I'm actually rather good with opamps and algebra, the part I can't figure is how you interpreted the question. Even after I realised that "5,G,-5" isn't gibberish, it looks like fxmech needs the opposite slope out of the opamp. Then there is, "where did you come up with +/-10V" (that wasn't provided in the question) and how did you come to the conclusion that a person with only 2 posts can understand the answer and generate the necessary voltages?

    I can see that +/-10V is much more appropriate than the +/-5V that was provided in the question. I think you picked up the +/-24V off the advertisement for the scanner. That leaves, "Is the slope right?" and, "How did you know you weren't talking over his head?"

    This is really about, "how can I interpret the questions better"?

    Thank you for your patience.
     
    Last edited: Oct 22, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, there was a fair bit left off my post. I had a number of interruptions going on, and I was out until late yesterday evening.

    Expecting a TL082 opamp to output near the rails is just not realistic. You'll get within ~1.5v of the positive supply, but not much closer than the negative supply +3v. Since the desired output range is -5v to +5v, the minimum supplies would need to be +6.5v and -8v - but even there, you'd likely wind up with some distortion.

    There are rrio (rail-to-rail input and output) opamps, but they will fall a bit short if you try to run them right up to the rails.

    The first schematic was just the basic idea; how to translate an input of 0v to 5v to an output of -5v to 5v, and do so as simply as possible. Since fxmech is using an Arduino with a DAC as an input to the level converter, it's a given that fxmech is also writing the program for the Arduino. Flipping the bits around so that the output would range from 5v to 0v instead of 0v to 5v would be simple in comparison to everything else that's being accomplished. However, it might be advantageous to be able to select an inverted or noninverted output; so the attached schematic accomplishes that via U1b. An optional SPDT toggle switch can select between the two.

    I've also added positive & negative voltage regulators. R7 will need to be adjusted to get the correct output voltage. 845 Ohms should be approximately correct; you could use a 750 Ohm resistor in series with a 200 Ohm potentiometer to provide a suitable adjustment range.

    R8 isn't so critical. 820 Ohms or 910 Ohms would be OK.

    Not shown is 0.1uF capacitors across the supply pins for the TL082. I'm showing two individual TL071's, but for the purposes of the schematic, consider them as the two channels of a single TL082.

    The regulators, caps, and opamp should be located close to each other.
     
    #12 likes this.
  9. #12

    Expert

    Nov 30, 2010
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    OK. Your knowledge of the Arduino gave you the advantage in expecting fxmech to be able to manipulate the results. (I think that providing the voltage regulators will help fxmech quite a bit, right after he reads your explanation in post #8.)

    ps, contrary to popular opinion, most of us have a "real" life with real people.

    Thank you.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I received an E-mail asking about the circuit I posted; they wanted to use it with a range of 0v to 3.3v input rather than 0v to 5v in; what changes would be necessary to do this?

    You will still need a bipolar supply for the opamp. If you're using a TL07x or TL08x opamp, you will need at least +8v and -8v supplies (you can go somewhat higher; see your opamp's datasheet for it's recommended operating range).

    [​IMG]
     
  11. evengravy

    New Member

    Oct 6, 2012
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    Thank you for this, much appreciated!
     
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