Rail to rail op amplifier

Discussion in 'General Electronics Chat' started by johnkaz77, Sep 21, 2010.

  1. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    Hi,i have an op. amp. (Ltc6244) and i want to amplify a signal.My signal ranges from 0.8V to 1.8V and i want 0V to 5V at the output of the amplifier.The signal has a dc level of about 0.8V which i want to remove,can anyone show me how can this be done?If anyone could make a drawing i would really appreciate it because i'm just beggining in electronics...
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you tell us a bit more about your input signal? What is the approximate frequency range?

    You know that this opamp is not quite rail-to-rail, correct? It will get within about 15mV of the supply rails when there is no load on the output, but gets worse if you have a load.

    Are you planning on using a single 5v supply, or a dual rail supply?
     
  3. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    I don't know the frequency range.Here is an image of how they told me it should be but it's not finished.Does it make any sense or is it all wrong?How do i choose the values of R and the voltage at the input of the op amp in the bottom?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I've created a schematic and simulation for you using LTSpice.

    LTSpice is a free download from Linear Technology's website. Google "LTSpice download"

    Also, join the LTSpice Users' Group on Yahoo! Groups. It's free help, and they have LOTS of models available.

    See the attached files. The .asc file is the LTSpice simulation file. Once you have installed LTSpice, you can open the simulation by simply clicking on the link to the file.

    V1 is a wild guess at your input signal, a 30Hz sine wave that has a peak-to-peak amplitude of 1v, offset from 0v by 1.3v.

    U1a is configured as a voltage follower, or buffer. R1 and R4 are a summing network; U1a's output is summed with -0.8v, giving a range of 0v to 1v. The output of the summing network is the input to U1b, which is wired as a non-inverting amplifier.

    R2 and R3 give U1b a gain of 5.
     
  5. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    So,if i got it right,V3 is a negative voltage?I must supply the circuit with -0.8V at V3?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    That is correct.

    You didn't specify what power you had available, so I just kept things as simple as possible.
     
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  7. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    I have a regulator that provides -5V so,i guess i will use a voltage divider to get -0.8V.Thank you very much!!!!
     
  8. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    One last thing if you can...How didi you come up with the 100k value for R1 and R4?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I missed your prior reply. A simple voltage divider from -5v down to -0.8v will introduce a fair amount of error. The -0.8v supply needs to be low-impedance. You could obtain that low impedance by using yet another opamp that is stable at unity gain; connect the output to the inverting input, and the center of the divider to the non-inverting input. The opamp will attempt to keep the output voltage stable at whatever the input voltage is, except for the input offset voltage.

    You needed to subtract 0.8v from your input signal to get a range of 0v to 1v, which then needed to be multiplied by 5. The easiest way to do that was to sum your input and -0.8v using a resistive summing network.

    R1 and R4 can actually be any value from about 10k to perhaps 200k; just as long as they have identical resistance. If you go a lot lower than 10k, you start using a fair amount of current. If you go higher than ~200k, you will make the circuit more susceptible to noise. 100k was just a nice round number that's easy to obtain, and it worked well for the x5 amplifier.
     
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