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Integrator, Differentiator & I-to-V converter, why use opamp anyway?

  1. Jul 17, 2012 #1
    We all know the Differentiator and Integrator configurations of the Operational Amplifier.
    I realized that the RC circuits used as low pass and high pass filters can also be used as integrators and differentiators. Its obvious from their differential equations actually.

    As for current to voltage converter. We just need a resistor and a current source right? Than why bother making an opamp current to voltage converter? I think its called the buffer configuration.

    What I do not understand also is what does buffer means here? In digital circuits we may have buffer that stores binary data before it is passed on. But what does buffer mean in an analogue circuit?
     
  2. jcsd
  3. Jul 17, 2012 #2

    vk6kro

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    An analog buffer would have a high input impedance and a low output impedance.

    So, it would load the previous circuit as little as possible (so that the previous circuit would work properly) and still produce an accurate version of the input voltage at its output, even if the output is loaded with a fairly low impedance.

    This can make filter design much easier. A simple RC filter may use a large resistor (perhaps 500 K ohms) and the high impedance buffer will allow this to function properly whereas a lower impedance would form a voltage divider with the 500 K resistor and affect its function.
     
  4. Jul 17, 2012 #3

    jim hardy

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    ever study meter shunts?
    Whatever is measuring the voltage across the resistor is connected in parallel with it.
    So it bypasses a teensy bit of current around the resistor, lowering its voltage drop.
    No problem so long as the measuring device is high resistance compared to your current sampling resistor , the drop will be too small to notice. But, if you aren't sure what somebody will connect there, best put an opamp voltage follower in between - the opamp can deliver substantial current to whetever your customer connects.

    A buffer is something used to separate an analog signal from anything that could degrade its integrity.

    Usually just an opamp voltage follower with substantial resistance in its input lead.
    But some places you need more.

    To that point - here's
    "
    Another boring anecdote"
    - In my plant we used 4 to 20 milliamp current loops
    and nominal 250 ohm resistors to convert current into a 1 to 5 volt signal.
    We calculated how much each resistor was "loaded down" by the measuring devices it supplied, and the nominal 250 ohms was adjusted up to 251.25 or 252.5 etc as required to maintain 1 to 5 volts. We had an assortment of precision wirewound resistors for that purpose.
    Wherever a signal exited a safety system to go someplace less vital like the datalogging computer,
    we installed a "buffer" so that an unfortunate event in the non-safety system couldn't reach backward and upset the safety system. Our buffers were kinda super-opamps with stout transformer isolation between input and output terminals. You could electrically "stomp on" the output over at computer and not affect the safety system.

    That's the two extremes of analog buffering.

    old jim

    EDIT Thanks VK6 !
     
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