How to correctly supply power to an TL081CN OPAMP?

Discussion in 'General Electronics Chat' started by LastStarDust, Jul 20, 2014.

  1. LastStarDust

    Thread Starter New Member

    Jul 20, 2014
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    Hello,
    I am new to this forum and, more in general, to the world of electronics. This is the very first time I am trying to build a small circuit because I have read that one can turn the sound card of a PC into a small oscilloscope. In order to screen the card from over-voltage or amplify the signal I need this small circuit that I have found some time ago over the internet, but now I have lost the site address and all I am left with are this and my notes.
    [​IMG]
    Before attempting the assembling, I would like to simulate it with Qucs 0.0.15 (I use Ubuntu).
    This is the circuit as I think might look in reality (without the switches S1 and S2 that I don't need to simulate).
    [​IMG]
    The problem I think lies in the OPAMP alimentation. If I strip the capacitors C2 and C3 away the simulated behavior is what I would expect, but with that capacitor the output voltage is killed at high frequences.
    [​IMG]
    My question is:
    • how do I supply power correctly to the OPAMP?
    • Where do I have to put the capacitors C2 and C3 showed in the paper schematics?
    Thanks
    OPAMP Datasheet
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The zip-file contains some files that I (and many other) can not read or see.
    Can you post the schematic in a graphics format, like .png?

    Bertus
     
  3. to3metalcan

    Member

    Jul 20, 2014
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    Hey, LastStarDust...are you using something like this circuit? Because if so, the roll-off at high frequencies isn't due to an error...C2 is intentionally limiting input bandwidth (probably because a probe lead going to the input of a fet op-amp is pretty much an antenna for interference), and the TL08x op-amp has a limited slew rate, especially in the 10x gain mode (don't know if your sim accounts for that.) You could try the circuit with a higher-bandwidth op-amp,but it may not make much difference in real life...your soundcard is set up to deal with audio-range signals, so its op amps and bandwidth limiting circuitry may undo the difference...bottom line, soundcard-as-scope hacks are neat, but they're a compromise at best!
     
  4. LastStarDust

    Thread Starter New Member

    Jul 20, 2014
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    I have attacched the images, but they might be retrieved if you click on the links in the first post.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think the capacitors are meant to be bypass capacitors so they should go from Vcc and Vdd to ground and not be in series with the supply voltages.
     
  6. LastStarDust

    Thread Starter New Member

    Jul 20, 2014
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    Thanks for your answer. Yes, I am trying to build something like that, even if not the very same. I know that sound cards have a limited band-width (let's say 20 Hz to 20 kHz) and I have considered that while doing my simulations. But the cut-off I get is way before the 20kHz limit, is at 100 Hz or something and I think it is not acceptable. Anyway The schematic is from a trusted source so I am quite sure it is me that is doing wrong.
    Finally I acknowledge that sound-cards are not even close to ideal when it comes to probe signals but it seemed fun to try ...
     
  7. to3metalcan

    Member

    Jul 20, 2014
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    Ronv is correct...you have your bypass caps completely blocking your DC supplies. It's actually sort of weird you're getting any results at all, except that maybe as your input voltage approaches DC it starts discharging the caps. Both 100nf caps should be running from the respective power pins of the op amp to GROUND.

    It's an easy transcription error to make, if you weren't sure what they were supposed to be doing!
     
  8. to3metalcan

    Member

    Jul 20, 2014
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    To be clear...+9 should run straight to Vcc, AND ALSO connect to ground through a 100nF cap. -9 should run straight to Vee AND ALSO connect to ground through the other cap. Only AC can pass through a capacitor (to varying degrees, depending on frequency and the size of the cap.) To DC, a capacitor is like an infinitely big resistor.
     
  9. LastStarDust

    Thread Starter New Member

    Jul 20, 2014
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    Thanks a lot. Now all is working fine.
    Out of curiosity, what are those capacitors meant to do? I cannot figure out what "bypass capacitors" means exactly ...
     
  10. bertus

    Administrator

    Apr 5, 2008
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  11. LastStarDust

    Thread Starter New Member

    Jul 20, 2014
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