project help!!

Discussion in 'General Electronics Chat' started by saghar19, Dec 14, 2012.

  1. saghar19

    Thread Starter Member

    Jul 16, 2012
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    Hi
    I'm trying to run this kind of buffer with single supply but when i measure the input(10uv amp sinusoidal)i see that it is not sinusoidal!(It has a lot of ripples)
    Please tell me what is wrong in my schematic.
    (This buffer work with dual supply but i must run it with just one supply)
    My project is about making EEG active electrode,so i should work on micro-volt amplitude.
     
    Last edited: Dec 14, 2012
  2. MrChips

    Moderator

    Oct 2, 2009
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    The ripples you see are not real but represent the quantization of the simulator.
    I have never seen this in any analog simulator before.

    For EEG recording, you need to use a differential input instrumentation amplifier with very high CMRR.
     
  3. saghar19

    Thread Starter Member

    Jul 16, 2012
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    It is just an active electrode that we put on the scalp,to increase input impedance and reduce output impedance and many other goals.
    So you mean that simulator just has error and it is not because of my circuit??Am i right?
     
    Last edited: Dec 14, 2012
  4. MrChips

    Moderator

    Oct 2, 2009
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    The waveform pictures you have posted show digital steps.
    Where did the steps come from? There is no digital signal shown in your circuit diagram.

    The common-mode signal is going to kill your project. You have to use an instrumentation amplifier.
     
  5. saghar19

    Thread Starter Member

    Jul 16, 2012
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    Excuse me but you did not understand my meaning.I'm trying to design active electrodes(instead of passive one) not an EEG amplifier.I need low noise, low input offset, high input impedance op amp not an instrumentation amplifier.please take a look at this PDF file that attached, to understand me better.I have a question about their design too.how did they work with single supply without adding dc to input?????:confused:
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Excuse me, you're not listening.
     
  7. saghar19

    Thread Starter Member

    Jul 16, 2012
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    Oh,why????:eek:
     
  8. saghar19

    Thread Starter Member

    Jul 16, 2012
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    Excuse me if I'm bothering you with my questions.I'm a beginner in electronic design and i have a lot of mistakes,I'll become so happy if you help me to find the right way in my project.
     
  9. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
    0
    As i know an instrumentation amplifier,amplifies difference between two inputs but here i don't need a gain, i just want to buffer input.
    I forgot to say that when input is 1v out put is correct,but as i reduce input voltage, ripples appear both on input and out put!(attachment)
    Any idea could help me.
    thanks for your answers before.:)
     
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    Last edited: Dec 15, 2012
  10. Audioguru

    New Member

    Dec 20, 2007
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    If you use a single-polarity supply then the patient must be biased at half the supply voltage (+2.5V) by the output of U28 on your schematic. It is called a "virtual ground".
     
  11. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
    0
    Io
    Yes sir you're completely right.I'm going to do that.This schematic shows data aquisition of one point on the sculp,i 've modeled brainwave just with sinusoidal signal in this schematic but i get this error!I'm confused with this error!How can i solve that?
     
    Last edited: Dec 15, 2012
  12. Audioguru

    New Member

    Dec 20, 2007
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    We already told you that we think the "steps error" is caused by your simulation program (when the level is extremely low).
     
  13. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
    0
    Thanks alot for your help:)
     
  14. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
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    Dear MrChips
    Could you explain it more to me ?
    I'm so willing!please tell me.I'll listen carefully!:D
    thanks:)
     
    Last edited: Dec 19, 2012
  15. MrChips

    Moderator

    Oct 2, 2009
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    What part do you not understand?
     
  16. Sparky49

    Active Member

    Jul 16, 2011
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    Just a thought... the steps you see might be a result of too few points being calculated.

    If you tell it to calculate more points, you might find you get a smoother graph.
     
  17. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
    0
    Where should i use an instrumentation amplifier?(Instead of lmc6482 or after that?)
    And,where is the origin of common mode signal?
     
    Last edited: Dec 19, 2012
  18. saghar19

    Thread Starter Member

    Jul 16, 2012
    38
    0
    I reduced step ceiling to 1us but i didn't see no change.I think there is a big mistake that i don't know what is it!
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    There is common mode signal all around you, AC line frequency, radio, TV, cell phone, wi-fi, computers, etc.

    The common mode signal is stronger than your signal you are trying to detect and is going overwhelm your signal. That is why you use an instrumentation amplifier at the electrodes off the scalp.
     
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