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amplifier and unwanted oscillations

Discussion in 'General Electronics Discussion' started by indigo, May 6, 2011.

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  1. indigo

    indigo

    3
    0
    May 6, 2011
    Hello,

    To record mouse's brain activity, I am using a non-inverter amplifier (gain 1) whose output and power supply come to through 70 cm long wires tied together (the amplifier is a TL071 supplied with 9V). This configuration produces a massive oscillation at 500 kHz with a amplitude about 3V, which is dramatically important compared to the 1mV signal coming from the mouse's brain.

    As I understand form information read on internet, the oscillations are produced by the capacitance of the wires which prevent the amplifier to work properly. Also, the power supply is likely to be affected by induction from the output signal passing in the adjacent wires.

    I am looking for a solution to prevent this oscillation. The main issue results from the fact that the amplifiers is carried by the mice therefore it is not really possible to add many electronic components there... I need to solve the problem on the other end of the wire.

    I would appreciate any advice or reference to article which can help me to fix this.
    Thanks
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Oscillations occur when there is feedback in the amplifier system. This can happen by feedback in the chip or outside the chip. To reduce feedback in the chip, place a capacitor (10microF?) directly across the chip power supply. To stop external feedback, both capacitive and inductive, use sreened (coaxial) cable on input and output and perhaps also power supply. A small capacitor across the input will reduce the high frequency gain, how fast is your mouse? Poor mouse.
     
  3. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    sad indeed for little mouse. don't want to help.
     
  4. indigo

    indigo

    3
    0
    May 6, 2011
    I look at frequencies between 1 to 200 Hz so the 500 kHz oscillation is far from it. I already tried with a capacitor of 670pF across the chip power supply without success. I maybe need to use an higher capacity?

    Unfortunately, I can't put coaxial cable, a mouse is about 30 g, it will never be able to walk around with an heavy weight on the head.

    Do you know where can I get information about the capacitance and the effect to reduce feedback? I am not an electronics engineer and I will be glad to understand how that work ;)!
     
  5. indigo

    indigo

    3
    0
    May 6, 2011
    Hi poor mystic,

    I study Alzheimer and Huntington's disease in mice, I am sure, if you would have already seen one person in an advanced stage of one of these diseases, you would change your mind very quickly...
     
    Last edited: May 6, 2011
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Are you sure that you are not picking up a radio signal, how about a medium wave radio station, fluorescent light, ultrasonic bath, induction heater etc.
     
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