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ECG device

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Zener_Diode, Mar 13, 2007.

  1. Zener_Diode

    Zener_Diode New Member

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    Hey.
    I am trying to do an ECG decive.
    I am useing AD620 amp.
    http://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdf
    I built the circuit at page 12.
    The leg I connected to the ground, and put 2.2K (or 100 om) between pins 1 and 8.
    I am trying to see the ECG on scope. I don't see it, what I see is a straight line or 50Hz noise sin wave.
    As electrodes I am useing a coins.
    can some one help me plz?
     
  2. akg

    akg New Member

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    are you getting any signals if you touch the terminals. if yes try using lab grade connection pads
     
  3. Roff

    Roff Well-Known Member

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    Did you build the circuit in Figure 36?
     
  4. dave

    Dave New Member

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  5. Zener_Diode

    Zener_Diode New Member

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    I add the circuit I built.
    I tried to add a nother amp with A=68, it didn't change a thing.
    When I touch Right Arm electrode, left Arm electrode, or both of them I do see any wave, when I touch the ground too, every think it gone.
    what can be the problem?
     

    Attached Files:

    • ECG.jpg
      ECG.jpg
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  6. Zener_Diode

    Zener_Diode New Member

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    If I touch only the right arm electrode and left arm electrode (without leg electrode - GND electrode) I see something looks like ECG, but it very noise.
     
  7. Optikon

    Optikon New Member

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    The circuit you show is not the same as shown in Figure 36 of the datasheet. You should consider driving the leg electrode with the AD705 as shown. Also, that circuit shows overall gain of 1000 and you are only providing 495. That could be part of your problem.
     
  8. Zener_Diode

    Zener_Diode New Member

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    I don't have AD705, but I can add driving leg with TL071.
    if I add another amp (serial connection) with A=68 (now Atot=500*68) I still see nothing.
     
  9. Zener_Diode

    Zener_Diode New Member

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    I added driven leg (based on TL071), it didn't help
    :(
     
  10. Zener_Diode

    Zener_Diode New Member

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    If I touch any cobination of 2 electrodes I see someting look like ECG.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The gain of your AD620A is much too high causing its output to saturate at 50Hz.

    The circuit in the datasheet has the AD620A with a gain of only 7 then a highpass filter to remove hum and noise. You don't have a highpass filter so you have plenty of hum. You also have plenty of noise. The circuit in the datasheet then amplifies the hum-removed and noise-removed signal 143 times.

    The resistors (R2 and R3) dividing the input to the leg-driver opamp must have exactly the same value (1% or better) to cancel some of the hum and noise.
     
  12. Zener_Diode

    Zener_Diode New Member

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    I changed the ciruit, it don't work too, with or without the driven leg.
     

    Attached Files:

    • ECG.jpg
      ECG.jpg
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  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I mentioned the "highpass" filter because it is on the datasheet.
    IT IS WRONG!
    It is a lowpass filter that removes the 50Hz hum, its harmonics and noise.
    Its cutoff frequency is a very low 0.03Hz so it attenuates 50Hz very much.

    Wait a minute. 0.03Hz is a heartbeat each 33 seconds. The datasheet must be wrong again or the patient is hibernating.
     
  14. Zener_Diode

    Zener_Diode New Member

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    It should be band stop filter to 50 Hz.
    and low pass filter to 130- 150 Hz.
    but even without filters I don't see what I should see, even not clother.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Here is another one:
     

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  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Here is another another one where they use an external very sharp 6Hz to 30Hz bandpass filter:
     

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  17. Zener_Diode

    Zener_Diode New Member

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    does anyone have any idea???
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Audioguru has posted you various suggestions - you MUST have filtering, if you don't it won't work!.
     
  19. Zener_Diode

    Zener_Diode New Member

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    Should this work?
     

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  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Certainly looks more hopeful!.
     
  21. kokos

    kokos Member

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    ECG Devise

    I have attached a schematic diagram of ECG device that i builded. it is based on ECG device given from AD620 instrumentation amplifier datasheet..i ve tested the first part of the circuit (AD620 and AD706 right leg driver)using oscilloscope but the signal i was getting it was not a proper one.when i attached the RA and LL electrodes i could see a signal looking like an ecg but not a proper one but when i was connecting the RL (ground) the signal disapeared.can anybody tell me why? now i have builded the whole circuit with a high pass filter after the AD620 of cut off frequency 0.025Hz and an operational ampliier of G=143.does the circuit looks ok or shall i make some changes? shall i include a resistor at the non inverting input of the last op-amp for input impedance?
     

    Attached Files:

    • ECG.jpg
      ECG.jpg
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