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Instrument amplifier or just operational amplifier?

  1. Dec 22, 2012 #1
    i have a project to design ECG system:
    read ECG signal by using (DAQ)data aquestion and the Signal processing(bandpass& notch filter) will be done by using matlab program.

    I was Intended to use instrument amplifier to obtain I,II or III signals
    as this figure:


    but my supervisor told me:
    amplify each lead separately (LA,RA or LL , and RL as reference) and read it by DAQ
    then inside MATLAB we can obtain I,II and III signals, mathematically according to these equations:

    so,what's your opinion about this method(of my supervisor), can succeed??
    should i use instrument amplifier or just operational amplifier?
  2. jcsd
  3. Dec 22, 2012 #2

    jim hardy

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    Science Advisor
    Gold Member

    I'd use instrumentation amplifiers.

    You're probably aware of the lessons learned in late 60's by early medical instrument manufacturers, but i'll mention two of them just in caes...

    1. If your machine is connected to a patient who is also connected to one of those defibrillators with the high voltage paddles to get a stopped heart going, it's apt to fry your input amplifiers. Provide protection.

    2. Isolate all your input leads, even the "common", from the electronic amplifiers by at least a megohm and preferably ten.
    Reason is an electrical fault inside your machine must not reach out and touch the patient.
    That's the reason hospitals have separate grounds for instuments and , say, vacuum cleaners. You dont want a defective vacuum cleaner to electrocute a patient through your ECG machine.

    Analog Devices was a leader in developing isolated op-amps for the medical instrument industry. Their Peter Zicko was at the forefront back around 1973.
    Intronics is another name brand, do some searching on "isolated input amplifier"

    end of nagging

    old jim
  4. Dec 22, 2012 #3
    thank you "jim hardy"
  5. Dec 29, 2012 #4
    Tough call.

    Whatever the boss is, he's always the boss. It's generally better to gather evidence than argue.

    Time allowing, I'd run both methods in parallel. Then, give a short effort towards the op amp method, record some notes regarding it's behavior, and then, if neccesary use your fall back circuit.

    If your manager wants to commercialize this circuit, he'll have to deal with UL601 or equivalent. No repectable hospital or clinic will allow your device in without the precious safety agency sticker. They'll require that the connections have a current limiting impedance and be galvanically isolated from other circuits and of course, power.

    For simple experiments, an isolated laptop is your best friend. Operating it from the battery with no external connections (i.e. Ethernet line, USB devices, etc...) allows you to have a power source (via USB) and signal proccessor. It's best to clear it of all possible connections to ensure that no one is tempted to charge it etc...
  6. Dec 29, 2012 #5


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    Staff: Mentor

    Please also review UL544 for this project. As mentioned, you will need to get safety approvals if you want to actually attach this to any person. Even if this is a simulation-only project, you need to research the safety considerations (and immunity issues that Jim brings up), in order to write a complete project report.
  7. Dec 30, 2012 #6
    In addition to what has already been said, I'd like to suggest that you provide substantial notch filtering, bandpass limiting and transient protection to your input lines and amplifiers. Touch the probe of an oscilloscope to see how high an induced power line voltage could be on the input leads. This could easily drive your amps to either rail preventing amplification of the signal.
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