suggestion, comments, corrections on EEG circuit

Discussion in 'General Electronics Chat' started by WAN24, Jan 5, 2008.

  1. WAN24

    Thread Starter Member

    Jun 27, 2006
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  2. hgmjr

    Moderator

    Jan 28, 2005
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    Looks reasonable to me.

    Do not ignore the Disclaimer!

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    We haven't built it either. There is nothing there to prevent it from working, though. Certainly a bit more advanced than 100 years ago when you had one foot in a bucket of water for the reference, and the electrodes made a galvanometer movement twitch.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The old circuit was designed before instrumentation amp ICs were invented to do the job properly.
    An instrumentation amp IC has highly matched opamps and resistors. It is difficult to match parts yourself.
    Google has links to many ECG circuits that use an intrumentation amp IC.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    If you are going to build this circuit for the experience look up 'guard ring' on Google or similar.

    The circuit itself certainly has more instructional value than buying an off the shelf solution. You have direct access to the guts of each stage to experiment with to examine the whys and wherefores.

    Good luck and post again if you have further questions during the project.
     
  6. WAN24

    Thread Starter Member

    Jun 27, 2006
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    thank you guys for the comments!!! i'll try it first then i'll let you know what happen... one thing.. can i replace the 1st stage by OP177? a high precision amp.. again... tnx!!
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    One of the reasons I suggested staying with your proposed circuit was the first stage as it is a conventional 3 amp differential amplifier and using it will give experience with it. There is no substitute for playing and puzzling out why certain configurations are as they are to gain a designer feel for things.

    For instructional purposes it is best to concentrate on the circuit configuration, not the particular 'wonder IC' of the moment. These will alter throughout you career, and even before you have finished your course.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Doesn't the safety of these kinds of circuit revolve around leakage current? I think medical equipment is required to have something less than 100uA. I had an old prof that was really into medical equipment and he was nuts about making sure it was safe, etc.

    Steve
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    In line with what Scubasteve mentioned, I'm a bit concerned that the inputs to the preamp stage do not have current limiting resistors. Yes, they're JFET input opamps, which are very high impedance - however, if an input circuit failed...

    The resistors in the preamp should be matched to 0.01% or better in order to ensure a good CMRR. This can be tough to do. You could replace that amp with an instrumentation amp like an INA128, which is an industry standard, and won't be going away anytime soon. It has precision matched amps & resistors that were laser trimmed inside the case; you just need one resistor to set the gain.

    If you wanted more precision and less noise, you might opt for TL072s. These are dual low-noise low-offset JFET input op amps. You could also go with Linear Technology's LTC1057, which are also dual op amps with even better noise figures.

    I'm just not very enthusiastic about quad op amp packages, as they are noisier and have a greater chance of phase reversal than dual or single packages.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The TL084 is an older quad opamp with FET inputs. Like the TL071, TL072 and TL074 opamps which are selected for low noise, they all have phase-reversal when their input voltage approaches within about 2V from the negative supply voltage.

    If you don't match the opamps and resistors extremely well then the output of this project is nothing but mains hum.
     
  11. Distort10n

    Active Member

    Dec 25, 2006
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    I know this is offtopic from the OP, but what have you seen that makes you conclude this (phase reversal?).

    I had a customer at my old job ask me about how isolated each channel is from one another in a dual/quad package. The issue was, if the inputs on one channel were overdrive, how would if affect the other channels?

    One of the senior apps engineers tested a quad in such a fashion and looked at the THD of all the channels. The other 3 channels were not affected. Although, I am sure there are plenty of other tests that could have been implemented.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Older FET-input opamps like the TL07x and TL08x have 'phase-reversal" when an input goes lower than the allowed common-mode voltage limit. Newer FET-input opamps don't do it and show the old problem on their datasheets.
    Try it. The effect is weird. The output suddenly inverts.
     
  13. Distort10n

    Active Member

    Dec 25, 2006
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    This is true, but Sgt. Wookie specifically mentioned the number of channels exasterbating the problem.

    Dual/Quad package op-amps would be the same single channel design. So if an op-amp does not have phase reversal for a single channel, I do not see it happening on duals or quads.
     
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