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Re: [buildcheapeeg] TinyEEG

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  • Doug Sutherland
    Andreas, Now you re speaking my language, the language of tiny, low power, unobtrusiveness. I want the whole EEG on my thumbnail, but then I want my whole
    Message 1 of 11 , Jan 7, 2002
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      Andreas,

      Now you're speaking my language, the language of tiny,
      low power, unobtrusiveness. I want the whole EEG on my
      thumbnail, but then I want my whole system on my thumb
      nail (freaky thing is: it's getting close).

      > fits in less than 1/2 a euro-board. Don't expect the
      > through-hole version to be that small.

      Sounds awesome, but hard to build.

      > It runs on four 1.5V batteries.

      Assuming I already have regulated 5V from another
      supply (PT6302A) ... can I use that? My battery will
      be 12V, powering an embedded PC through 3A 5V
      switching regulator.

      > I've cut away everything non essential, so forget
      > about expandability...

      Here's an idea. Leave two pins on the Uc set up for
      TTL serial to another uC. A second board can grab
      temp, gsr, ekg, emc, and sent data to the other uC
      over TTL serial. The first uC just sends the data
      down the line to the PC. A simple protocol can show
      the data source (do we have this in the protocol in
      use now?). The second uC (multi-parameter data
      acquisition module) can also accept commands from
      the first to set modes etc. A little command language
      similar to what GPS receivers use could be set up
      to program the modules from the PC. For example you
      send an escape code sequence followed by a letter
      and number combination. This could set the sampling
      rates and other tweaks. I've done this before with
      a single microcontroller connected to GPS, humidity,
      barometer, and up to 256 1-wire temp sensors. The
      PC protocol can be programmed in software, but also
      works in a terminal session like hyperterminal or
      minicom. Press you escape code, cay CTL-S for the
      sampling rate then 300 then a termination char and
      the system configures for that rate. The other
      channels for temp, gsr, etc can be done the same
      way, and can be shut off or on when desired.

      -- Doug

      PS. Please proto the TinyEEG to make sure it works.
      I want this one! I may strugle with making the
      board, but it will be worth it.
    • sleeper75se
      ... Not really, if your fine-motor skills are in order. They say the secret is in the sauce. Well, in this case, it is in the flux...
      Message 2 of 11 , Jan 7, 2002
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        --- In buildcheapeeg@y..., Doug Sutherland <wearable@e...> wrote:
        > Andreas,
        >
        > Now you're speaking my language, the language of tiny,
        > low power, unobtrusiveness. I want the whole EEG on my
        > thumbnail, but then I want my whole system on my thumb
        > nail (freaky thing is: it's getting close).
        >
        > > fits in less than 1/2 a euro-board. Don't expect the
        > > through-hole version to be that small.
        >
        > Sounds awesome, but hard to build.

        Not really, if your fine-motor skills are in order.
        They say the secret is in the sauce. Well, in this case, it is in the
        flux...
        http://www.geocities.com/vk3em/smtguide/smtguide.htm

        >
        > > It runs on four 1.5V batteries.
        >
        > Assuming I already have regulated 5V from another
        > supply (PT6302A) ... can I use that? My battery will
        > be 12V, powering an embedded PC through 3A 5V
        > switching regulator.
        >

        Hmm, it really does need a dual supply, so if you have -5V on hand
        and regulate that and the +5V to +/- 3V...
        The power-system is in a ever-changing state I'm afraid. Right now
        I'm considering splitting up the amplifier and microcontroller so
        that the amplifiers can run on higher voltages, but the minimum
        requirement will probably remain +/- 2.5 or 2.7 volts.

        > > I've cut away everything non essential, so forget
        > > about expandability...
        >
        > Here's an idea. Leave two pins on the Uc set up for
        > TTL serial to another uC. A second board can grab

        What two pins? :-)

        If you can do high-voltage (+12V) programming of AVR processors, the
        reset pin can be used for I/O or as an analog input (EMG maybe?). But
        other than that, there are no pins left. Ok, you can get rid of the
        1kHz reference clock and free another pin, but the microcontroller's
        internal oscillator has a 2% error margin....

        Next step up from the ATtiny15 is AT90S4433...I think.

        A better solution is probably to have another MCU "downstream" that
        collects the data from different sources.

        In this first prototype I will use a very simple 3-byte protocol:

        First byte:
        Bits 7-1: Left channel most significant bits
        bit 0: sync bit = 1
        Second byte:
        Bits 7-1: Right channel most significant bits
        bit 0: sync bit = 0
        Third byte
        Bits 7-5: Left channel least significant bits
        Bits 4-2: Right channel least significant bits
        bit 1: unused
        bit 0: sync bit = 0

        > PS. Please proto the TinyEEG to make sure it works.

        No problems. As soon as I get the little snags sorted out...

        /Andreas
      • Jim Meissner
        Dear Andreas: (and welcome back Doug) Since you asked for some comments, here are a few. These are not in any order. They are meant to be constructive! 1)
        Message 3 of 11 , Jan 7, 2002
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          Dear Andreas: (and welcome back Doug)
           
          Since you asked for some comments, here are a few.  These are not in any order.  They are meant to be constructive!
           
          1)  The RF bypass caps (C1 and C53) must go to a good solid ground like chassis or pc board ground plane. 
           
          2)  The inductor L2 and R56 must go in the (-) power bus.  I am not convinced the inductors are needed if you are running from batteries. 
           
          >  It runs on four 1.5V batteries.
          Do you intend to run the op amps from +/- 3 volts and then float the processor?  That could be big trouble!
          The processor power will take some extra care and may need the inductors to isolate the power glitches.  A separate battery for the processor would be ideal.  The digital and analog ground should be connected together at one star ground point. 
           
          3)  The .1 uf bypass caps C27-37 etc need to be distributed with resistors throughout the various op amp stages.  As Joerg shows in his schematics, they must be on each op amp power pin to ground.
           
          4)  With such a small board, keeping the processor noise out of the input will be take some careful design and layout.  This should be breadboarded first before committing to a pc board layout. You might think of this as two projects, the preamp, and rest of the circuit.  The circuit after pin 6 of U2 and U3 can be built with confidence and possibly gamble with a pc board layout without a breadboard stage.  The preamp will take more tweaking.  This design may change several times.  Consider a shielded box for the input stage 1NA114 and associated components.
           
          5)  The Bessel filter may be an enormous overkill.  I found no significant brain wave frequencies above 60 Hz.
           
          6)  Your input protection diodes should also go to a good ground.  I used a 2N3904 for a reason.  Don't arbitrarily change transistors unless you know why.
           
          7)  What is your decision to use the 1NA114 rather than the lower noise LT1012.
           
          8)  Have you or Joerg actually used the left leg drive?  I have had no experience using it.  I do know why it is used for ECG.  Do you have any information that it works better than the traditional circuit ground reference for EEG. 
           
          >  It uses a LED for data transmission into a plastic
          >  fiber. I've posted a schematic for the receiver
          >  previously.
          Could you tell me the date when you posted it, so I can look at it?

           
          Juergen P. (Jim) Meissner
          Check out my Website at www.MeissnerResearch.com
          Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
          ----- Original Message -----
          Sent: Monday, January 07, 2002 3:33 AM
          Subject: [buildcheapeeg] TinyEEG

          Here's what I've been working on for a while. It still
          needs a lot of adjustments. A surface-mount PCB-design
          for the current schematic is 90% done - it fits in
          less than 1/2 a euro-board. Don't expect the
          through-hole version to be that small.

          The LT1012 design got put on hold a bit - its only
          advantage was that it can use larger resistors at the
          inputs (though not large enough).

          Please read the design notes on the schematic and
          comment on what I've done so far.

          Various stuff:

          It runs on four 1.5V batteries.
          It uses a LED for data transmission into a plastic
          fiber. I've posted a schematic for the receiver
          previously.
          I've cut away everything non essential, so forget
          about expandability...

          /Andreas

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        • yaniv_vi
          hey andreas i m impressed , very impressed !!!! you re the man!!! tell me more : specs, prices ,etc.. ? great !! yaniv.
          Message 4 of 11 , Jan 7, 2002
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            hey andreas
            i'm impressed , very impressed !!!!
            you're the man!!!
            tell me more : specs, prices ,etc.. ?

            great !!
            yaniv.
            --- In buildcheapeeg@y..., Andreas Robinson <sleeper75se@y...> wrote:
            > Here's what I've been working on for a while. It still
            > needs a lot of adjustments. A surface-mount PCB-design
            > for the current schematic is 90% done - it fits in
            > less than 1/2 a euro-board. Don't expect the
            > through-hole version to be that small.
            >
            > The LT1012 design got put on hold a bit - its only
            > advantage was that it can use larger resistors at the
            > inputs (though not large enough).
            >
            > Please read the design notes on the schematic and
            > comment on what I've done so far.
            >
            > Various stuff:
            >
            > It runs on four 1.5V batteries.
            > It uses a LED for data transmission into a plastic
            > fiber. I've posted a schematic for the receiver
            > previously.
            > I've cut away everything non essential, so forget
            > about expandability...
            >
            > /Andreas
            >
            > _____________________________________________________
            > Do You Yahoo!?
            > se.yahoo.com
          • sleeper75se
            ... Hi Yaniv, I have not made any cost estimates yet, but I think much less than $100 for the parts is possible. But I ll get back to you about that when the
            Message 5 of 11 , Jan 7, 2002
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              --- In buildcheapeeg@y..., "yaniv_vi" <yaniv_vi@y...> wrote:
              > tell me more : specs, prices ,etc.. ?
              >
              > great !!
              > yaniv.

              Hi Yaniv,

              I have not made any cost estimates yet, but I think much less than
              $100 for the parts is possible. But I'll get back to you about that
              when the design is more stable.

              The specs: Probably very similar to modular eeg. The difference lies
              in the power system (simpler design) and I'm using a cheaper
              microcontroller, thats all.

              /Andreas
            • sleeper75se
              ... Hello Jim, thanks for your comments, they are very helpful to me. ... Acknowledged. I won t be using the shield driver then. ... Me neither, but I ll keep
              Message 6 of 11 , Jan 7, 2002
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                --- In buildcheapeeg@y..., "Jim Meissner" <jpmeissner@m...> wrote:
                > Dear Andreas: (and welcome back Doug)
                >
                > Since you asked for some comments, here are a few.
                > These are not in any order. They are meant to be constructive!

                Hello Jim,

                thanks for your comments, they are very helpful to me.

                >
                > 1) The RF bypass caps (C1 and C53) must go to a good solid ground
                > like chassis or pc board ground plane.

                Acknowledged. I won't be using the shield driver then.

                > 2) The inductor L2 and R56 must go in the (-) power bus. I am not
                > convinced the inductors are needed if you are running from
                > batteries.

                Me neither, but I'll keep the inductors and then try to remove them
                and see what happens.

                > > It runs on four 1.5V batteries.
                > Do you intend to run the op amps from +/- 3 volts and then float the
                > processor? That could be big trouble!

                I've built a few mixed signal devices this way before, and I always
                get this 1-10mV, 400MHz ringing all over the place. It comes in 8MHz
                pulses - matching the switching of the 4MHz oscillator. I'm half
                expecting the same here, so its definitly time for a new approach.
                Will try to add a 4.5V battery for the MCU.

                > 3) The .1 uf bypass caps C27-37 etc need to be distributed
                > with resistors throughout the various op amp stages.

                The caps are placed like that on the board. That they are placed
                together on the schematic is just for convenience.
                About the resistors, I suppose what you are recommending here is
                10ohms or so in series with each capacitor?

                > 4) With such a small board, keeping the processor noise out of
                > the input will be take some careful design and layout. This should
                > be breadboarded first before committing to a pc board layout.

                Won't breadboarding cause a lot of trouble in itself, like high
                parasitic capacitances?

                I usually build on PCB directly (preparation and etching of a 2-sided
                board takes about 2 hours) and then build and patch whatever needs
                patching. That's hard to do with ground planes of course.

                > You might think of this as two projects, the preamp, and rest of
                > the circuit.

                Ok, I'll draw a distinct line between the preamplifier and the rest.
                If big changes are necessary, I can just cut the board in two.

                > Consider a shielded box for the input stage 1NA114 and associated
                > components.

                will do that.

                > 5) The Bessel filter may be an enormous overkill.
                > I found no significant brain wave frequencies above 60 Hz.

                Do you mean that it perhaps can be omitted entirely? It is not that
                expensive...

                > 6) Your input protection diodes should also go to a good ground.
                > I used a 2N3904 for a reason. Don't arbitrarily change transistors
                > unless you know why.

                What property/ies of the transistors is the most important in this
                case? Can I make a first selection using datasheets or is trial and
                error the only way? BC547 has a leakage of a few picoamps near zero
                volts base-emitter voltage.

                > 7) What is your decision to use the 1NA114 rather than the lower
                > noise LT1012.

                Good question.

                LT1012 is superior but would make the input stage more complex as it
                is a dual opamp and not an instrumentation amplifier.

                Now that you have made me look at it again... Linear manufactures
                LT1167 - an instrumentation amp that can handle 13kV ESD (human body
                model) using only two 5k resistors at the inputs. It is a drop-in
                replacement for INA114 as far as I can see, but with better
                performance.

                Here's the datasheet: current-noise figures are on page 9.
                http://www.linear.com/pdf/1167f.pdf

                > 8) Have you or Joerg actually used the left leg drive?
                > I have had no experience using it. I do know why it is used for
                > ECG.

                I think Joerg has used it. I'm still at the drawing board. :-p

                > Do you have any information that it works better than the
                > traditional circuit ground reference for EEG.

                The leg driver is used and recommended by Biosemi, a dutch EEG-
                equipment maker:

                http://www.biosemi.com

                Basically, they say that it should be used always, by everyone. :-)

                A relevant question from their FAQ:
                http://www.biosemi.com/faq/limit_current.htm

                All of Biosemi's research articles are here:
                http://www.biosemi.com/publications.htm

                Quoting from one of their articles (third one in the list I'm linking
                to) on the subject:

                "A proper driven right leg circuit (see Fig. 2) offers a large
                reduction of common mode voltage magnitude in both isolated and non-
                isolated measurements by actively reducing the voltage difference
                between patient and amplifier common; a reduction between 10 and 50
                dB is usually accomplished. A driven right leg circuit is the most
                practical way to reduce the common mode voltage if a reduction of
                interference current through Zrl is not feasible.

                In addition, the driven right leg circuit makes measurements
                reasonable safe in a non-isolated situation (switch closed in Fig. 1
                and Fig. 2) because a rather large impedance between body and ground
                can be achieved by selecting a large resistor R0 (several MOhm) and a
                small feedback capacitor Cfb (< 1 nF). This feature can be used to
                omit isolation amplifiers in experimental situations in which safety
                standards are not as critical as in clinical situations.

                The main drawback of a driven right leg circuit is it being
                potentially unstable (Winter and Webster, 1983). In practical
                designs, a compromise between common mode suppression and possible
                instability - depending on circumstances - must be found. This
                problem is worked out in more detail in the appendix."

                And, of course, the appendix is missing .... :)
                Hmm, I think I can solve that with a trip to the university library,
                they should be able to find the article for me.

                > > It uses a LED for data transmission into a plastic
                > > fiber. I've posted a schematic for the receiver
                > > previously.
                > Could you tell me the date when you posted it, so I can look at it?

                Please do, but it is available in the files-section under hardware
                design / PDF_and_PNG_schematics.

                http://groups.yahoo.com/group/buildcheapeeg/files/hardware%
                20design/PDF_and_PNG_schematics/

                It's been a while since I first built it so I'd like to make a few
                amendments: The 1K resistor is probably unnecessary and the
                word "flawless" a bit too much - the capacitance in the photo diode
                distorts the incoming pulses a bit and that can perhaps cause
                problems for some receivers. If I could give more current to the
                transistors, perhaps by stealing power from the PC's keyboard-
                connector, it would work much better.

                Also, I'm referring to a toslink-transmitter. It is designed for
                digital audio and can handle a transfer rate of several megabits per
                second. Since that is overkill for this application so I'm going to
                try using a LED instead.

                Whoa, a lot of typing there.
                Thanks again for your help.

                /Andreas
              • (no author)
                rDA Protocol Stack Handler Targets DCE Applications From: X-Yahoo-Group-Post: member; u=43636143 X-Yahoo-Profile: yaniv_vi Date: Mon, 7
                Message 7 of 11 , Jan 7, 2002
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                  Date: Mon, 7 Jan 2002 13:21:15 -0800
                  MIME-Version: 1.0
                  Content-Type: text/plain;
                  charset="iso-8859-1"
                  Content-Transfer-Encoding: quoted-printable
                  X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4522.1200

                  yaniv vilnai (yaniv_vi@...) has sent you an article.=0D=0D=0D=0D
                  IrDA Protocol Stack Handler Targets DCE=20
                  Applications
                  =0DRead the full article at: =
                  =0Dhttp://www.e-insite.net/ednmag/index.asp?layout=3Darticle&articleid=3D=
                  CA189927&spacedesc=3Dnews&
                • Jim Meissner
                  ... In my days of designing tube amplifiers we would run into a problem called motor boating. This is a very low frequency feedback coupled through the B+
                  Message 8 of 11 , Jan 7, 2002
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                    Dear Andreas:
                     
                    >  I suppose what you are recommending here is
                    >  10ohms or so in series with each capacitor? NO!
                    In my days of designing tube amplifiers we would run into a problem called motor boating.  This is a very low frequency feedback coupled through the B+ power supply line.  The bypass capacitors would be large enough to short the audio frequencies to ground, but as the frequencies became lower the capacitors became ineffective.  The solution was to add resistors between stages of bypass capacitors to make R-C filters.  Of course by the time you got to the input stage the B+ would be down to a third or half of the power stage voltage.  A phonograph preamp stage and power amplifier is similar in gain to what you are building here.
                     
                    >  Won't breadboarding cause a lot of trouble in itself, like high
                    >  parasitic capacitances?
                    Just the opposite.  PC board will have much higher (but controlled) capacitance than point to point wiring.
                     
                    >  I usually build on PCB directly (preparation and etching of a 2-sided
                    >  board takes about 2 hours) and then build and patch whatever needs
                    >  patching. That's hard to do with ground planes of course.
                    Hey, if you can do it that fast, great!  Then do it and redo it.  You may be lucky and it works the first time!
                     
                    > 5)  The Bessel filter may be an enormous overkill.
                    > I found no significant brain wave frequencies above 60
                    Hz.
                    >  Do you mean that it perhaps can be omitted entirely? It is not that
                    >  expensive...
                    You will need some filter ahead of the A/D to get rid of aliases.  My application involved biofeedback and listening to the brain on speakers.  I wanted full range frequency response.  That may not be of interest to you.


                    > 6)  Your input protection diodes should also go to a good ground.
                    > I used a 2N3904 for a reason.  Don't arbitrarily
                    change transistors
                    > unless you know why.
                    size=2>What property/ies of the transistors is the most important in this
                    >  case? Can I make a first selection using datasheets or is trial and
                    >  error the only way? BC547 has a leakage of a few picoamps near zero
                    >  volts base-emitter voltage.
                    If I were not retired I would not tell you the trade secret.  It took a long time to find this solution.  Any regular diode is light sensitive (generates stray voltage) and when you pot it then it is pressure sensitive and causes drift.  Metal can transistors have glass seals and are also light sensitive.  Regular diodes make good RF detectors.  I ran into the 2N3904 solution by accident.  This is a good light tight epoxy package.  You will not find this in the spec sheets.  The base emitter is not as good as the base collector.  The emitter collector makes a good low leakage zener diode for op amp bounding.  Now, the BC547 might be better ( I have no idea ), but I would try the 2N3904 first because I have had good experience with it.
                     
                     
                    Juergen P. (Jim) Meissner
                    Check out my Website at www.MeissnerResearch.com
                    Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
                    ----- Original Message -----
                    Sent: Monday, January 07, 2002 4:09 PM
                    Subject: [buildcheapeeg] Re: TinyEEG

                    --- In buildcheapeeg@y..., "Jim Meissner" <jpmeissner@m...> wrote:
                    > Dear Andreas: (and welcome back Doug)
                    >
                    > Since you asked for some comments, here are a few.
                    > These are not in any order.  They are meant to be constructive!

                    Hello Jim,

                    thanks for your comments, they are very helpful to me.

                    >
                    > 1)  The RF bypass caps (C1 and C53) must go to a good solid ground
                    > like chassis or pc board ground plane.

                    Acknowledged. I won't be using the shield driver then.

                    > 2)  The inductor L2 and R56 must go in the (-) power bus.  I am not
                    > convinced the inductors are needed if you are running from
                    > batteries. 

                    Me neither, but I'll keep the inductors and then try to remove them
                    and see what happens.

                    > >  It runs on four 1.5V batteries.
                    > Do you intend to run the op amps from +/- 3 volts and then float the
                    > processor?  That could be big trouble!

                    I've built a few mixed signal devices this way before, and I always
                    get this 1-10mV, 400MHz ringing all over the place. It comes in 8MHz
                    pulses - matching the switching of the 4MHz oscillator. I'm half
                    expecting the same here, so its definitly time for a new approach.
                    Will try to add a 4.5V battery for the MCU.

                    > 3)  The .1 uf bypass caps C27-37 etc need to be distributed
                    > with resistors throughout the various op amp stages.

                    The caps are placed like that on the board. That they are placed
                    together on the schematic is just for convenience.
                    About the resistors, I suppose what you are recommending here is
                    10ohms or so in series with each capacitor?

                    > 4)  With such a small board, keeping the processor noise out of
                    > the input will be take some careful design and layout. This should
                    > be breadboarded first before committing to a pc board layout.

                    Won't breadboarding cause a lot of trouble in itself, like high
                    parasitic capacitances?

                    I usually build on PCB directly (preparation and etching of a 2-sided
                    board takes about 2 hours) and then build and patch whatever needs
                    patching. That's hard to do with ground planes of course.

                    > You might think of this as two projects, the preamp, and rest of
                    > the circuit.

                    Ok, I'll draw a distinct line between the preamplifier and the rest.
                    If big changes are necessary, I can just cut the board in two.

                    > Consider a shielded box for the input stage 1NA114 and associated
                    > components.

                    will do that.

                    > 5)  The Bessel filter may be an enormous overkill.
                    > I found no significant brain wave frequencies above 60 Hz.

                    Do you mean that it perhaps can be omitted entirely? It is not that
                    expensive...

                    > 6)  Your input protection diodes should also go to a good ground.
                    > I used a 2N3904 for a reason.  Don't arbitrarily change transistors
                    > unless you know why.

                    What property/ies of the transistors is the most important in this
                    case? Can I make a first selection using datasheets or is trial and
                    error the only way? BC547 has a leakage of a few picoamps near zero
                    volts base-emitter voltage.

                    > 7)  What is your decision to use the 1NA114 rather than the lower
                    > noise LT1012.

                    Good question.

                    LT1012 is superior but would make the input stage more complex as it
                    is a dual opamp and not an instrumentation amplifier.

                    Now that you have made me look at it again... Linear manufactures
                    LT1167 - an instrumentation amp that can handle 13kV ESD (human body
                    model) using only two 5k resistors at the inputs. It is a drop-in
                    replacement for INA114 as far as I can see, but with better
                    performance.

                    Here's the datasheet: current-noise figures are on page 9.
                    http://www.linear.com/pdf/1167f.pdf

                    > 8)  Have you or Joerg actually used the left leg drive?
                    > I have had no experience using it. I do know why it is used for
                    > ECG.

                    I think Joerg has used it. I'm still at the drawing board. :-p

                    > Do you have any information that it works better than the
                    > traditional circuit ground reference for EEG. 

                    The leg driver is used and recommended by Biosemi, a dutch EEG-
                    equipment maker:

                    http://www.biosemi.com

                    Basically, they say that it should be used always, by everyone. :-)

                    A relevant question from their FAQ:
                    http://www.biosemi.com/faq/limit_current.htm

                    All of Biosemi's research articles are here:
                    http://www.biosemi.com/publications.htm

                    Quoting from one of their articles (third one in the list I'm linking
                    to)  on the subject:

                    "A proper driven right leg circuit (see Fig. 2) offers a large
                    reduction of common mode voltage magnitude in both isolated and non-
                    isolated measurements by actively reducing the voltage difference
                    between patient and amplifier common; a reduction between 10 and 50
                    dB is usually accomplished. A driven right leg circuit is the most
                    practical way to reduce the common mode voltage if a reduction of
                    interference current through Zrl is not feasible.

                    In addition, the driven right leg circuit makes measurements
                    reasonable safe in a non-isolated situation (switch closed in Fig. 1
                    and Fig. 2) because a rather large impedance between body and ground
                    can be achieved by selecting a large resistor R0 (several MOhm) and a
                    small feedback capacitor Cfb (< 1 nF). This feature can be used to
                    omit isolation amplifiers in experimental situations in which safety
                    standards are not as critical as in clinical situations.

                    The main drawback of a driven right leg circuit is it being
                    potentially unstable (Winter and Webster, 1983). In practical
                    designs, a compromise between common mode suppression and possible
                    instability - depending on circumstances - must be found. This
                    problem is worked out in more detail in the appendix."

                    And, of course, the appendix is missing .... :)
                    Hmm, I think I can solve that with a trip to the university library,
                    they should be able to find the article for me.

                    > >  It uses a LED for data transmission into a plastic
                    > >  fiber. I've posted a schematic for the receiver
                    > >  previously.
                    > Could you tell me the date when you posted it, so I can look at it?

                    Please do, but it is available in the files-section under hardware
                    design / PDF_and_PNG_schematics.

                    http://groups.yahoo.com/group/buildcheapeeg/files/hardware%
                    20design/PDF_and_PNG_schematics/

                    It's been a while since I first built it so I'd like to make a few
                    amendments: The 1K resistor is probably unnecessary and the
                    word "flawless" a bit too much - the capacitance in the photo diode
                    distorts the incoming pulses a bit and that can perhaps cause
                    problems for some receivers. If I could give more current to the
                    transistors, perhaps by stealing power from the PC's keyboard-
                    connector, it would work much better.

                    Also, I'm referring to a toslink-transmitter. It is designed for
                    digital audio and can handle a transfer rate of several megabits per
                    second. Since that is overkill for this application so I'm going to
                    try using a LED instead.

                    Whoa, a lot of typing there.
                    Thanks again for your help.

                    /Andreas




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                  • sleeper75se
                    ... So distributing power for LF-circuits through a bus, is better than a star formation as long as there is filtering between the stages? Or is it just
                    Message 9 of 11 , Jan 8, 2002
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                      --- In buildcheapeeg@y..., "Jim Meissner" <jpmeissner@m...> wrote:
                      > Dear Andreas:

                      > The solution was to add resistors between stages of bypass
                      > capacitors to make R-C filters.

                      So distributing power for LF-circuits through a bus, is better than a
                      star formation as long as there is filtering between the stages?
                      Or is it just another tradeoff?

                      > > Won't breadboarding cause a lot of trouble in itself, like high
                      > > parasitic capacitances?
                      > Just the opposite. PC board will have much higher (but controlled)
                      > capacitance than point to point wiring.

                      Ok. Well, I am planning to put a more or less unbroken ground plane
                      on the top side and run all signals on the bottom. That creates a lot
                      of capacitance...and it is not exactly a star ground, so it is
                      perhaps not a good idea? Also, whenever two signals intersect I will
                      use a piece of wire so that I don't need to break the ground plane.
                      One might call it a 2.5-layer board.

                      > You will need some filter ahead of the A/D to get rid of aliases.

                      I've designed the filter for just that. If the sample rate is 500Hz
                      or more, there is no theoretical possibility of aliasing up to 100Hz,
                      with a 10-bit ADC. 1kHz samplerate is sufficient for 16-bit precision.

                      > You will not find this in the spec sheets. The base emitter is
                      > not as good as the base collector. The emitter collector makes a
                      > good low leakage zener diode for op amp bounding.

                      Thanks, I'll remember this.

                      /Andreas
                    • Jim Meissner
                      Dear Andreas: Take another look at the EEG preamp stage that I posted on my website. This is a shielded box that is worn on a belt. The output cable can be
                      Message 10 of 11 , Jan 8, 2002
                      • 0 Attachment
                        Dear Andreas:
                         
                        Take another look at the EEG preamp stage that I posted on my website.  This is a shielded box that is worn on a belt.  The output cable can be run 10 to 100 feet without degradation to the A/D converter.
                         
                        Think about your preamp in those terms.  It could be an inch away, but think of managing your power and signals as though it is 100 feet away.
                         
                        > The solution was to add resistors between stages of bypass
                        > capacitors to make R-C filters.
                        The op amps have a 100 nf cap from (+) power bus to ground, and a 100 nf from (-) power bus to ground.  As the power enters the board, there are 10 mf tantalums from the power bus to ground.  This allows me to put a resistor between the preamp and the power source.  The larger this resistor the better the RC filter.  Too high a value and too much voltage is lost and voltage regulation-stability becomes a problem.

                        > >  Won't breadboarding cause a lot of trouble
                        in itself, like high
                        > >  parasitic capacitances?
                        > Just the
                        opposite.  PC board will have much higher (but controlled)
                        >
                        capacitance than point to point wiring.
                        >  Ok. Well, I am planning to put a more or less unbroken ground plane
                        >  on the top side and run all signals on the bottom. That creates a lot
                        >  of capacitance...and it is not exactly a star ground, so it is
                        >  perhaps not a good idea?
                        Look at the schematic and notice that I purposely added a 10 nf capacitor from the signal output line to ground!  Also notice the op amp configuration necessary to be able to do that.  The purpose was to reduce the impedance of the line to reduce noise pickup.  Capacitance in the right place is good.
                         

                        >   Also, whenever two signals intersect I
                        will
                        >  use a piece of wire so that I don't need to break the ground plane.
                        >  One might call it a 2.5-layer board.
                        Yes that works.  But that might be an overkill.  Think of a separate preamp for all the fancy stuff.  Then the other board is a "normal" project.

                        > You will need some filter ahead of the A/D to
                        get rid of aliases. 
                        >  I've designed the filter for just that.
                        If the sample rate is 500Hz
                        >  or more, there is no theoretical
                        possibility of aliasing up to 100Hz,
                        >  with a 10-bit ADC. 1kHz
                        samplerate is sufficient for 16-bit precision.
                        My sampling rate was 130 Hz and I was able to send 5 bytes at 9600 Baud.  That generated megabytes of data in a few minutes.  Why are you sampling at 1KHz?
                         

                         

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