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Re: [buildcheapeeg] Re: My project - status report

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  • Joerg Hansmann
    Hi Andreas, ... From: sleeper75se To: Sent: Thursday, February 28, 2002 4:55 PM Subject: [buildcheapeeg]
    Message 1 of 14 , Mar 1, 2002
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      Hi Andreas,

      ----- Original Message -----
      From: sleeper75se <sleeper75se@...>
      To: <buildcheapeeg@yahoogroups.com>
      Sent: Thursday, February 28, 2002 4:55 PM
      Subject: [buildcheapeeg] Re: My project - status report


      ...
      >... the latest schematic is uploaded now.

      I have just downloaded them.

      ...
      > As you can see some things have changed, and some things need to be
      > changed. I hope I have not added any bugs... Ok I know I may have
      > added one bug, that may already have reared its ugly head, but I can
      > patch it if necessary.
      >
      ...
      > > > By the way: When I touch the amplifier inputs once,
      > > > the noise is replaced by a high level (200mV) 50Hz hum
      > > > that remains even when I remove my hand. The 50Hz
      > > > signal stays until I power-cycle the amplifier.
      > >
      > > IMO this test says nothing.
      >
      > Do you mean touching the amplifier?

      No. Doing tests with inputs floating.

      ...

      ...
      > > Did you use a differential probe or a single (referenced to
      > > earth) probe ?
      >
      > Single-ended ... :-p (Anyone got a FET-probe lying around?)

      If you have a 2-channel scope you have the option to add both channels
      while one is inverted. If you use the same gain for both channels
      you effectively get the difference between both probes.
      This setup eliminates much of the ground noise without
      need to have expensive FET probes ...


      > > > The RLC-filters are not nearly as
      > > > efficient as calculated for some reason...

      Did you include series inductances of some 10nH for each C of your simulation ?


      > >
      > > Provide schematics / layouts. Then I can say more.
      >
      > Ok, it is uploaded to the files section. The DC/DC converter is not
      > specified on the schematic, but I'm using this one:
      > http://www.dcdc.com/pdf/ha&hp.pdf (model HP22-060-18)
      >
      > Some notes:
      >
      > I tried patching the circuit by paralleling the 2200uF cap with a 1uF
      > film cap - no difference at 20MHz (where the disturbance is).


      For 20MHz you need 10nF ceramics (with very short wires) very near
      to the DCDC outputs.

      Treat your bypassing Cs in your layout as if they had 4 poles
      (2 input wires and 2 output wires)
      The layout traces of a bypass C should look like a "X" and not
      like a "H" to minimize trace inductance in series with the C.
      (each cm of trace has about 10nH)



      > Most of the noise comes from the switching spike generated in the
      > power supply, which is a Mascot 9921 medical power supply:
      > http://www.mascot.no/pdf/9921.pdf
      >
      > Just measuring its unloaded output shows a single spike occurring at
      > the switching frequency (40kHz).
      >
      > Connecting it to my circuit, produces terrible ringing at around
      > 20MHz. I could successfully simulate the ringing in spice - the cable
      > works as a transmission line (1.8m unshielded cable) with a 20MHz
      > resonance peak.
      >
      > I tried terminating the cable by using an impedance (22 ohms + 100nF
      > in series) across the terminals but it didn't help.

      Try to reduce the problem at its source.

      > Coiling the cable and putting my hands around it reduces the ringing
      > visibly (about 50%), so I'm guessing a large ferrite bead like the
      > ones used in VGA-cables could help here? Unfortunately, my local
      > supplier (ELFA) is permanently out of stock, of the models I need
      > (though they catalogue them), or I would have tried it already.


      BTW: For dual power supplies it is always a good idea to put a reversed diode
      (capable of handling the regulator output shortcut current)
      from each regulator output to GND. Otherwise in case of a pcb failure with
      shortcut between +5V and -5V one regulator will win and pull a part of the
      attached circuit to reverse polarity - and destroy it.


      amplifier:

      You write on the schematic that the HP-coupling is useless due to
      the long time constant. That is not true.
      With 1meg and 1uF you get T=1 sec or fc=0.16 Hz and not 0.016 Hz.


      Regards,


      Joerg
    • sleeper75se
      ... Hi Scott, thanks! I ll look into it. Regards, Andreas
      Message 2 of 14 , Mar 2, 2002
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        --- In buildcheapeeg@y..., "scottelofson" <scottelofson@y...> wrote:

        > Hi, Andreas
        >
        > Linear has modified the recommened values for the input filter:
        > Cxd= 1 nF
        > Cxcm1, Cxcm2 = 100 nF
        >
        > See specification notice: http://www.linear.com/pdf/sn1167.pdf

        Hi Scott,

        thanks! I'll look into it.

        Regards,

        Andreas
      • Andreas Robinson
        Hi Joerg, ... Ah! Clever! I ll try that. Thanks. ... Hmm, no. Only resistors for the electrolytics. (Checking simulation) Yikes, from good to almost useless.
        Message 3 of 14 , Mar 2, 2002
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          Hi Joerg,

          > If you have a 2-channel scope you have the option
          > to add both channels while one is inverted. If you
          > use the same gain for both channels you effectively
          > get the difference between both probes.
          > This setup eliminates much of the ground noise
          > without need to have expensive FET probes ...

          Ah! Clever! I'll try that. Thanks.

          > > > > The RLC-filters are not nearly as
          > > > > efficient as calculated for some reason...
          >
          > Did you include series inductances of some 10nH
          > for each C of your simulation ?

          Hmm, no. Only resistors for the electrolytics.

          (Checking simulation) Yikes, from good to almost
          useless. :-p

          > For 20MHz you need 10nF ceramics (with very short
          > wires) very near to the DCDC outputs.

          Unfortunately is the cable is not detachable and the
          supply looks very difficult to open without breaking
          something (there are no screws). However I can try
          shortening the cable to 5-10 cm.

          I've attached a switcher-CAD schematic of a filter I
          hope will work. The 10uF tantalums are there to
          attenuate a 500kHz resonance peak caused by the 10uH +
          10nF LC-combination. Please tell me what you think, is
          it "realistic"?

          The series inductances and resistances are entered
          into the capacitor-models and are not visible in the
          schematic.

          > Treat your bypassing Cs in your layout as if they
          > had 4 poles (2 input wires and 2 output wires)
          > The layout traces of a bypass C should look like
          > a "X" and not like a "H" to minimize trace
          > inductance in series with the C.
          > (each cm of trace has about 10nH)

          Oops. I already knew that, yet somehow failed to
          implement it (for the 2200uF capacitor anyway). :-p

          > BTW: For dual power supplies it is always a good
          > idea to put a reversed diode (capable of handling
          > the regulator output shortcut current) from each
          > regulator output to GND. Otherwise in case of a pcb
          > failure with shortcut between +5V and -5V one
          > regulator will win and pull a part of the attached
          > circuit to reverse polarity - and destroy it.

          Acknowledged...

          > amplifier:
          >
          > You write on the schematic that the HP-coupling is
          > useless due to the long time constant. That is not
          > true.
          > With 1meg and 1uF you get T=1 sec or fc=0.16 Hz and
          > not 0.016 Hz.

          Hmm, maybe I was unclear. The filter is not performing
          well, because the high-gain stage (G=100) in front of
          it will cause saturation of the output (after both
          gain stages) for fairly low offsets (15mV). Lowering
          the gain before the filter and increasing it after,
          will produce better results. G=20 is the equivalent of
          what you are using now and your design handles 100mV
          offsets.

          Regards,

          Andreas

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        • Joerg Hansmann
          Hi, ... From: Andreas Robinson To: Sent: Saturday, March 02, 2002 6:43 PM Subject: [buildcheapeeg] Re:
          Message 4 of 14 , Mar 4, 2002
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            Hi,

            ----- Original Message -----
            From: Andreas Robinson <sleeper75se@...>
            To: <buildcheapeeg@yahoogroups.com>
            Sent: Saturday, March 02, 2002 6:43 PM
            Subject: [buildcheapeeg] Re: My project - status report


            ...
            > > For 20MHz you need 10nF ceramics (with very short
            > > wires) very near to the DCDC outputs.
            >
            > Unfortunately is the cable is not detachable and the
            > supply looks very difficult to open without breaking
            > something (there are no screws). However I can try
            > shortening the cable to 5-10 cm.

            What cables ? I thought your DCDC were pcb mounted ?
            (at least I had this impression from the pcb layout)

            And don't get me wrong: I did not want you to break anything
            open...

            > I've attached a switcher-CAD schematic of a filter I
            > hope will work. The 10uF tantalums are there to
            > attenuate a 500kHz resonance peak caused by the 10uH +
            > 10nF LC-combination. Please tell me what you think, is
            > it "realistic"?

            It looks good.

            BTW: The thing with putting the 10nF C close to the output (with X
            shaped traces) assumes that the DCDC has some output impedance
            (should be listed in the datasheet of the DCDC)

            ...
            >
            > Oops. I already knew that, yet somehow failed to
            > implement it (for the 2200uF capacitor anyway). :-p

            It is somewhat questionable to me what sense makes a
            2200uF C in a device with switching frequencies of
            many 10 to 100kHz anyway.


            ...
            > > amplifier:
            ...
            > Hmm, maybe I was unclear. The filter is not performing
            > well,

            In theory or in practical experiments ?
            Perhaps with better electrodes the offset handling
            capacity is enough ?

            > because the high-gain stage (G=100) in front of
            > it will cause saturation of the output (after both
            > gain stages) for fairly low offsets (15mV). Lowering
            > the gain before the filter and increasing it after,
            > will produce better results. G=20 is the equivalent of
            > what you are using now and your design handles 100mV
            > offsets.

            I would like to know, what DC offset the Abhayamudra unit
            can handle:

            Excerpt from the article about the Abhayamudra unit
            (http://www.pocket-neurobics.com/)
            that Doug has posted some days ago:

            "...
            We also include a number of features to try to ensure eeg sessions are
            successful (with these big amplifications required, there's lots of
            scope for things to go wrong). We have constant monitoring of the
            "offset potential" - probably the most common cause of eeg signals
            going 'flat-line'.
            ..."


            Regards,

            Joerg
          • sleeper75se
            Hi Joerg, Thanks for your input, it is very valuable! ... I wish I had a camera.... There are two dc/dc converters. Connected to the mains is a medical power
            Message 5 of 14 , Mar 5, 2002
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              Hi Joerg,

              Thanks for your input, it is very valuable!

              --- In buildcheapeeg@y..., "Joerg Hansmann" <info@j...> wrote:

              > What cables ? I thought your DCDC were pcb mounted ?
              > (at least I had this impression from the pcb layout)

              I wish I had a camera.... There are two dc/dc converters. Connected
              to the mains is a medical power supply.

              Data: IEC60601-1 compliant, 13.2V 3A output, 50mV ripple, 40kHz
              switching frequency, switching noise energy in the 15-20MHz range.

              Connected to the medical power supply is a PCB holding filters and a
              dual output DC/DC converter given to me for free, by Scott Elofson.

              Data: 500V isolation, +/-12V, +/-300mA, 20mV ripple with a 3uF
              capacitor, 200kHz switching frequency.

              I didn't put much effort in designing the filter. It was like "oh,
              and by the way...", basically ignoring Jim's advice on capacitors
              (sorry) and not following any common sense rules. So much for rushing
              things... :-p

              I'll get some EMC-parts tomorrow and backtrack a bit (new PCB). The
              battery and the mains-powered solutions are worked on in parallel,
              because they share 5V linear regulators and EMI filters.

              > And don't get me wrong: I did not want you to break anything
              > open...

              Don't worry, I realized that. This medium (text only) is not very
              suited for this kind of collaboration.

              > BTW: The thing with putting the 10nF C close to the output (with X
              > shaped traces) assumes that the DCDC has some output impedance
              > (should be listed in the datasheet of the DCDC)

              The medical power supply has a long cable. Shortening it to 10-15 cm,
              and winding it a few turns on a ferrite toroid creates a 10-20uH
              inductor that will be part of the filter.

              > > Hmm, maybe I was unclear. The filter is not performing
              > > well,
              >
              > In theory or in practical experiments ?

              In theory, (I can hear someone on the list cringe... ;-) the
              electrodes are not finished yet. I want decent offset handling,
              because I don't think the electrodes will work as well as the ones
              Jim use. Then again +/-15mV might be sufficient. We'll see...

              > I would like to know, what DC offset the Abhayamudra unit
              > can handle:

              The designer is taking questions over at the Mind-L list. Considering
              his openness on the specs, I think you will get answers to any
              questions you may have (please share his answers if you ask him
              anything).

              Follow this link to get his e-mail address:
              http://groups.yahoo.com/group/mind-l/message/19812

              Regards,

              Andreas
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