ModularEEG Design Modification tips

Table of Contents

• Design Modifications

Design Modifications

modularEEG build tips 
---------------------

NOTE: THIS FILE WAS LAST UPDATED IN MAY-2003
(In April 2005 it was extracted from the build tips, but otherwise left untouched)

Introduction
------------

This is a collection of bits of information, build tips and so on,
that have appeared on the openEEG mailing list regarding the
modularEEG.  Eventually these will be formatted up into a proper build
document.  They are in latest-first order, which means the most
accurate information should be nearest the top!

Authors are indicated with initials:

AR == Andreas Robinson
JH == Joerg Hansmann
MvB ==  Moritz von Buttlar 

DESIGN CHANGES/IDEAS
--------------------

[MvB] (possible problem?)

  In case somebody else has similar problems: I had to change R124 on
  the digital board of modularEEG V0.06 from 1k to 500 ohms. Otherwise
  the optocoupler didn't transmit the serial data stream. This
  resistor should be changed in the schematic as well.


[AR] (re filter design)

  First of all, I would call this a "Besselworth" filter (my name for
  it) because it has a blend of Butterworth and a Bessel filter
  properties. If you want more details on what those are just let me
  know.
  
  It is based on the so-called Sallen-Key filter structure, which lets
  you make 2nd order filters with one amp. This one is modified so
  that it gets G = 16 (R229 and R230) and improved Q (C232).
  
  Strictly speaking, it isn't a 3rd order filter, so maybe it should
  be relabeled. A 1st order filter has an increase in attenuation of
  6dB per octave, a 2nd order filter has 12dB per octave, a 3rd order
  filter - 18dB per octave, etc. This filter has a 14.3dB increase per
  octave, meaning the order is closer to 2.4...
  

[JH] (design)

  DRL, VGND, 2 test signal outputs will only be required on the first
  amplifier board.  That means also, that some of the not needed
  circuit parts do not need to be inserted on the 2nd and 3rd
  amplifier board. (e.g. DRL circuit)

  > Somehow using 3.5mm or 1/4" jacks would be nice, because then the
  > input signal lines could be connected to ground automatically on
  > disconnection

  This is a really good idea!

  Instead connecting to ground (VGND), disconnected signal lines could
  be connected to calibration signal + and -. The calibration signal
  is quite near to VGND and could be switched off by software (if
  required at all). This setup would spare the additional calibration
  signal connectors.

  > Is the current drain going to be too much for a laptop keyboard
  > port, do you think ?
  
  I think it is OK. You can also drain power from gameport, if you
  have one.  Keyboard or gameport have stabilized +5V, so you have to
  bypass the 7805 regulator on the modularEEG digital board.

[AR] (design changes)

  Anyway, for good measure I incorporated a 10k trimpot that was used
  to patch my unit. Most people +will just want to replace it with a
  short piece of wire...

{This one may be out of mailing list order}
[JH] (re using for ECG recording)

  yesterday I have done some ECG recording with the modularEEG.
  
  ECG has a R-spike amplitude of about 1mV. Because the strong
  asymmetry of the signal, a total range of at least 2mVp-p for
  fullscale is necessary. To have headroom for movement artifacts
  etc. 4mVp-p would be desirable.  So in order to record ECGs with the
  modularEEG, the gain has to be lowered. This can be easily done by
  replacing e.g R214 and R215 (for channel2, modEEGamp_v0.05) with
  100k. The gain of the first stage is thereby lowered from 12.2 to
  about 1.(BTW: This is not the best way to lower the total gain,
  because the signal to noise ratio of the INA114 is better when using
  higher gains).
  
  Pictures and recorded ECG raw data (at the end of the page):
  
  http://www.jhansmann.de/eeg/data/

// END Design Mod Tips //