ModularEEG Parts Substitution tips

Table of Contents

• Parts Substitutions

Parts Substitutions

modularEEG Parts substitution and Tolerances
--------------------------------------------

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 parts substitutions in 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
JB == Jim Bilderback

PARTS SUBSTITUTION AND TOLERANCES
---------------------------------

[JH] (alternatives to BAT48)

  The BAT48 is not so critical. You can use any Shottky Diode with
  parameters equal or better:

  Repetitive Peak Reverse Voltage, VRRM=40 V
  Forward Continuous Current at Tamb = 25C, IF=350 mA

  [JB: 33C2687 $0.027 at Newark seems to be an ok substitute.]

[AR] (re using less accurate components)

  In many places 10% resistors and 20% capacitors are not good
  enough. The filter for example, could produce awful results with the
  wrong values.
  
  I showed this a while ago:
  http://openeeg.sourceforge.net/modeeg/cap_tolerances.gif
  
  This graph shows the effect on the frequency response in the filter
  depending on whether you use 5% or 10% capacitors. Needless to say,
  you want as flat response as possible.

[JH] (re regulator on digital board)

  Any 7805 with TO220/3-case and 1A current rating will work.

[AR/JH] (re low-ESR capacitor on digital board)

  Recommended ESR for C201 is 1 ohm (0.8-1.2 ohm)
  Recommended reference parts:

  Kemet T350K476K025AS or T356K476K025AS
  Digikey part no 399-1422-ND or 399-1347-ND
  47uF/25V, ESR = 1.0 ohm @ 100kHz.
  Lead pitch 5mm, price $5.07

  C201 is a component that is critical for the stability of the
  virtual ground generated by IC201A (1/2 TLC277 opamp).
  
  Generally opamps do not like being loaded with a few 100nF (these
  are the many power line bypassing Cs) and begin oscillating or at
  least ringing due to low phase margin.  An even bigger C in series
  with a defined R (the internal ESR of the capacitor) placed in
  parallel with the other 100nF Cs _increases_ the phase margin, so
  that stability is achieved (no oscillation, no ringing).  My SPICE
  (LTspice) simulation results (posted here some time ago) have shown,
  that best stability is achieved with ESRs from 800mOhms..1200mOhms.

  So I can only recommend to get the reference part (see above) or
  something with similar ESR.  Tantalum Cs are more recommended than
  aluminum electrolytic Cs, because the latter tends to have less
  defined ESRs that vary over time (aging) and temperature.

  > however, if the cap / resistor in series solution is acceptable
  > then that would put the price well under $1.

  If a tantalum cap had e.g. 200mOhms ESR and you would place a
  800mOhms in series, the result would be sufficiently equivalent to
  the above mentioned reference part. For the e.g. 800mOhms additional
  series R be sure to use a low inductivity type like a smd resistor.


[AR] (part tolerances)

  Btw, using anything worse than 5% (e.g. 10%) capacitors is not not
  recommended. I made this little graph today:
  http://openeeg.sourceforge.net/modeeg/cap_tolerances.gif

  (This is regarding using lower tolerance capacitors in the analogue
  board)

[JH] (part substitutions)

  > The closest they had to an AT90S4433P is an AT90S8515-8PC. Can
  > those in the know comment whether this chip is a suitable
  > replacement please.
  
  No. The AT90S8515 neither has the right package (DIP 28-3) nor does
  it have on chip ADC.

[JH] (part substitutions)

  > Firstly, is it necessary to use Tantalum capacitors?
  
  Not really. I have built the RS232EEG and modularEEG with aluminum
  electrolytic capacitors and they seemed to work. Generally aluminum
  types have a greater internal series resistance (ESR, equivalent
  series resistance) than tantalum types and are therefore less
  efficient at high frequencies and have limited capability to buffer
  current spikes.
  
  The Cs around the MAX232 are uncritical (=aluminum electrolytics can
  be used)
  
  C101 for bypassing TL431 seems to be uncritical too.

  > capacitor (C201). Why does this capacitor have resistance and what
  > is it doing?
  
  C201 has 47uF capacitance and should have an equivalent series
  resistance of about 800m..1200mOhms.

  > Is it draining static voltage and if so would a zena diode do the
  > job?
  
  No. You can imagine it as an ideal capacitor (of 47uF) in series
  with a resistor (800m..1200mOhms)

  > Does anybody know where I can get this part from
  > (internationally).
  
  Simply use a normal 47uF aluminum electrolytic capacitor any hope
  that the ESR is quite usable.
  
  Or better: Use a 47uF tantalum type and connect a small resistor in
  series so that the total ESR gets about 1000mOhms.  BTW: Does anyone
  have ESR values for 47uF tantalum Cs? I guess it is about 100mOhms.

  > 50v capacitors are a lost easier to find than 100v ones. This is a
  > low voltage circuit so it should be ok to use 50v capacitors
  > right?
  
  OK.
  
  However C216 and C212 (1nF at DRL output) could be more easily
  destroyed by ESD into the DRL output if they have low voltage
  rating.

  > Pin 3 on the ribbon cable (J201) does not seem to go
  > anywhere. What is it doing?
  
  My intention when feeding the buffered +2V (now called VGND) back
  into the bus cable was that only one analog pcb would need the +2V
  buffer and the other 2 analog pcbs would simply reuse the +2V so
  that IC201A(+B) could be (or better _must_ be) omitted on the other
  pcbs.

[JH] (parts substitutions)

  > is this the right Voltage Regulator?
  > http://focus.ti.com/docs/prod/folders/print/ua7805.html
  
  The 7805 regulator is quite uncritical. You can use any 7805-type
  that has TO220/3 case and 1A current capability.


  
// END Parts Substitutions //