From: Andreas Robinson (sleeper75se_at_yahoo.se)
Date: 2002-01-01 15:30:21
Hi Joerg,
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann"
<info_at_jhansmann.de> wrote:
> This could be a solution. However you lose the
> enhancement of the protection circuit impedance by
> the open loop gain of the input buffer. But you get
> at least a better common mode impedance (what would
> be adequate IMO)
Yes, connecting the transistors to the buffer output
would exhibit least possible leakage, but the
transistors leak very little at low voltages (< 100mV)
so that it may not make much difference?
>> It is supposed to be entirely passive. The
threshold
>> voltage is determined by the transistors alone so I
>> don't see any reason why it should not.
>
> My question was more intended to see, if the
> the whole www.biosemi.com/publications/artikel7.htm
> circuit "eegfig3.gif" works at low voltage.
Well, the only thing I can see is limited by low
voltage is common mode range, but that has nothing to
do with the protection. Can you think of anything
else?
>> I've attached the new attempt.
>> I hope this works better.
>
> R7 will not really protect X1 because the (-) input
> could still be overloaded.
Ahem, you are right. It looks a lot better if the
transistors are connected to the other end of the
resistor (the resistor is supposed to be there
anyway).
>
> For what purpose are the 1nF C's C1..3 ?
> For shunting HF they must be connected to a low-
> impedance node (what they are seemingly not)
<confused> They don't attenuate HF? My intention here
was to let the capacitor act like a short for high
frequencies between a channel and the reference. It
looks good on paper/in simulation. Could you please
clarify a bit why this is so and how?
> I have also attached an input/user protection
> circuit for the INA114 based amplifier:
>
> Worst case current through any port should be
> ca. 40uA.
Please consider using transistors rather than diodes.
See the attached schematic where I've replaced the
diodes with transistors.
The transistors don't not protect the user very well
due to their higher turn-on voltage, but the diodes,
while an effective protection, leak far too much.
BAR42 schottky diodes (Vf = 0.4V supplied with
Simetrix, that's why I used them) leaked 1nA with a
1mV signal. The transistors leak only a fraction of
that, around 2pA. It gets worse when you add an
offset. At 100mV offset the diodes let 100nA through
while the transistors stay at a lower 20pA. Of course,
all these calculations depend somewhat on what
capacitors are used...
I'm beginning to think we would need to measure the
current through the electrodes and have an active
emergency-shutoff...
That is, if we should still aim for FDA/IEC
compliance? I'm leaning towards just putting something
small and simple together now, so that people can get
started writing software as soon as possible. FDA can
perhaps wait a bit... and the paranoid can always
check the amplifiers with a multimeter before
connecting themselves.
> BTW: Do you have an idea how the PSPICE AMB
> equations for OP-amps looks like ? (gain and
> three poles would be enough ...)
Unfortunately no.
I'm guessing here that your spice-simulator lacks
built-in support for opamps? You have previously
mentioned that you were forced to simulate opamps
using voltage controlled voltage sources.
In that case, take a look at Simetrix:
http://www.newburytech.co.uk/
The user interface is a bit different, and the free
version limits circuit size, but other than that, it
is very good.
> It seems to me that this could be done with the
> LAPLACE transform...
If you have plots of the open-loop gain and phase for
an opamp you could try to make a transfer function
from that, but I think it would be a lot of work...
> or is this only for transient analysis ?
Since the laplace-transform was designed for frequency
calculations, it should work fine for AC-analysis as
well.
Regards,
/Andreas
PS
I made a little practical experiment the other day:
I've built some electrodes of 18mm silver discs, and
using two of them I measured the impedance on the
lower arm.
First it was cleaned with alcohol and salt water was
poured onto it. The electrodes (non chloridized, also
cleaned with alcohol) were applied and pressed hard
onto the skin. The resistance was measured with a
multimeter.
The results: 160Kohms when they were about 10 cm apart
and 24Kohm when they were 1 cm apart.
I would have gotten lower values if I had scrubbed the
skin, but when you are supposed to measure EEG on
yourself, that's hard to do to and is not very
comfortable.
DS.
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