From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-12-17 21:48:01
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Saturday, December 15, 2001 3:28 PM
Subject: [buildcheapeeg] Re: Necessity of ir isolation
> --- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
>
> Hi Joerg,
>
A list of my mistakes:
> > 1)In order to get two bipolar channels (as in the modularEEG)
> > the circuit above would have to be built 4 times (more than
> > 4 x 10$ in cost) and in a further stage the difference
> > between each two channles would have to be calculated.
> >
> > 2)Inevitable tolerances in the feedback networks (of the active
> > electrodes)of lets say 1% would produce a very poor CMRR of
> > only 40dB.
> >
> [snip]
> > 3)Further you can not use this design with driven shielded cables,
> > because there is no output with gain=1 in this circuit.
>
> They don't tell the whole story in the article I sent you, so maybe I
> was misleading, I'm sorry.
You were more or less right ;-)
> artikel1.htm depicted what could be used as one half of an in-amp.
Yes I see. My negative impression of the artikel1.htm -circuit was a bit
premature. In fact it is very similar to the EEGfig2.gif in
http://www.biosemi.com/publications/artikel7.htm
Also I was mistaken about the influence of resistor tolerances
on CMRR in this particular circuit:
Assuming 1% tolerances and a differential gain of 20000
CMRR in the passband would be 120dB, what is very good.
> In another article, they claim 136 dB CMRR for the complete circuit.
> The feedback network differs slightly, but it's the same idea. They
> don't use a symmetric setup as suggested by (?) artikel1, but a
> reference electrode without any special feedback. Together with the
> integrator-setup described in artikel1, it forms a 2-opamp
> instrumentation amp. The signal from the reference electrode is also
> fed to the shield driver and right-leg driver.
>
> http://www.biosemi.com/publications/artikel7.htm
>
> Here are direct links to the circuit(s) in question:
>
> http://www.biosemi.com/publications/eegfig2.gif
> http://www.biosemi.com/publications/eegfig3.gif
The more I think about this circuit the more I am beginning to like it...
The full gain of 20000 is archieved in one stage. DC offsets are
fully compensated by the second integerator. No additional
coupling capacitors are required.
Difference and common mode input impedance are equally high
(however I can't get the exact value from the LT1012 datasheet).
The LT1012 GBW is 10^6, so at G=20000 the bandwidth would be limited
to 50Hz (not a problem if the gain is lowered to 10000 we get 100Hz
and do not interfere with the 75 Hz anti-alias filter)
> As you can see, you only need three pairs of opamps for two channels
> (+ one additional shield
>
> driver amplifier).
I do not quite agree. My prior posting about the necessity of calculating
the difference between between two channels to get one differential channel
was complete nonsense - perhaps this has irritated you...
In EEGfig3.gif the inputs attached to V0 and V1 would form one complete differential input.
So for V0 1x LT1012 would be needed
and for V1 1x LT1012 and 2/2 TLC1078,
making a total of 4 opamps in 3 packages per differential channel.
In order to get 2 differential channels as in the current
INA114 based design, the above is needed twice, giving
8 Opamps or 6 packages.
Guard and DRL would require 2/2 TL062, making 1 additional package.
That makes a total of 7 packages without anti-alias filter.
Until here the INA114 based design needs 5 packages.
> > > the only
> > > problem with it lies in the unlikely event of one electrode lead
> > > being shorted to V+ and another to V- or ground.
> >
> > Unlikely but more or less dangerous ...
>
> Agreed. And I can't think of a good way to fix it either, except
> those noisy resistors. By the way, 470k is a bit unecessary. Using a
> power supply of +/-3V you only need 2*62k to meet the 50uA limit
> requirement => 2 uVp-p current noise (actually a bit more since the
> signals are not limited to 10Hz) for INA114. Is that low enough for
> EEG?
Less noise would be better. However the LT1012 has only 1/20
noise current of the INA114, so the current noise in the above
mentioned protection resistors can be neglected.
> There are opamps with very low input noise currents available,
> but they have higher voltage noise and require careful layout.
> Example: http://www.national.com/ds/LM/LMC6001.pdf
Is it better than LT1012 ?
Regards,
Joerg
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:35 BST
