From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2001-12-14 21:08:11
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
Hello Joerg,
> IMO this is a bad idea. It will introduce 18uV p-p noise due to
> the drop of INA114 18pA p-p input noise current (for simplification
> over a frequency range from 0.1 to 10Hz) over the two 470k
protection
> resistors.
You got me there, I only calculated the Johnson noise in the
resistors and overlooked the noise current.
> Do you have a URL ?
> I only know the old bm input stage.
Here it is. No information besides "10Gohm + 110 dB CMRR" though:
http://www.brainm.com/productinfo/bmr2emodule/systemspecs/superinput/s
uperinput.html
> The difference-mode impedance is really reduced to 10Mohm - however
this
> type of impedance will only result in a minor amplification error
> (about 0.1% loss) due to a virtual voltage divider between the
> electrode impdance of ca. 10k and the diff-mode input impdance of
10M.
But what happens if the electrode impedances are higher, and are
unequal?
In message 1340, Jim Meissner wrote:
> The other problem is the electrode source impedance. When you start
> it is easy to get 10 K ohm with conductive paste. For medical or
> clinical settings that is no problem. But if the person tested goes
> into an altered state, the skin resistance changes and the
conductive
> paste has no effect.
[snip]
> In a meditative state the skin resistance will rise to 100 K ohms
and
> sometimes several meg ohms.
Wouldn't higher input impedance (difference-mode) help reduce the
problem of large electrode impedances as the voltage-divider effect
would be much smaller?
Biosemi's approach to high-pass filtering on the input stage retains
the higher difference-mode impedance. I'm sure you have read it
before:
http://www.biosemi.com/publications/artikel1.htm
Since they use a right-leg-drive, and not passive ground, the only
problem with it lies in the unlikely event of one electrode lead
being shorted to V+ and another to V- or ground.
Biosemi solves this by turning the input stage off if the voltage at
the ADC becomes too high or low.
They give a brief description here,
http://www.biosemi.com/faq/limit_current.htm
but do not explain how they tell the difference between large input
signals that cause clipping and a fault condition... :-p
What do you think, could we use the Biosemi design? Not like their
active electrodes, but as a regular input stage.
/Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:33 BST