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PICList Thread
'EEG impedance checking'
1998\03\26@153650 by Craig Webb

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Is there anyone who has experience with impedance checking EEG electrodes,
and could help to redesign a low-noise front end for medical purposes
(signals in the low microvolt range). The system we have has a million
Megaohm (10^12) input impedance, 130dB CMRR, and 12 bit resolution

Craig Webb

1998\03\26@205020 by Octavio Nogueira

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>Is there anyone who has experience with impedance checking EEG electrodes,
>and could help to redesign a low-noise front end for medical purposes
>(signals in the low microvolt range). The system we have has a million
>Megaohm (10^12) input impedance, 130dB CMRR, and 12 bit resolution
>
>Craig Webb

I would be very interested in this too!!

Octavio
======================================================
Octavio Nogueira  - e-mail:   spam_OUTnogueiraTakeThisOuTspammandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================

1998\03\27@175901 by Alex Torres

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> From: Craig Webb <.....lucidKILLspamspam@spam@MAGNET.CA>

> Is there anyone who has experience with impedance checking EEG
electrodes,

It could be done as trivial ohmmeter, but not at DC (becouse electrodes
potential   is fluctuating).
We use 20/70Hz with 50-100mV amplitude.

> and could help to redesign a low-noise front end for medical purposes
> (signals in the low microvolt range). The system we have has a million
> Megaohm (10^12) input impedance, 130dB CMRR, and 12 bit resolution

Craig, this is not a problem. The problem are work-inpedance-calibration
switching, noise and multychannals of the EEG amplifier.
There are 2 types of EEG amps - general purposes (16 and more channels,
0.3-100Hz) and
amlifier for the evoked potentials measurment (1-2 ch, 1-2kHz).
For the general EEG amps there are 2 ways  - using N equal instrumentation
amplifiers that have
good CMMR and low noise, then MUX then ADC.
We use Analog Device's AD620 chip.
The second way is using N low noise non-differential OP without DC coupling
capicitors, then MUX and then 22-24 bit ADC.
The input signal typicaly includes a variable dc component of about 300 mV
and a much smaller ac signal and up to 600 mV common mode signal. At the
1st type of EEG/ECG amplifiers the dc component reject by the input
capicitors and common signal by the high CMMR, at the 2nd type ADC digitize
ALL signal,
andt the dc offset and common mode rejects by the digital processing.

If you want additional information write me a private e-mail.

Alex Torres, Kharkov, Ukraine (exUSSR)
altorspamKILLspamchat.ru
2:461/28@FidoNet
www.geocities.com/SiliconValley/Lab/6311

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