From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-02-18 21:33:24
Dear Jim:
----- Original Message -----
From: Jim Meissner <jpmeissner_at_mindspring.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Monday, February 18, 2002 6:19 PM
Subject: Re: [buildcheapeeg] Loose ends: Power
>Dear Joerg:
>
>Have not heard much from you for a while, and all of a sudden it looks
>like you have been "very" busy!
>
>Seeing such good data, I must try this Driven Right Leg circuit.
>
>The 500 K is for safety?
Yes.
>The C5 causes decreased gain with frequency,
Yes. C5 and R9 with U2 is an inverting integrator.
>C4
>causes increased gain with frequency. The right leg impedance affects
>gain. Do you have to tune these to keep it from oscillating?
No. The components values are _very_ uncritical and the DRL is stable
over a wide range of load conditions.
However there is one thing that _is_ critical:
R4 must match R5 _very_ exactly.
This can be avoided if the voltage (at node "cm")
is buffered with a 2nd opamp.
(see like it is done in the new schematic of the
modularEEG amplifier at http://www.jhansmann.de/eeg/eeg.html )
>A question, where exactly to you put the right leg drive?
It is connected to a plastic clamp with a 3 x 3.5cm^2 steel plate contact area. (IIRR an ECG electrode clamp from NIHON KOHDEN)
I simply clamp it to my right lower leg ca. 10cm over the
ankle.
However I suppose it could be attached anywhere as long as it
is sufficiently far away from the active electrodes.
>Do you use
>paste?
No. It does not seem to make a difference. The moisture thats
gets trapped beneath the electrode seems to be sufficient.
>It seems your "ground" is the ear lobe?
It is neither the analog ground nor the Vref/2.
Ground is completely replaced by the DRL output.
The (-) input of the amplifier is connected to the right earlobe
(with a little plastic clamp , a pure silver electrode and 10/20 paste)
> Where were the two input
> leads connected Cz and what? Please explain.
The (+) input of the amplifier is connected to Cz.
> > (BTW: The spectral view was done with cooledit)
>
>What is cooledit? How can I get it?
http://www.syntrillium.com/download/
A few years ago they have had a free 30 days trial version.
However now it seems that they want to have money.
>Can you increase the resolution around 10 Hz? There seem to be two
>frequencies there?
Yes, indeed.
(see attached picture)
Regards,
Joerg
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