EOG (electro-oculography)

with 2 electrodes, you can pick up one signal and can't tell which eye it came from....

with 3 electrodes you can get two signals, one from each eye.

Also even if you only want one signal, a third electrode can be used to help reject 50Hz pickup

Mark

Hi.

You should be able to do the whole thing with a conventional

3-opamp instrumentation amplifier. Just insert your HPF between the input stage's output and the final stage's input. I state this from having seen a number of different EKG amplifiers and observed their performance under common field conditions. I see no reason for the common mode signals to be much better or worse and, from the gain you mention, it appears the differential signal is similar. EKG amplifiers typically handle differential signals of a couple mV riding on common mode signals in the neighborhood of several Vpp when a 3rd electrode is driven as stated below and the amplifier is well isolated to keep common mode input impedance high.

Probably smart, as long as you don't mind replacing batteries. An isolated circuit is not too hard to do, but may be a bit much for a hobby project.

As Mark said, it helps reject line pickup. A technique used in EKG amplifiers, (which handle signals with similar level and bandwidth when used for monitoring), is to drive the

3rd wire with the mean of the other two signals. This helps reduce current flowing through the electrodes into the common mode impedance. Because the electrodes typically have poorly controlled resistance, reducing the current flowing thru them reduces conversion of common mode noise at the skin surface into differential input noise. That is the main reason to keep the common mode input impedance high.

I would think so.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

I have some experiments with this. Basically run a PIC at a low clock rate to save on batteries. Store data into external EEPROM via I2C. Choose sample rates appropriate to the data you are mesearuning.

Add a button to dump all of the data to a PC after the fact via rs-232.

Run entire unit on standard 9volt battery for amplifies, and use low power regulator for the 5 volts to the PIC.

Use spare op-amp section for ttl/rs232 level converter. +/- 4.5 volts is plendy good for RS-232 in most cases.

--
Luhan Monat (luhanis \'at\' yahoo \'dot\' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

Oh, really? Are there 2-opamp or 4-opamp IA's?

Not accessible, idiot...

You are a TOTAL crock of bs...snip remainder of your crap...

[Derf transform applied.]

"Fred Bloggs" wrote >

Yes. Really.

There is a 2-opamp configuration that is sometimes called an instrumentation amplifier. I've never seen one with 4 opamps.

Apparently, you have never seen this circuit, or did not know what it was if you did see it:

|\\ In+ -----|+\\ ___ ___ | >----o-----|___|--o---|___|--. .-|-/ | | | | |/ .-. | === | | | | GND | | | | | '-' | | | | '---------o | | | |\\ .-. '-|+\\ | | | >-----o---- Out | | .-|-/ | '-' | |/ | | | | .---------o | | | | | | | .-. | | | | | | | | | | | | | |\\ '-' | | '-|-\\ | ___ | ___ | | >----o-----|___|--o---|___|--' In- -----|+/ |/

It has 3 op-amps, is often used in EKG amplifiers, and its 2nd stage can readily be AC coupled by inserting 2 capacitors into *accessible* branches.

[derf]

(That was in one of my professional design jobs.)

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com...

[Drivel cut.]

transistor opamp.

That is a very parochial point of view.

Below is a circuit that implements an instrumentation amplifier in order to reduce input errors due to common mode signals reaching the electrodes. It uses 3 opamps in the classic IA configuration, but exploits a normally inaccessible (and nonexistent) node in integrated IAs to derive the signal used to drive the 3rd lead. With 4 0.1% resistors (in the diff-amp), it will have sufficiently low artifact due to common mode line pickup that the 30 Hz LPF mentioned by the OP can handle what remains. There is a 0.1 Hz HPF implemented to reject the DC that the OP wanted to discard.

|\ 1.00k 24.9k In+ -----|+\ ___ ___ | >---o---|___|-o--|___|----. .--|-/ | | | | |/ .-. | | | | |39.2k | | | | | | |\ | | '-' | GND-|+\ | | | | | >-o 3rd '---------o | .----|-/ | Lead ___ | | | |/ | --o-|___|-GND .-. | | | | | |1.00k | | || | --- 22M | | | o--||-----' --- '-' | | || 2.2u | 1u /| | | | | /+|----o | | ___ '--o-< | | | '--|___|--. | \-|-. .-. | | | \| | | | | 750k | | | | |1.00k | |\ | '------' '-' '--|+\ | | | >---o-o--- .---------o .--|-/ | | | | |/ | | .-. | | | | | | | | | |39.2k | | | |\ '-' | | '--|-\ | ___ | ___ | | >---o---|___|-o--|___|--' In- -----|+/ |/ 1.00k 24.9k (created by AACircuit v1.28.4 beta 13/12/04

[Name-calling cut.]

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Larry Brasfield wrote: [..snip moronic Brasfield trash...]

No one would use a discrete opamp IA any more than they would build a discrete transistor opamp. You are a pathetic idiot.

[Fred's drivel, posturing, and invective cut.]

That is a lie.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

transistor opamp.

Oh? Is it "paraochial"-hoo tee doo- another misused word by the pretentious pseudo-intellectual, hopelessly outclassed and desperately trying to save face...

Really? You are driving the 3rd lead with a HPF of the buffered VCM from the Rg center tap- since when is the Rg center tap inaccessible on the monolithic IA, you worthless p.o.s.? This is just another example of where you are exposed as a good-for-nothing fake and incompetent.

You're not going to do better than 60dB CMR with 0.1% resistors on the DA, and that is on a good day. Compare that to the standard 120dB of the readily available monolithic IAs- with no expensive externals added.

Yeah- I see that, retard- what a totally incompetent reject you are. The only redeeming feature of that type of output is the elimination of a large DC block output capacitor in the case of inordinately low HPF cut-in frequency, and that is not the case here-but the price you pay is the requirement for expensive precision matched external components with inferior performance by modern standards. Looks like it is beyond your sorry-excuse for intelligence to realize that working with the single-ended signal is a much relaxed requirement, and you seem to have no concept of the relationship of AC/DC and differential/common mode signal components.

The archaic circuit cut-and-pasted from a book by Brasfield idiot to follow:

Nope- you are a poser- that much is clear- you *understand* absolutely nothing- as I said before, you are a muddle-headed jackass who can't put two and two together. Keep up with your cut and pasting of circuits and snippets of terminology- when called to task you always *skulk* off like the little rat you are- you are a total fake, pseudo-intellectual, and incompetent. Everyone on SED is well aware of your posing, you fool no one.