From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2001-12-21 03:16:47
Dear Group:
I have hand drawn the input stage from the EEG that I built some years ago.Please look at my webpage under Brain Wave Monitor. EEG01.gif
This circuit is mounted in a "shielded" die cast Bud box CU123 mounted to the belt. Box dimensions are 3.5 x 1 x 1.5 inches. Even with a massive solid box I still had trouble from "microphonics". If you tap on the box you can hear that in the biofeedback speakers! RCA plugs and shielded wires goto the head band that holds the shielded electrodes. The output is a standard 6 wire telephone wire and can be run for 50 to 100 feet from the preamp to the A to D converter.
If you look at the schematic you will see that this circuit is capable of more than 10 giga ohm input impedance, but ultimately I chose to use 10 meg ohms to ground. This is where the theory-simulation and practical collide.
You will also see that this is capable to drive the shields and it works better with the shields grounded. Again practical.
You will see a 10 K ohm protection resistor that is shorted out. Too much noise.
The common mode rejection is adjustable to near infinity. It turns out that in practice this adjustment helps very little. The problem is "not" common mode rejection. Common mode rejection will help only a little to get rid of 60 Hz. The unequal source impedance is part of the problem. Think ofit as unequal antennas picking up the 60 Hz.
Note my clever guarded driven protective diodes. Back biased diodes have way too much leakage unless you pay over $10 per diode. Even back to back diodes across the input are light sensitive and pick up radio stations.
Note that I chose to DC couple the preamp.
Note that I chose to run from +/- 12 volts and tried +/- 5 volts first. You need some common mode compliance to reduce overload saturation from real world events. This is not a problem in the clinical testing where the person is lying down and nobody in the room moves.
I am sharing some of this information to help the group. This is what I ended up with after months of tinkering. If this information helps, fine, ifnot ignore it.
I have the highest regard for Joerg's engineering skills and simulation tools, BUT the design has to be tested in the real world.
Please ask if you have any questions. I was sort of rambling.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:35 BST