I based my design on the openEEG layout. The thing I changed is the instrumental amplifier, I went with the LT1167, a high quality IC, which I sampled for free. I also changed the way the virtual ground signal is generated. I used an IC I had lying around, a MAX931 a comparator chip which happens to have a 2% accurate reference voltage on board. So I just used the chips internal ref voltage, which I amplified to the needed value with an op-amp for the use as analog reference voltage, AnalogRef. I then divide AnalogRef by two with a simple resistor divider and buffer and low pass this signal again to get my final virtual ground (VGND).
|The Virtual Ground Circuit|
VGND is needed to measure positive and negative voltage swings with just a positive power supply. Here is the link.
One EEG channel connects two electrodes, the channel's signal is then the difference between these Inputs. I read a good article called "Instrumentation Amplifiers for bioelectric events: a design with a minimal number of part". It explains all design choices in great detail and I believe served as inspiration for the openEEG design.
My design has two channels thus 4 electrodes. This is the link to the image below.
|One EEG input channel which connects to two electrodes, IN_A and IN_B|
|The DRL circuit|
I will post pictures of the prototype as soon as i take them!
Here are the links to the EAGLE files:
The board file, which is routed in 2.54mm grid, to transfer it easily to breadboard.