Assembling the Arduino EEG Circult

(Click on the figures to enlarge):

Do this on a humid day. If it’s a crisp dry spring or fall day, use a ground strap.

The amplifiers require a neutral reference, or floating ground, halfway between the Arduino’s 5V positive and ground supplies. The second 3130 (farthest to the right in the photo, with the two 15K resistors) provides this. Wire it up, plug in the Arduino (I assume you will be powering it from the USB cable), and check with the voltmeter to be sure that the 3130 is putting out 2.5 volts at pin 6. This output will be “neutral” for the other two chips.

Next wire up the other 3130, which is the high-gain amplifier. But don’t put the 1 M trimmer in quite yet. You will need to bias the output to the middle of the input range for the Arduino’s analog input, which will be referenced to “internal” and has a range from 0 to 1.1 V. We'll aim for halfway between 0 and 1.1 volts. To do this, use the ohmmeter to set the 1 Meg trimmer to around 500 Kohm. Then put it in the circuit, connect up the power, and use the voltmeter to compare the pin 6 output of the 3130 with the Arduino ground. Adjust the trimmer so this reads 0.55 volts.

Finally, install and wire up the AD 620. The 2.2K resistor sets the gain of this chip to about 23, so the total amplification of the circuit is around 1500. As an initial check, temporarily short the two AD 620 input pins (2 and 3) to the neutral wire (pin 5). The output pin 6 should then read about 2.5 volts. Turn off the power and remove the shorting wires.

The two low-pass filter segments are helpful for EEG, but if you don't need it, leave off the capacitor marked with *.