MAKE YOUR OWN EEG DEVICE-BIOMEDICAL PROJECTS

Overview

An EEG signal is usually acquired through silver-chloride covered electrodes, though sometimes other materials like pure silver, tin, steel or gold are used. The signal amplitude is only a few microvolt and needs to be amplified several thousand times before it can be captured. Because it is faint, the signal can very easily drown in noise, particularily 50/60Hz hum from the mains which is transmitted capacitively (i.e by an electric field) from the wiring in your house.

To handle this, the signal is first amplified by a high quality instrumentation amplifier, which measures the voltage difference between two locations on the scalp. In the example in the previous section, we used C3 and P3. This ensures that a large percentage of the mains hum never enters the system, because the level of the mains hum on those two locations is essentially the same.

Afterwards the signal strength is increased further by normal amplifiers, and passed through a low-pass filter which minimizes distortion caused by so-called aliasing that may occur when the signal is converted to digital samples.

Below is the block diagram of one EEG amplifier channel, and the Right-leg driver (DRL-circuit).

Simplified block diagram of the ModularEEG amplifier

Some parts are not included here. The schematic gives you all the details if you are interested.

The EEG signal is picked up by the two topmost electrodes and passed through the protection circut. It serves two purposes: First, it protects the circuitry from electrostatic discharge (ESD) and second it protects the user from failing circuitry. In theory at least.

Leaving the protection circuit, the signal enters the instrumentation amplifier where it is amplified 12 times. After that, the signal is amplified about 40 times in a second amplifier stage. You can't see it in the diagram, but there is a reason for splitting the amplification into two steps like this. Between the two stages there is a high-pass filter which removes DC-voltage offsets.

Some electrode materials, such as gold or steel, are polarizable. This means that electric charge can accumulate on the surface of the electrode, building up a relatively large DC-voltage, sometimes several hundred millivolts if you are unlucky. In theory, you would amplify a 200mV signal 480 and get a 96 volt output. In reality, the circuitry can handle about 2.5V so the output signal would be stuck at at a maximally high or low level, usually +/- 2.5V and not contain any EEG. The highpass filter tries to solve this problem.

Finally, the signal is amplified 16 times more and lowpass filtered. The filtering is done to prevent aliasing effects later on, when the signal is digitized.

Below the signal amplifiers, and the filter, sits a third amplifier pointing the other way, seemingly sending a signal to the user. This is the right-leg driver. It is named like this for historical reasons. The driver is, and was, previously only used by ECG meters, which measures the electrical activity in the heart. During ECG sessions, the driver (also abbreviated DRL, for Driven Right Leg) is attached to the right leg, as far away from the heart as possible.

The purpose of the DRL is to reduce common-mode signals such as 50/60Hz mains hum, by cancelling them out. It replaces a ground electrode which older EEG designs use, and can attenuate mains hum up to 100 times more than the instrumentation amplifier can do by itself.

After the filtering, the signal is ready for acquisition by the analog-to-digital converter which in our case is located inside a microcontroller. The microcontroller sends the digitized EEG to a PC via a standard serial cable. To protect the user from electrical faults, the EEG device is electrically isolated from the PC and external power sources. The block diagram below shows this.DOWNLOAD MORE INFORMATION ABOUT THE PROJECT FROM THE LINKS BELOW

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