| Electroencephalography is the neurophysiologic measurement of the electrical activity of the brain by recording from the electrodes placed on the scalp, or in the special cases on the cortex. The resulting traces are known as an electroencephalogram (EEG) and it represent the brainwaves. The EEG helps to detect any brain damage, epilepsy or other problems of the brain. The signal amplitude is only few microvolts and needs to be amplified several thousand times before it can be captured. The work presented in this Masters thesis includes the design and development of a compact amplifier circuit for amplification of these very small amplitude EEG signals. In this process the signal is first amplified by a high quality instrumentation amplifier, which measures the voltage difference between two locations on the scalp.;The amplifier not only amplifies the EEG signals but also lowers the interface-impedance, making it less sensitive to noise. At this stage, the amplification is about 12 times that of the original signal. Thus the differential amplifier setup cancels most of the noise that interferes with these low amplitude signals. At the next stage a High Pass Filter, consisting of RC network is placed in order to remove the do offset. Afterwards the signal strength is increased further by standard non-inverting amplifier, where the gain can be adjusted from 6-100. Finally, there is a 3rd order low pass filter which also amplifies the signal to another 16 times, with consists of an operational amplifier that minimizes distortion caused by aliasing that may occur when the signal is converted to digital samples. Thus these digital signals can then transmitted using any wireless arrangement for ambulatory measurement of real time EEG without the patient actually being in the hospital. |
