Bioelectric signal measurement

In the medical diagnosis, ECG, EEG and EMG play a major role. Some skin electrodes are located in the body of the patient for this purpose. Three electrodes are required here of which one serves as ground. An ECG signal is about 1 milli volt in its amplitude. Its frequency is ranging from 0.2 to 100 Hz. An EEG signal is in the range 1-10 micro volt and of a frequency range 3 to 30 Hz. An EMG signal is in the range 0.1 to 10 milli volt with a frequency range 5 to 500 Hz. While measuring these signals care has to be taken to block all the dc potentials due to membranes. The signal which is to be measured has to be properly amplified by some mechanism. For EEG signals, a 50 Hz notch filter is used to eliminate 50 Hz components. These filters are not preferred for ECG and EMG signals as they cause output signal distortion. For amplification, differential amplifiers having high common-mode rejection ratio are preferred. An amplifier with high input impedance and with the provision for connection of the ground electrode to the input-side ground terminal is used here. Suitable ground isolation eliminates possible shock hazards.