The proposed system consists of four units: sensor unit,
signal conditioning unit, power supply unit and Microcontroller
unit.
A. Sensor Unit
For sensing ECG, EMG and EOG signal, silver chloride
electrodes are used. A commercially available disposable
electrode named Bio Protech T716 was used, which is shown
in figure 4 (b) [8].
B. Signal Conditioning Unit
Signal conditioning unit consist of instrumentation
amplifier, op-amp and filter. In the top of figure 3 signal
conditioning circuit is shown. In this system, a commercially
available low cost instrumentation amplifier AD620 is used,
which gives Common-mode rejection ratio (CMRR) greater
than 100dB at nearly 1 kHz [9]. Total gain of 840 is achieved
by two stage amplification. First stage amplification was done
by AD620 and the gain was 13.7. Second stage amplification is
done by CA3130 op-amp and the gain of second stage was 51.
CA3130 was used as a non-inverting amplifier. To avoid the
risk of ampli fier saturation two stage amplification is done.
Saturation of ampli fier generally occurs due to ampli fication of
electrode offset voltages [10]. To set the gain of AD620, a
resistor R
is used and its value was 3kΩ. The gain expression
for AD 620 is given in Eq. (1).