EoG based Biopotential Instrumentation Amplifier

. To examine the biomedical signals from the human body is of a challenging task. Signal acquisition of the biosignal needs proper design methodology with the estimation of consequences further. In this paper we discuss about the signal acquisition of EoG using the surface electrodes and to amplify the weak biopotential signal an amplifier is proposed with the additive noise reduction filter. The biopotential instrumentation amplifier is designed using standard operational amplifier that posses the high voltage swing. However the major artifacts present in the EoG signal acquisition is because of power line interference can be eliminated by notch filter.


Introduction
Eye is probably the most important of the five senses to human beings.The eyes which make us to view and focus on objects those are around us.For moving it around, several muscles help for eye movements.It is necessary to know the functioning of eye, for that EoG is one of the observed techniques which give us information about eye movements.EOG-Electrooculography is one of the new technologies used to sense the eye.It is a technique which measures the potential difference between the cornea and the retina .The signal obtained by this technique is called electrooculogram.These signals have the frequency range of 0-50 Hz and amplitude range about 15-200 micro volts [1-2].By using EoG we can record the eye movements and the position of eye which is provided by the potential difference between the two electrodes that are placed on either side of eye.Simply if we consider the eyeball as a small battery, the cornea will be the positive pole and retina will be the negative pole.

Electrode configuration
For acquiring the EOG signal from the eye, we use five surface electrodes .To measure the signal for eye movement in horizontal direction-one set of electrodes are placed at user's eye and for vertical direction-another set of electrode are placed at above and below the user's eye, the fifth electrode is taken as reference electrode placed at forehead as in Fig. 1.Electrodes are devices which pick up biosignals i.e.; converts ionic potentials into electrical potentials.Based on the site of measurement, the type of electrode is to be decided [3,9,10].The amplifier is designed using three op-amps where op-amps 1 & 2 are connected as non-inverting amplifiers and op-amp 3 as normal difference amplifier.Op-amp is a basic building block is many analogue circuits which are used for mathematical operations and amplification functions.In signal acquisition standard opamp-741[6-7] is preferred because of high voltage gain that consists of eight pins-pin 2 and 3 are inverting and non inverting input terminals and pin 6 is for output, pin 4 is negative power supply pin and pin 7 is positive power supply.Henceforth three standards op-amp are used in construction of an instrumentation amplifier where the outputs of op-amps 1 and 2 are fed to op-amp3 as input, the amplified final output will be taken from the op-amp3.The second order low pass filter removes the higher frequencies that are above the frequency range of EoG signal [8] as in Fig. 5, the input and output signal voltage will be equal by making gain =1 considering the C1=100nF, C2= 100nF, R1= 22kΩ, R2=120kΩ as in Fg.5

Stage 3: Design of Notch Filter
Notch filter, which is known as band stop or reject filters that attenuates a specific frequency band and allows the signals below and above that frequency band, primarily used remove the power line interference i.e., 60Hz, in the signal as designed in Fig.

Results
Noise analysis of the proposed design is stimulated for the frequency range of 0-200 Hz that is suitable standard frequency of EoG specifications.The measure values of noise with respective to frequency is of minimum as 794.187nV/√Hz @1.0Hz and maximum as 133.0563 nV/√Hz @200Hz in Fig. 7.

Conclusion
The signal acquisition of EoG is very much useful in examining the different pathological activities of eye related issues.The proposed noise reduction biopotential instrumentation amplifier is widely used in the signal acquisition of both EoG as well as EEG applications.The notch filter which allows the composed frequency signal amplitude and reduce the power line interference that makes low power consumption of the design also help to retrieve the amplified output signal.
a). Surface Ag-Agcl electrode, (b) gold cup shaped electrodes for acquiring the EOG signal.

Fig. 4 .
Fig.4.Biopotential instrumentation amplifier Fig.4 demonstrates the biopotential instrumentation amplifier used for amplifying the EOG signal.It rejects the noises and interferences in the signals contains has high CMRR, and also open-loop gain is high.The amplifier is designed using three op-amps where op-amps 1 & 2 are connected as non-inverting amplifiers and op-amp 3 as normal difference amplifier.Op-amp is a basic building block is many analogue circuits which are used for mathematical operations and amplification functions.In signal acquisition standard opamp-741[6-7] is preferred because of high voltage gain that consists of eight pins-pin 2 and 3 are inverting and non inverting input terminals and pin 6 is for output, pin 4 is negative power supply pin and pin 7 is positive power supply.Henceforth three standards op-amp are used in construction of an instrumentation amplifier where the outputs of op-amps 1 and 2 are fed to op-amp3 as input, the amplified final output will be taken from the op-amp3.