SENSING CIRCUIT :
The electrical signals from the heart characteristically precede the normal mechanical functions and monitoring of these signals has great clinical significance. These signals provide valuable information about a wide range of cardiac disorders such as the presence of an inactive part ( infarction ). Or an enlargement ( cardiac hypertrophy )of the heart muscle. Although the electric field generated by the heart can be best characterized b vector quantity, it is generally convenient to directly measure only scalar quantities, i.e., a voltage difference of mV order between the given points of the body. The amplifier should faithfully reproduce the signals in this range and a good low frequency response is essential to ensure the stability of the base line. The interference of non biological origin can be handled by using modern differential amplifiers, which provide excellent rejection capabilities. The CMRR of the order of 100-120db is a desirable feature for sensing machines in addition to this in specially adverse circumstances, it becomes necessary to include a notch filter tuned to 60 Hz to reject hum due to power mains. When ever we put electrodes ( silver ) to measure the pulses of the patient there is formulation of air gap between the skin and electrodes. This air gap prohibits proper sensing of pulses. To avoid the air gap we apply gel on the patient as well as electrodes. Even though we apply gel there is a possibility of having air bubbles and this will lead to small gap between the skin and the electrodes. To balance this capacitance one has to connect capacitance at the input side. The typical value of these capacitance’s is 0.01 - 0.1microF. By connecting these capacitance’s we are structure next we connect resistance in series which is required to compensate silver electrode resistance, wire resistance. The value of resistance should be so chosen that it does not lower the input impedance of the circuit. Typical range is 50K-10K. This bridge formed due to resistance and capacitance somewhat cancels the making a brick negating effect of the air gaps formed. Next block is instrumentation amplifier. The instrumentation amplifier provides high input impedance, gain and CMRR. At this stage we would like to keep the gain of instrumentation amplifier moderate. This is just to prevent instrumentation amplifier to go to saturation. Hence we have total gain of instrumentation amplifier as 66.6 using three op-amp differential amplifiers. The reason for using instrumentation amplifier in place of normal op- amp is that it gives it selectable gain with high accuracy and good linearity. Differential input capabilities of instrumentation amplifier give high CMRR. It gives high selectivity of gain with low temperature coefficient. It also provides with low dc offset and drift errors referred to input. Inspite of this the output of instrumentation amplifier may have some dc offset. To remove this we put a small low pass filter of timing 1 sec so that dc will be blocked and the pulse will be passed on to the next stage. The next block is single inverting amplifier . the gain of the amplifier is 30 . the next is notch filter for 50Hz. This is to remove 50 Hz line frequency noise picked up by the body. The notch filter has a low figure of merit and hence is followed by voltage follower . it is preferred over emitter follower due to high input resistance and output amplitude exactly equal to the input. It provides buffering action
The total gain of the circuit is 66.6 * 30 *4 =8071.
TECHNICAL DESIGN APPROACH
Instrumentation Amplifier :
1) Instrumentation system is needed to measure the I/P signal produced by a transducer and often to control a physical signal producing it.
2) To amplify low level o/p signal of the transducer so that it can drive the indication or display.
1) Selectable gain & high gain accuracy & linearity
2) Differential i/p capability with high gain common mode rejection.
3) High stability of gain with low temp. coefficient.
4) Low dc offset & drift errors referred to I/P .
5 )Low it O/P impedance
6) high I/P impedance
· In practical applications instrumentation amplifier is a specific combination of a suitable de op-amp wired up with feedback.
· The high impedance in achieved by the non inverting configuration of 2 i/p op-amps. The Common mode rejection in achieved by following stage connected as differential amplifier.
· Differential Op-amp can also provide some gain ( nominal ) for the whole stage.
· The gain accuracy can be increased by using high precision MFR’s.
If R4 = R5= R6 =R1
e3 = ( 1+ R2/R1 )e1 - R2/R1 ( ecm + e2 )
e4 = ( 1+ R3/R1 ) e2 - R3/R1 ( ecm+ e1 )
& e5 = e4- e3
where : ecm +e1 i/p to A1
ecm +e2 = i/p to A2
if R2=R3 then Vo/p is given as :
e5 = ( 1+ 2(R2 /R1)) x ( e2 - e1 )
· CMRR can be adjusted to optimum by ensuring R5/R4 = R7/R6
· A1 & A2 act as i/p buffers with unity gain for common mode signals & with gain of ( 1+ 2(R2/R1)) for differential signals.
· Gain of instrumentation amplifier can be varied by changing R1 alone.
· In short instrumentation amplifier is required for precise low level signal amplification i.e. where low noise, low thermal drift, high i/p resistance and accurate closed loop gain is required.
· We have designed our instrumentation stage based on the prototype and its formulas since our ECG signed is also very much prone to noise and low in strength.
Notch Filter :
The notch filter is an example of a narrow band reject filter . This is used for attenuation of a single frequency. This twin T-network is most commonly used notch filter. Twin t-network is a passive network consisting of 2 T-shaped networks One T-network is made up of 2-ressistor & a capacitor while the other is made of two capacitor & a resistor. The frequency at which maximum attenuation occurs is called notch out frequency given as :
The major disadvantage of twin T-network is that it has a relatively low figure of merit ‘Q’. This affects the selectivity if filter .This can be increased by increasing the Q value .This is done by using a voltage follower whose o/p is fed back to the Junction of R/2 and 2C. The notch filter is commonly used in communication & biomedical instrumentation for eliminating undesired frequencies (here we use notch filter to remove the 50Hz hum of the ac line .)
The C value of notch should be chosen less than 1 microfarad & then R can be calculated from the above formula for the desired notch out frequency. Hence in place of voltage followers we can also use an emitter follower but advantage of voltage follower in the it has much higher i/p resistance and the o/p amplitude is exactly equal to the i/p in magnitude and in phase .
Design values for 50 Hz hum rejection :
If c= 0.1 microfarad & Fn = 50 Hz
Then R= 1/2Pifn C = 31.8 Kiloohms
Gain Amp b/w instrumentation ampr & notch filter :
Inverting gain amp A( gain = 30.3)
Gain formula = -Rf /R1
Cc in coupling capacitor used for the signal coming from the o/p instrumentation amplifier. Moreover Cc & R from a RC filter with RC time constant = 1Sec .Total gain provided by this section is around 30.3
Final gain amp at o/p of voltage follower :
This is non inverting amplifier designed as a final stage in amplification with a gain 3. The o/p (max) obtained is 1+Rf /R1.The gain for this stage is kept low since this is the last stage for the circuit . Hence we should try to avoid the discrepancies like non linearity for such high gain being provided by a single stage .
The total gain provided by each stage is :
1) instrumentation amp stage : 66.6
2) first gain amp ( inverting ) :30.3
3) last gain amp ( non inverting ) : 3
The total gain thus amounts = 66.6 X 30.3 X 3 = 6053.94