Buffer circuit.
The buffer
circuit with an operational amplifier makes the input of the amplifier equal to
the output, it increases the impedance of the circuit and decrease current’s
value. it’s used to protect the patient from electrical currents. It was built
with an operational amplifier lf353 connecting the electrodes to the positive
input (right arm), the second positive input of the amplifier (left arm) and
finally the left leg’s electrode to another amplifier in the positive input.
Wilson network.
This
network is the reference to get the bipolar derivations. The voltage in each
side is the respective electrode biopotential. The Wilson network is composed
by nine 10kΩ resistors with a low tolrance.
This
network is the mix of the 3 electrodes of right arm, left arm and left leg like
Einthoven explained , getting the
differential of potential between two electrodes and registering DI DII and
DIII derivations:
·
DI:
differential of potential between right arm(-) and left arm(+).
·
DII:
differential of potential between right arm(-) and left leg(+).
·
DIII:
differential of potential between left arm(-) and left leg(+).
Amplification.
With an instrumentation
amplifier INA 114AP, each derivation signal was amplified with a gain value of
200 . the resistor´s value calculated from the equation 
is 250 ohms, but in a commercial way, the resistor
value used was 240ohms. INA 114 inputs came from wilson network:
In the first ina (DI) right arm is connected to the inversor input and the left
arm to the not inversor input. In the second INA (DII) the right arm is
connected in the inversor input and the left leg in the not inversor input.
Finally the third INA (DIII), the left arm is connected to the inversor input
and the left leg in the not inversor input.
is 250 ohms, but in a commercial way, the resistor
value used was 240ohms. INA 114 inputs came from wilson network:
In the first ina (DI) right arm is connected to the inversor input and the left
arm to the not inversor input. In the second INA (DII) the right arm is
connected in the inversor input and the left leg in the not inversor input.
Finally the third INA (DIII), the left arm is connected to the inversor input
and the left leg in the not inversor input.
Band-Pass Filter .
To clean
the EKG signal obtained from the body of a patient, the active filter is one of
the best options because is a low-cost filter in analog circuits. In this case,
the band pass filter is built in two parts: a high pass filter and a low pass
filter.
·
High
pass filter:
where
fc is the chosen frequency, R is the resistor´s value and C is the capacitor´s
value. To build the filter, the capacitor´s value is set with a commercial
value, in this case C = 1μf, replacing in the equation,
looking for the resistor´s value we obtain that
1.061MΩ.
·
Low
pass filter:
The low pass filter is built with a center
frequency of 30hz, stopping all the frequencies above this value, and allowing
all the frequencies under the value. The math operations to know the resistor´s
value is the same with a capacitor´s commercial value of C = 1μf,
5.30KΩ, but the resistor´s value found is not
a commercial value, for that reason, 5.6KΩ is the chosen value in the construction of the filter.
5.30KΩ, but the resistor´s value found is not
a commercial value, for that reason, 5.6KΩ is the chosen value in the construction of the filter.
Figure 2, Low pass filter schematic
Band-stop filter.
Also is
called Notch filter, is used to clean the ECG signal from electrical noise of
the light bulbs, currents of the voltage sources and the oscilloscope.
·
Band-stop
filter 60Hz
The center
frequency chosen is 60Hz. The equation to get the values Is the same, but in this case, the
filter is a second order filter, to get a exactly filter of frequencies. With a
capacitor´s value C= 100nf the
resistor´s value calculated is
26.525kΩ but in a
commercial way, the R´s value is 27kΩ.In this filter, is necessary calculate
the quality factor, if the quality factor is high, the signal is better, but if
the quality factor is low, the signal can lost energy .the quality factor is
expressed by the following equation :
where
, w0 is the frequency of the notch filter, w2 and w1 are the frequencies
of the high pas filter and low pas filter respective. The quality factor in
this case is 
obtaining a value of 2.
26.525kΩ but in a
commercial way, the R´s value is 27kΩ.In this filter, is necessary calculate
the quality factor, if the quality factor is high, the signal is better, but if
the quality factor is low, the signal can lost energy .the quality factor is
expressed by the following equation :where, w0 is the frequency of the notch filter, w2 and w1 are the frequencies
of the high pas filter and low pas filter respective. The quality factor in
this case is obtaining a value of 2.
·
Band-stop
filter 50 HZ:
The center frequency chosen is 50Hz. The equation to get the values Is
the same, but in this case, the filter is a second order filter, to get a exactly filter
of frequencies. With a capacitor´s value
C= 1μf the resistor´s value calculated
is
3.18kΩ but in a
commercial way, the R´s value is 3.3kΩ. In this filter, is necessary calculate
the quality factor, if the quality factor is high, the signal is better, but if
the quality factor is low, the signal can lost energy .the quality factor is
expressed by the following equation :
where
, w0 is the frequency of the notch filter, w2 and w1 are the frequencies
of the high pas filter and low pas filter respective. The quality factor in
this case is 
obtaining a value of 1.67.
is
3.18kΩ but in a
commercial way, the R´s value is 3.3kΩ. In this filter, is necessary calculate
the quality factor, if the quality factor is high, the signal is better, but if
the quality factor is low, the signal can lost energy .the quality factor is
expressed by the following equation : where, w0 is the frequency of the notch filter, w2 and w1 are the frequencies
of the high pas filter and low pas filter respective. The quality factor in
this case is obtaining a value of 1.67.
Figure 4,Band-stop filter 50Hz schematic
Alarm system.
The alarm system was built with an Arduino UNO board and three-color leds, yellow, green and red identifying each electrode connection, the left arm was the yellow led, the right arm was the red led and the green led was left leg connection.
In the first derivation exist a potential difference between right arm and left arm, to check if the right arm electrode was connected, the change of voltage in the first INA (derivation I), was used to create an interval of voltage, when the right arm was connected the voltage was lower than 1v (voltage<1v) and disconnected the voltage was higher than 1v (voltage>1v). If the voltage < 1v the red led was off but if the voltage > 1v the red led turned on. In the second derivation there is a potential difference between right arm and left leg and was used to identify the left leg connection with the same steps, when the left leg was connected the voltage in the INA output was higher than 1.5v but when the voltage was lower than 1.5 v the green led turned on. Finally to check the left arm connection, the third derivation show the potential difference between left arm and left leg, if the left arm electrode was disconnected the voltage was higher than 3v turning on the yellow led but if the voltage was lower than 3v, the led turned off.
Bipolar EKG schematic
Figure 6, EKG schematic
Results.
The EKG circuit was proved in the two students of the group, first in Christian Charry obtaining the following signals:
DI
Them, to make a comparisson, the ciruit was proved in the other student, Natalia Orozco, with her respectives results.
DI
Comparisson between the student´s signals
As you can see in the Christian signals, the t wave is bigger than in the natalia´s signals, it colud be a pathologie to analize in the future with a specialist and a real diagnostic but in these signals, the frecuency is different for the two students, where CHristian had a lower frecuency than natalia.
Results.
The EKG circuit was proved in the two students of the group, first in Christian Charry obtaining the following signals:
DI
DII
DIII
Them, to make a comparisson, the ciruit was proved in the other student, Natalia Orozco, with her respectives results.
Figure 7,DI bipolar derivation
DII
Figure 8, DII bipolar derivation
DIII
Figure 9, DIII bipolar derivation
Comparisson between the student´s signals
Figure 10,DIII natalia´s derivation
Figure11,DIII Chirsitan´s derivation
As you can see in the Christian signals, the t wave is bigger than in the natalia´s signals, it colud be a pathologie to analize in the future with a specialist and a real diagnostic but in these signals, the frecuency is different for the two students, where CHristian had a lower frecuency than natalia.
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