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Electrocardiogram (ECG) Detection Circuit Design 2020 -------------------------------------------------------------------------------------------------------------------------------

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1. Introduction An ECG or also known as Electrocardiography is a procedure of generating an Electrocardiogram. It is an analysis that measures the electrical movement of the heartbeat with every beat an electrical wave transits through the heart. Typically it occurs with an oscillation frequency of 1.3 hertz which is equivalent to 80 beats/minute. The waves generate the muscle to crunch and pump blood from the heart. For the last decade of the 19th century, people have verified the growth of a new era in which physicians used a technology as well with academic history and physical examination for the investigation of heart disease. With the introduction of the chest X-ray back in 1895 on a medical field as well as the Electrocardiograph back in 1902, it provided unbiased data (information) about the function and structure of the heart. The first technique of using Electrocardiograph is applying a string galvanometer to note the possible variation between the extremities resulting from the heart’s electrical activation. In these days, Electrocardiography has been known worldwide especially in the medical department as it is an essential part of the initial interpretation especially for patients with cardiac complaints. ECG is a very typically used technique to support the evaluation of many illnesses or epidemics. The complete ECG setup contains a minimum four electrodes which each of those being placed on the chest or at the four extremities correspond to standard nomenclature (RA , LA , RL , LL as right arm, left arm, right leg, left leg continuously). There are variations of the ECG setup where it is more flexible in order to grant the patient's needs. For example, the electrodes can be attached on the forearms and legs. Normally ECG electrodes are wet sensors, it requires the use of conductive gel to boost the conductivity between electrodes and skin, in which are cupped with collodion and gauze. ECG measurements are taken by attaching leads to the skin of a patient, because of the physical constraint of monitoring the heart through the surface of the skin, the detectable signal is incredibly small,while an electrical noise can be relatively large. Which real world ECG instruments need to use amplifiers and filters to condition the signal. Amplifiers convert energy from a power source to increase the amplitude of an input signal In the following report, we will discuss the design process of an ECG. Additionally we will also reproduce an ECG using an electronic circuit simulator, in this case LTspice, to produce an ECG reading. This will be followed by a brief discussion as to how each component values are calculated along with why a specific design has been chosen e.g. why an inverting filter was chosen instead of a non-inverting.

2. Design As mentioned in the introduction, an ECG circuit is used to monitor electrical activities of heartbeats but the signals generated by them are extremely small compared to the electrical noise. Firstly, an amplifier is needed to amplify the signals that are generated by heartbeat. After that, filters are needed to remove the electrical noise because it might interfere with the waveform created by the heart pulse. In the ECG circuit, an instrumentation amplifier will be used to amplify the waveform generated by heart pulse. Furthermore, 3 types of filters will also be featured which are high pass, low pass and notch (band stop) filters. A high pass allows signals to pass through when frequency is higher than a certain amount

2 and it is dependent on the design of the circuit. On the other hand, a low pass filter would only allow signals to pass through when the frequency is lower than a fixed amount. Lastly, a notch filter allows frequency with certain range which means it filters the signals that are not needed.

Figure 1 Purpose of different components

2.1 Instrumentation Amplifier An Instrumentation Amplifier gives engineers the ability to alter the gain of an amplifier circuit without the need to switch more than one resistor value. The difference between Instrumentation Amplifier with Differential Amplifier is that Differential Amplifier needs to arrange a multiple resistor value. The Circuit below is composed from a buffered differential amplifier with two new resistors linking the two buffer circuits together while the R gain (R1) connects the two new resistors (R2). All the resistors have the same value except for the R1. Figure 2 Schematic of Instrumentation Amplifier

The negative feedback of the upper left op amp generates the voltage at point Va to be equal to V1. Also, the Voltage at point Vb is equal to V2. This initiated a voltage drop across R1 to be equal to the Voltage difference between V1 and V2. That voltage drop generates a current pass R1, and because of the feedback loops of the two input op amps attract no current, the same amount of current must be through both the R2 at the above and below. This action resulting a voltage drop at point between R2 and R3 equivalent to:

V a−b=(V 2−V 1 )(1+

2R2 ) R1

The formal differential amplifier that is located on the right-hand side of the circuit will take the voltage drop at the point of Va and Vb and amplify it by a gain of 1. There are huge advantages of using an instrumentation amplifier, although it may look like a complicated approach to build a differential amplifier, it has a specific edge of acquiring extremely high input impedances on the V1 and V2 inputs and flexible advance that can be arranged by single resistor. In the use the Instrumentation amplifier, it can be demonstrated by its gain formula:

V out =(V 2−V 1 )(1+

2 R2 R4 ) )( R1 R3

3 Instrumentation amplifiers can be analysed from two parts which are input buffer and differential amplifiers. An input buffer amplifier is combined with two non-inverting amplifiers. The gain of a buffer amplifier:

1+

2 R2 R1

For differential amplifier, the gain is shown below:

V out=(V 2−V 1 )(

R3 ) R1

2.2 Cut-off frequency Cut-off frequency (break frequency) can be realized as the limit of a system. When it reaches its cut-off frequency, its energy starts to decrease. The equation of cut-off frequency is shown below:

fc=

1 2 πRC

2.3 High pass Filter

Figure 3 Schematic of High Pass Filter

Figure 3 shows the schematic of a high pass filter. It simply connects a capacitor and a resistor in series. Furthermore, the order is important. Capacitor is needed to install before the resistor since the impedance of the capacitor (Zc) will be used to control the cut-off frequency. The impedance of a capacitor is dependent on the frequency-in. This means that if the frequency-in is low, the impedance of the capacitor will be large. It can be demonstrated by the impedance formula of a capacitor:

1 jwc

w=2 πf

Low frequency might lead to a small value of w, since w and C are denominators and C is a fixed value, the value of Zc will be large since the value denominator is small. Thus, the capacitor would act like an open circuit when the frequency-in is low since the impedance is large. In conclusion, having a high pass filter in a circuit may filter out the nonessential frequency which is lower than the cut-off frequency.

2.4 Low pass filter

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Figure 4 Schematic of Low Pass Filter

Figure 4 shows the schematic of a low pass filter. It is the opposite of a high pass filter. Resistor is needed to install before the capacitor. Since the value of R is fixed, the impedance of the capacitor (Zc) will control the cut-off frequency. It can be demonstrated by the impedance formula of a capacitor:

1 jwc

w=2 πf

High frequency may lead to a small value of Zc. Therefore, the capacitor will act like a short circuit. This means that Vin would only go through the capacitor since there is a potential difference and Vout will be 0. To sum up, installing a low pass filter in the circuit filters out the frequencies that are higher than the cut-off frequency.

2.5 Notch filter

Figure 5 Schematic of Notch Filter

___ shows the schematic of a notch filter. It is used to filter out a specific frequency (in the case of an ECG, 50Hz). The circuit above employs both positive and negative feedback providing high degree performance. Figure ___ shows an example of how a notch filter works. This can also be demonstrated by a formula:

fc=

1 2 πRC

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Figure 6 Waveform of Notch Filter

3. Results and discussion 3.1. Instrumentation Amplifier Circuit

Figure 7 Instrumentation Amplifier circuit

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Figure 8 Bode plot for Instrumentation amplifier

Given the formula for

V o=

R2 2R3 )(V 2−V 1) , Vo ended up equating to 0.008V. ∗(1+ Rg R1

This is the instrumentation amplifier using differential amplifiers. The reason why an instrumentation amplifier is useful in ECG is due to their input buffer amplifiers within the differential amplifiers, which is why the instrumental amplifier circuit is optimal for the ECG. The 3 op-amp design is quite common in instrumentation amplifiers, where each op-amp acts as a buffer for the inputs, positive and negative, and a third op-amp is used to get the desired output. The Vcm is a voltage offset common to both the noninverting and inverting inputs. The difference amplifier measures the difference between the inputs and rejects any voltage that is common to the two. Since the instrumentation amplifier is implemented with buffer amplifiers, the V+-, it removes the need for an input/output impedance matching which makes the circuit a lot simpler. Furthermore, this allows to higher gain, higher impedance, better accuracy, and stability which reduces noise and allows for more correct readings. This is especially important when dealing with the heart, as the more accurate the reading is, the better doctors can monitor the ECG. Due to these circumstances, the instrumentation amplifier is usually used first within the ECG, and that’s the same with our design. This allows the small voltage input from the ECG probes to be greatly amplified to the desired output, then filtered through the high, low and notch filters to filter out the unnecessary noise and frequencies.

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3.2. High Pass Filter Circuit

Figure 9 Non-inverting high pass filter

Figure 10 Bode plot for High pass filter

An active high-pass filter was considered for this circuit as the main disadvantage of a passive filter is that the amplitude of the output signal is less than the input signal. With the multiple sages in a passive filter, the loss of attenuation can become quite problematic. This is solved by using an active filter. An active high pass filter uses an op-amp where the maximum frequency response is limited to the gain/bandwidth of the op-amp used. In a real-life scenario, the main advantages of an active filter are that there are no resonance issues, provides reliable operation, and is used for voltage regulation however it is also more expensive and more complex. Unlike the passive high pass filter, the maximum band frequency is limited by the bandwidth of the opamp used. In the frequency response of the filter, the maximum pass band frequency is limited by the open loop characteristics of the op-amp. This will cause it to appear like a band-pass filter. Common opamps such as the TL071 reduce at a roll off rate of -20dB/dec as the input frequency increases. The gain reduces until it reaches the unity gain of 0dB or transition frequency which is usually around 1Mhz. For standard clinical purposes, the frequency range reads from 0.5Hz to 100Hz or 0.5Hz to 50Hz for ambulance and intensive care patients, thus the desired cutoff frequency for the high pass filter is 0.5Hz.

8 1 where fc is equal to 0.5Hz and C is equal to 2 π∗R∗C 100μF, after rearranging and calculating, the value for R is 3183.0988Ω . Using the formula for cutoff frequency,

fc=

3.3. Low Pass Filter Circuit

Figure 12 Non-inverting low pass filter

Figure 11 Inverting low pass filter

Figure 13 Bode plot for low pass filter

This circuit consists of a passive RC filter, allowing low frequencies to pass through while blocking high frequency signals. The amplifier is configured as a buffer, giving it a gain, Av = +1. This design prevents excessive loading on the filter’s output while its low output impedance limits the filter’s cut-off frequency from being overwhelmed. The inverting low pass filter was chosen due to the input being flipped. The low pass filter inverts the input signal as well as filtering out frequencies above the threshold calculated. As mentioned before, the frequency range for standard clinics is 0.5Hz - 100Hz, therefore we would have a cutoff frequency of fc=100Hz. Using the same formula, fc= a value of R =

4973.59 Ω .

1 , we can rearrange it to find 2 π∗R∗C

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3.4. Notch Filter

Figure 14 Notch filter circuit

Figure 15 Bode plot for Notch filter

The calculations for the notch filter are fairly straight forward. The formula to calculate the frequency cutoff

is

the

same

as

the

high

pass

and

low

pass

filters,

fc=

1 2 π∗R∗C

,

R = R13=R12 and C = C4=C5. With this, the value for R can be calculated when fc = 50Hz and C = 47nF, resulting in R = 67725.5077 Ω . 68000 Ω resistors were chosen as it fits the 1% or better tolerance.

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3.5 Final circuit

Figure 16 ECG circuit

Figure 17 Bode plot of ECG circuit

The waveform depicts a filtered output at the notch. The first few milliseconds of impulse seem to be conducted without filtering before it proceeds to show a filtered waveform. According to Tomas B. Garcia, “this initial conduction through the normal electrical conduction system creates an initial deflection that is in the exact same direction as a normally conducted beat. In other words if the normal beat starts off with a negative deflection, the aberrant beat will start off with a negative deflection.” This is later met by the refractory period causing the waveform to display correctly.

4. Conclusion The final results depicted a waveform imitating the form of an ECG reading. However, there were some challenges and constraints throughout the construction process. The biggest challenge was emulating the small electrical charges on human skin due to heart activities along with the noises coming from the human body or surrounding electronic machines. These electrical charges and noise were depicted by voltage sources which resulted in a waveform looking similar to that of heart activity. In future works, this challenge could hopefully be overcome by using real components, creating a more authentic ECG reading. However, in the end, the filters and instrumentation amplifier worked as intended, creating a waveform displaying heart activity.

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5. References NTS.ECG Circuit-Project lab #3. (Jun. 29, 2018).Accessed: May. 21, 2020. [Online Video]. Available:https://www.youtube.com/watch? v=lwfvtS4v8NM&t=130s H-salaman, “Electrocardiogram (ECG) Circuit,” Instructables Circuits, Dec. 13, 2017. [Online]. Available: https://www.instructables.com/id/Electrocardiogram-ECG-Circuit/ “Electrocardiogram (ECG or EKG).” American HeartAssociation. https://www.heart.org/en/health-topics/heart-attack/diagnosing-a-heart-attack/electrocardiogram-ecg-or-ekg (accessed May. 20, 2020). A. Majd and L. Joseph, “A brief review: history to understand fundamentals of electrocardiography,” 2012. Accessed:May. 25, 2020. [Online]. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3714093/ B.Farnswoth. “What is ECG and How Does It Work?” IMOTIONS. https://imotions.com/blog/what-is-ecg/ (accessed May. 19, 2020). Basic Electronics Tutorials 2018, “Passive Low Pass Filter,” AspenCore, https://www.electronics-tutorials.ws/filter/filter_2.html? utm_referrer=https%3A%2F%2Fwww.google.com%2F (assessed 15 May 2020) . Basic Electronics Tutorials 2018, “Passive High pass filter,” AspenCore, https://www.electronics-tutorials.ws/filter/filter_3.html (assessed 15 May 2020). Basic Electronics Tutorials 2018, “Band Stop filter,” AspenCore, https://www.electronics-tutorials.ws/filter/band-stop-filter.html (assessed 15 May 2020). Basic Electronics Tutorials 2018, “The Differential Amplifier,” AspenCore, https://www.electronics-tutorials.ws/opamp/opamp_5.html (accessed 31 May 2020). M.K. Nathan, “Electrocardiography Circuit Design,” May, 4, 2013. [Online]. Available: https://pdfs.semanticscholar.org/d413/4baa3ea1f19eb21e9dc9b9dc33ad0ba89a34.pdf Stanford. (2017). ENGR 40M Project 4: Electrocardiogram. [PDF]. Available: https://web.stanford.edu/class/archive/engr/engr40m.1178/labs/ecg.pdf Jambek, A., Yaacob, S., Pin, O. 2014, “Circuit Architectures Reviews for Portable ECG Signal Analyzer,” Malaysia, https://www.researchgate.net/publication/265336871_Circuit_Architectures_Reviews_for_Portable_ECG_Signal_Analyzer (assessed 16 May 2020). Watford, C. 2014, “Understanding ECG filtering,” http://ems12lead.com/2014/03/10/understanding-ecg-filtering/#gref (assessed 19 May 2020). JoVE Science Education Database 2020, “Acquisition and Analysis of an ECG (electrocardiography),” JoVE,https://www.jove.com/science-education/10473/acquisition-and-analysis-of-an-ecg-electrocardiography-signal (assessed 18 May 2020). Michael van Biezen, Electrical Engineering: Ch5: Operational Amp(25 of 28) The Instrumentation Amplifier. (Jun. 23, 2016). Assessed:May. 18, 2020. [Online Video]. Available: https://www.youtube.com/watch?v=kGbMBa5scU0 Michael van Biezen, Electrical Engineering: Ch5: Operational Amp(26 of 28) The Instrumentation Amplifier (Variable Gain). (Jun. 23, 2016). Assessed: May. 18,2020. [Online Video]. Available: https://www.youtube.com/watch?v=kZ0F65AwlAY “Instrumentation Amplifier,” Wikipedia Foundation, 2020, https://en.wikipedia.org/wiki/Instrumentation_amplifier (assessed 15 May 2020). All About Circuits, The Instrumentation Amplifier, EETech Media,https://www.allaboutcircuits.com/textbook/semiconductors/chpt8/the-instrumentation-amplifier/ (assessed 21 May 2020). Hess, B. 2019, “What is an instrumentation amplifier?,” https://e2e.ti.com/blogs_/b/analogwire/archive/2019/08/09/what-is-aninstrumentation-amplifier (assessed 24 May 2020). Garcia, T. 2013, 12-Lead ECG: The Art of Interpretation, 2nd edn, Jones & Bartlett Learning, United States of America. Texas instruments 2018, “Three op amp instrumentation amplifier circuit,” http://www.ti.com/lit/an/sboa282/sboa282.pdf? &ts=1590143035030 (assessed 20 May 2020). "Instrumentation amplifier in ECG", Slideshare.net, 2020. [Online]. Available: https://www.slideshare.net/keerthanachithanathan/instrumentation-amplifier (Accessed: 31- May- 2020)....