Analog Circuit Design for an ECG Measurement System

Note : This is a project done by two Juniors in biomedical engineering students for circuit lab. This is for educational purpose only.

Bio-signal can be defined as any signal produced in living beings that can be measured such as heart rate, blood pressure, muscle tension, respiration, etc. and it is an integral part for medical diagnosis and treatments[1].An ECG is a test used to read bio-electric signals produced during heart rhythm and electrical activity. It is used to investigate symptoms of heart problems such as chest pain, palpitations, dizziness, shortness of breath and helps to detect arrhythmia, heart attacks, coronary heart disease, cardiomyopathy, etc [2]. It was first measured in 1887 by Augustus Waller at St. Mary’s Hospital, London and has been studied and researched by more scientists since then allowing us to measure the heart rate easily and accurately[3]. Amplifiers used in ECG must have high input impedance, low output impedance, limited bandwidth, low power consumption with high gain [4]. During cardiac function, the heart is continuously pumping de-oxygenated blood in and oxygenated blood out of the heart and this can be seen by the heart beat pattern via ECG reading. During ECG reading different numbers of electrodes are used and the most basic common ones are 12 lead ECG, 5 lead ECG, and 3 lead ECG. 12 lead electrodes is the most standard form as it provides most information compared to 3 or 5 lead electrodes. Although there are 12 lead electrodes, only 10 lead are used: 6 of those electrodes are attached to the chest region and 4 of them are attached to the right and left arm and right and left leg [5]. In 5 lead electrodes, electrodes are attached in the right arm, left arm, left leg, right leg, and chest while in 3 lead electrodes; 3 of those are attached in the torso [6].                                

For our project, we will be using three electrodes for our ECG reading which will be attached to the right and the left ankle and right wrist. The electrode carrying ground signal is attached to the right leg, positive input voltage(Vin) is attached to the left ankle and the negative input voltage (Vin-) is attached to the right wrist. We will be designing an ECG device using Arduino and and circuits that can measure the heart rate of a person using 3 stage instrumentation amplifiers with a gain of 1209, low pass filter with a cutoff frequency of 250 Hz, and notch filter with a cutoff frequency around ~60 Hz. Instrumentation amplifier will help to amplify the bio-electrical signals received and suppress any unwanted noise and common mode-signals[4], low pass filter will be used to filter out high frequencies that is above the cutoff frequency of 250Hz, and a notch filter will be used to remove ~60Hz noise that is ideally present in the power supplies

Materials Required:                                                                                                                     

• 3 uA741 op-amp
• Resistors: R1 = 1KΩ, R2 = R3 = 15 KΩ, R4 = R5 = 10KΩ, R6 = R7 = 390KΩ.

Procedures:                                                                                                                                 

To design our instrumentation amplifier we first calculated the values required for our resistors, and capacitors by using the equation. We found our gain to be 1209. After finding the required values and gain desired we then created the circuit schematic using 3 uA741 op-amps, resistors with value R1= 1KΩ, R2 = R3 = 15 KΩ, R4 = R5 = 10KΩ, R6 = R7 =  390KΩ in LTSpice as shown in figure above and then ran the stimulation. After completing LTSpice simulation, we designed the circuit in the lab using breadboard, uA741 op amps, resistors with desired values and connected the circuit using jumper wires, alligator clips and different cables. We then displayed the input and output signal on the oscilloscope at different input frequencies to measure the magnitude and phase of the output signal.    

Materials Required:                                                                                                                     

• 1 uA741 op-amp
• Resistors: R1 = 11.4KΩ, R2 = 889KΩ
• Capacitors: C1 = 1nF, C2 = .04uF

Procedures:  Before designing the schematic circuit on LTSpice we first calculated the values required for our resistors, and capacitors by using the equation shown above ......... We found the value of our resistors to be R1 = 11.4KΩ, R2 = 889KΩ respectively and values for capacitor was found to be C1 = 1nF, C2 = .04uF and the cutoff frequency of the low pass filter used here is 250Hz. We then created the circuit schematic using one uA741 op amp, two resistors R1 and R2 with values mentioned above and 2 capacitors C1 and C2 on LTSpice which is shown in figure 5 below and ran the stimulation which is shown below in figure 14 below. After completing LTSpice simulation, we designed the circuit in the lab using a breadboard op-amp, resistors and capacitors with desired values. We then displayed the input and output signal on the oscilloscope at different input frequencies to measure the magnitude and phase of the output signal.                                                                                                                                

Materials Required:                                                                                                                     

• 3 uA741 op-amp
• Resistors - R1 = R3 = 1.65KΩ, R2 = 424.4 KΩ
• C1=C2 = .1uF, C3 = .2uF

Procedures:                                                                                                                               We first calculated the values required for our resistors, and capacitors by using the equation shown above..... The cutoff frequency of the notch filter used here is ~62 Hz. We then created the circuit schematic using 3 uA741 op amps, resistors with R1 = R3 = 1.65KΩ, R2 = 424.4 KΩ, and capacitors with C1=C2 = .1uF, C3 = .2uF using LTSpice which is shown in figure 1.5 below and ran the stimulation which is shown below in figure 16  below. After completing LTSpice simulation, we designed the circuit in the lab using a breadboard, resistors, capacitors, op amps, jumper wires, etc. which is shown in figure 1.6 below. We then displayed the input and output signal on the oscilloscope at different input frequencies to measure the magnitude and phase of the output signal.   

Materials required

• Low pass filter circuit, instrumentation amplifier circuit, and notch pass filter circuit from above
• 9V batteries
• 3 Electrodes 

Procedures: After successful testing of all three circuits above individually on LTSpice, we then connected the individual circuits on LTSpice as shown in figure above and then ran the simulation. After completing the LTSpice simulation of integrated circuits, and successfully running individual circuits we then combined the individual circuits together as shown in figure. We used two 9V batteries, one to supply Vin(+) and the other one to supply Vin(-). We then connected 3 electrodes to the circuit; the electrode carrying Vin+ was attached to left ankle, the electrode carrying Vin- was connected to right wrist, and the ground electrode was connected to right ankle. The output of the integrated circuit was connected to channel 2 of the oscilloscope. After adjusting the circuit and the scope, we recorded the ECG reading successfully.

Materials required:

1. Integrated circuit from above 
2. Arduino
3. USB cable
4. Electrodes
5. Arduino IDE software

Procedures: After testing the integrated circuit, we then wrote an Arduino code that would measure heart rate and show the ECG graph. Firstly, we connected the Arduino with the integrated circuit using jumper wires and then connected the Arduino with our desktop using a USB cable. Once the circuit is correctly set up,select the Arduino board on Arduino IDE software and import the code into the Arduino. After that is done, run the code as well as the scope to see the Arduino generated graph on the software and integrated circuit generated ECG wave on oscilloscope. Note: We would expect to see similar graphs in the scope and the Arduino. 

[1] T. Penzel, K. Kesper, and H. F. Becker, “Biosignal monitoring and recording,” Health 

Informatics, pp. 288–301, 2006. 

[2] “Electrocardiogram (ECG),” NHS choices. [Online]. Available: https://www.nhs.uk/conditions/electrocardiogram/. [Accessed: 01-Dec-2021].

[3] B. SS; “Willem Einthoven and the birth of clinical electrocardiography a hundred years ago,” Cardiac electrophysiology review, 07-Jan-2003. [Online]. Available: https://pubmed.ncbi.nlm.nih.gov/12766530/. [Accessed: 01-Dec-2021].

[4] L. Xiu and Z. Li, “Low-power instrumentation amplifier IC design for ECG System Applications,” Procedia Engineering, 15-Feb-2012. [Online]. Available: https://reader.elsevier.com/reader/sd/pii/S1877705812001786?token=1D4BB805ABD2CC375444FBD2C3432A80F6E7B57668A174CFAC67655D812107EFF5A2DCB5B59F8D57BEE8D675D4613101&originRegion=us-east-1&originCreation=20211129050857. [Accessed: 06-Dec-2021].

[5] “12-lead ECG placement guide with illustrations,” Cables and Sensors. [Online]. Available: https://www.cablesandsensors.com/pages/12-lead-ecg-placement-guide-with-illustrations. [Accessed: 06-Dec-2021].

[6] “5-lead ECG placement and Cardiac Monitoring,” Ausmed, 20-Apr-2020. [Online]. Available: https://www.ausmed.com/cpd/articles/5-lead-ecg. [Accessed: 06-Dec-2021].