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Electrocardiogram Amplifier Design Using Basic Electronic Parts Faculty Project Description I. Overall Scope This project is broadly centered around the topic of biomedical circuits. Its overall aim is to provide biomedical or electronic engineering students with a hands-on opportunity to develop an electrocardiogram (ECG) amplifier circuit from scratch and thereby learn more about the technical details of bio-potential measurement devices. By the end of this project, students are expected to achieve the following learning outcomes: 1. Explain to others the practical importance and technical details of amplifier circuits used for ECG potential measurements. 2. Develop a three-lead ECG amplifier on a breadboard using basic electronic parts such as op-amp chips, resistors, and capacitors. 3. Know how to reduce power-line interference in ECG measurements. 4. Discuss the dependence of detected ECG magnitude on the angle between a measurement lead and the actual direction of ECG potential. It is expected that the project can help develop students’ interest in biomedical or electronic engineering through examining how proper design of circuits can play an important role in measuring bio-potentials and assist in medical diagnoses accordingly. II. Background: The ECG Detection Problem The ECG is one of the vital signs of the human body. It originates from the sequential electrical activation of cardiac cells that are responsible for triggering heart contraction. In general, the ECG amplitude varies periodically based on the cardiac cycle, and its waveform shape typically comprises three distinct segments: 1) P-wave (atrial excitation), 2) QRS-complex (ventricular excitation and atrial recovery), and 3) T-wave (ventricular recovery). To measure ECG signals from the human body, one straightforward way is to place electrodes at two locations on the body surface (e.g. left arm, right arm) and measure the potential difference across these two electrodes. Nevertheless, the acquired ECG signals are often low in amplitude (at most a few milli-volts) and are distorted by the presence of electromagnetic interference due to radiations from nearby power lines. These pose concerns clinically because the ECG of a subject can provide critical insights on potential abnormalities in the subject’s heart functioning (e.g. for subjects with atrial fibrillation, their ECG pattern often lacks an organized P-wave due to the asynchronized excitation of atrial cardiac cells). If the ECG signal quality is poor, it would be difficult for clinicians to diagnose for cardiac problems on the subject. III. Project Overview Design Objective In this project, students will address the difficulties in measuring ECG signals by prototyping an electronic circuit that can amplify the potential difference across a measurement lead formed from two contact nodes. The circuit prototype will be developed using basic circuit components like op-amp chips and resistors, and the testing will be conducted using an ECG signal simulator. This project will be interdisciplinary in nature, and it involves technical concepts in three disciplinary areas: 1) electric circuits, 2) biomedical instrumentation, and 3) human physiology. Through working on this project, students will learn about the practical need to design ECG amplifiers so as to help clinicians make informed cardiac diagnoses and in turn benefit patients with heart problems. Resource Requirements To execute this project, a lab kit needs to be assembled for each of the student teams. The kit comprises an ECG signal simulator, a breadboard, connector wires, and some basic electronic parts. The following is a list of items needed for each lab kit and their estimated cost: Item ECG Signal Simulator (MCI -430) 9V Battery Battery Holder Breadboard Breadboard Wires Banana Jacks Alligator Clips Op-Amp Chips (TL074) Resistors (10, 100, 1k, 10k, 100k, 1M) Capacitors (50F, 500F) Number of Units 1 pc. 2 pcs. 2 pcs. 1 pc. 1 box set 5 pcs. 5 pcs. 2 pcs. 10 per value 3 per value TOTAL: Estimated Cost (USD) $249 $6 $1 $4 $4 $1 $1 $2 $1 $4 $273 Although the ECG signal simulator is the most expensive component of this project, it can be considered as capital cost because the simulator can be reused in future offerings of the project (same applies to some of the circuit prototyping supplies like the breadboard). In addition to these supplies, the project will require the use of an oscilloscope and a multimeter, both of which should be available in an undergraduate electronics lab. For this project, we will use the ECG waveform simulator to generate the signals needed for circuit testing. Human ECG recordings will not be conducted since the amplifiers developed by students are not certified medical devices. Delivery Structure This project will involve three main stages and an introductory lecture. It is expected that students can complete this project within 12-15 contact hours. Also, it is recommended that students should work in teams of at most three people to ensure that every student can have plenty hands-on opportunity to develop the ECG amplifier. IV. Project Details Introductory Lecture (2 hours) In this lecture, the instructor will present a high-level overview of the ECG amplifier design problem to the students. This will be delivered in the form of a background lecture, and it will cover two major engineering principles: 1) the basics of ECG detection (i.e. what is the origin of ECG potentials, how can we detect ECG signals from the human body), and 2) overview of ECG amplifiers (i.e. what are the building blocks and circuit principles involved in an ECG measurement device). It is expected that students will get a big picture of the ECG amplifier design project after this lecture. Depending on student interests, the instructor may wish to go over the circuit analysis of ECG amplifiers in more detail. Some supplementary slides have been provided for this purpose. Stage 1: Instrumentation Amplifier Design (5 hours) What students will do: After the instructor has explained the background of this project, the student teams will proceed to develop an amplifier circuit to boost the differential voltage detected across two circuit nodes. As illustrated in Fig. 1, they will be tasked to discover how to implement the circuit using a breadboard, op-amp chips, resistors, and 9V batteries provided to them in the lab kit. They will also determine how common-mode noise can be reduced via adding a third contact node. 2R R GD G G 1 2 4 R1 R3 G 1 R2 2 R2 R1 va R4 VS– R2 VS+ R4 R3 VS+ R3 R1 Input Conditioner G VS+ vo R3 R4 VS– Difference Amplifier vb VS– Fig. 1: Development of an ECG instrumentation amplifier on a breadboard (to do for Stage 1). Instructional remarks: The instructor can facilitate students in adopting a reflective approach to solve their amplifier design problem. In particular, he/she can first ask students to brainstorm a series of questions related to circuit implementation, such as “how many op-amp chips are needed”, “what resistor values should be used”, “what amplifier gain should be used”, etc. Students can then tie in these questions to the principles of instrumentation amplifiers presented in the introductory lecture. The instructor should also prompt students to consider the impact of common-mode noise on the ECG signal quality. In the absence of the third contact node, students will likely see significant common-mode voltage fluctuations rather than the actual ECG signal, so the instructor can take advantage of this opportunity to re-emphasize the concept of power-line interference to the students. Design trade-off to be observed: Through making measurements from the multimeter and the oscilloscope output, students will evaluate the impact of resistor values on the trade-off between amplifier gain, power consumption, and output noise level. They will also observe the trade-off between the number of contact nodes (two or three) and the power-line interference level (whether significant common-mode voltage is present). Stage 2: Power Source Reduction (3 hours) What students will do: In this project stage, the student teams will be required to fine-tune their amplifier design by reducing the number of power sources needed for the circuit. Specifically, they will be asked to use a single 9V battery (instead of using two) to drive the amplifier circuit, whilst still maintaining the amplifier’s functionality. This task will involve the creation of a virtual circuit ground using an op-amp-buffered voltage divider circuit, as shown in Fig. 2. ECG Amplifier Circuit Dual-Supply Op-Amp Single-Supply Op-Amp VS+ VS+ v– v+ vo VS– v– v+ Virtual Ground Circuit vo Physical Ground Single Power Supply Fig. 2: Fine-tuning of ECG amplifier into a single-supply-driven circuit (to do for Stage 2). Instructional remarks: Once again, the instructor can use the reflective learning paradigm to help students solve the power source reduction problem. He/she can first ask students to discuss with others on these questions: 1) what will happen if the negative reference voltage of an op-amp chip is changed from negative battery voltage to zero? 2) What should the ground voltage value be to sustain normal operation of the amplifier circuit? Afterward, the instructor can prompt students about the need for a virtual circuit ground when driving the amplifier circuit with a single power supply. When doing so, he/she should encourage students to explain the role of the op-amp voltage follower and the shunt capacitors in the virtual ground circuit. Design trade-off to be observed: The central message of this project stage is the trade-off between the number of power sources (single or dual) and the circuit complexity (whether or not a virtual circuit ground is needed). Stage 3: Multi-Lead ECG Measurements (3 hours) What students will do: In the final stage, students will use their amplifier circuit to find out the ECG potential propagation direction through measuring and analyzing the ECG signal from 12 different measurement leads, as illustrated in Fig. 3. This task will first involve acquiring ECG potentials from three frontal-plane leads formed from the limb nodes (RA, LA, LL) as defined by the Einhoven triangle. Subsequently, students will examine the ECG potentials from three augmented frontal-plane leads (aVR, aVL, aVF) that are formed via the three limb nodes and a central node called Wilson’s central terminal (created via summing the potentials at the three limb nodes). Also will be investigated in this stage are the ECG potentials from the six transverse-plane leads, which are created through pairing the six precordial nodes (V1, V2, V3, V4, V5, V6) with Wilson’s central terminal. Instructional remarks: The instructor can first steer students towards thinking about how the ECG signal can be reliably measured without prior knowledge of the actual ECG propagation direction in the human body. This helps to bring across the notion that measuring the true ECG magnitude is not a trivial task after all because the detected ECG magnitude depends on the angle between a measurement lead and the actual ECG propagation direction. Simulated Using MCI-430 Generator RA LA V1 to V6 Wilson’s Central Terminal RL LL Fig. 3: Measuring ECG from different simulated leads (to do for Stage 3). Design trade-off to be observed: Through working on this stage, students will observe the tradeoff between measurement complexity (whether single-lead or multi-lead measurements are involved) and measurement reliability (whether a strong ECG signal can be detected). It is expected to inspire students to reflect more deeply upon the complicated nature of the ECG instrumentation problem in practice.