Download Faculty Project Description - IEEE Real World Engineering Projects

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 (50F, 500F)
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.