Transcript Electrocardiogram Amplifier Design Using Basic Electronic

Electrocardiogram Amplifier
Design Using Basic Electronic
Parts
Summary Lecture
Outline of Discussion
• Project review: What did you do in this project?
• Recap of methodology: How did you do this?
– Technical principles involved
• Key tradeoffs: Did you discover these concepts?
• Conclusion: How to take things forward?
Project Review
Review of Project
• Project aim: Develop an ECG amplifier circuit
from scratch using op-amps & resistors
– Powered by a 9V battery
– Output displayed on oscilloscope
– Input from an ECG signal simulator (MCI-430)
• Challenge: ECG only 0.1 to 5mV in amplitude
– Signals often distorted by power-line interference
– Poor signal quality  Hard to obtain clinical insights
Project Structure
• Three main stages involved in your project
1. Instrumentation amplifier design
2. Design fine-tuning via conversion to single-supplydriven circuit
3. Multi-lead ECG measurements
Oscilloscope
Output
ECG Signal
Simulator
Dual Power Source
Amplifier Circuit
Ground Node
Learning Outcomes
1) Explain biopotential amplifier circuits to others
–
–
Their practical importance and technical details
How they can be used for ECG potential measurements
2) Develop an ECG amplifier
–
–
Implemented on a breadboard
Use only basic parts like op-amp chips, resistors, &
capacitors
3) Address the power-line interference problem
–
–
Why they appear as common-mode noise in ECG signals
How to reduce them
4) Describe the issue of measurement lead angle
–
Why the detected ECG magnitude depends on the angle
between a lead and the actual ECG potential direction
Recap of Methodology
Principle #1: Instrumentation Amplifier
• Aim: Boost the detected voltage across two ECG
electrodes
– Preferably will only amplify the differential voltage, but
not the common-mode voltage
• Method: Instrumentation amplifier
– Main design considerations:
1) Amplifier gain
2) Power consumption
3) Circuit noise level
R2
VS+
va
R4
VS+
R3
R1
vo
R2
VS+
vb
 2 R  R 
Gain  1  2  4 
R1  R3 

R3
R4
Virtual
GND
Principle #2: Common-Mode Noise
Suppression
• Aim: Remove noise inherent in ECG signals due
to distortions from power lines
– Emitted EM radiation induces current inside body 
Give rise to voltage as high as 50 mV
• Method: Shunting Power-Line
Radiations
displacement
current to ground through
3rd contact point (right leg)
+
–
Instrumentation
Amplifier
Displacement
Current
Circuit
Ground
Principle #3: Virtual Ground
• Aim: Establish a non-zero ground voltage so that
amplifier can be powered using one battery only
– Common feature for biomedical instruments
• Method: Use voltage divider circuitry to create a
V
virtual ground voltage
V
S+
S+
– Involve op-amp voltage follower
Ra
and shunt capacitors to
maintain virtual ground voltage Vvirtual
stability
Rb
Physical
Ground
VS+
Ca
Virtual
Ground
Ca
Principle #4: Multi-Lead ECG
Measurements
• Aim: Improve the ECG acquisition quality by
measuring from various lead angles
– Strongest potential when lead parallel to ECG field,
while zero potential when at 90°
• Method: Perform measurements from 12 leads
used in clinical practice
Three Basic
Frontal-Plane Leads
RA
LA
Einhoven
Triangle
LL
Three Augmented
Frontal-Plane Leads
+
+
aVR
–
Wilson’s
Central
Terminal
Six TransversePlane Leads
aVL
–
V6
V5
V4
aVF
+
V3
V2
V1
With Respect to
Wilson’s Central
Terminal
Principle #5: Wilson’s Central Terminal
• Aim: Facilitate augmented ECG measurements
by forming a central reference node
– Used to form nine ECG leads: aVF, aVL, aVR, V1-V6
• Method: Connect the RA, LA, and LL nodes to a
summing circuit
– This node is positioned at center of Einhoven triangle
R
RA
LA
R
R
LL
Wilson’s
Central
Terminal
Key Tradeoffs Observed
Tradeoff #1: Power Usage vs. Noise
• Key control component: Resistor value in the
instrumentation amplifier circuit
R1 ()
R2 ()
R3 ()
R4 ()
Gain
100
100
100
100
100
100k
100k
1k
10k
10k
100
100k
100
100
1k
100
100k
10k
1k
10k
2001
2001
2100
2010
2010
Output Noise
Voltage (mV)
6.44
7.61
22.32
10.14
18.05
Load Current
(mA)
9.63
1.43
1.43
2.38
1.43
• General trend observed
– Higher power consumption can reduce output noise
 Involves using resistors with smaller values
– Impact more significant in the difference amplifier
stage (i.e. adjusting R3 and R4)
Tradeoff #2: Num. of Contact Nodes vs.
Common Mode Noise
• Without using a third contact node (right leg),
common-mode noise is very significant
– QRS peak can still be seen, but not for the other parts
of ECG waveform
– Crucial to shunt common-mode noise to circuit
ground through extra contact node
ECG Signal Without
Third Contact Node
ECG Signal With
Third Contact Node Connected
Tradeoff #3: Circuit Complexity vs.
Num. of Power Sources
• If using only one power supply, then circuit
complexity increases
– Virtual ground needed to create a non-zero ground
voltage level
– But using a single power supply is more convenient
than using a dual supply
ECG
Amplifier
Circuit
Single
Power
Supply
Virtual Ground
Circuit
Tradeoff #4: Measurement Complexity
vs. Reliability
• Using more leads can gain more insights on the
ECG signal characteristics
– Need to form augmented leads via Wilson’s central
terminal  Measurements become more complicated
Lead I (LA-RA)
Peak-to-peak: 288mV
Lead II (LL-RA)
Peak-to-peak 780mV
Lead III (LL-LA)
Peak-to-peak: 500mV
aVR Lead
Peak-to-peak: 369mV
aVL Lead
Peak-to-peak: 131mV
aVF Lead
Peak-to-peak: 394mV
Concluding Remarks
Is This Project Relevant?
• Yes! ECG signal measurements is one of the
most widely accessed vital signs of human body
– Commonly used as first line of screening for cardiac
malfunctions
• Example #1: Heart rhythm disorder
– Technically known as arrhythmia
– Give rise to aperiodic ECG waveforms
Is This Project Relevant?
• Example #2: Atrial fibrillation
– Missing P waves due to asynchronized excitation of
atrial cardiac cells
• Example #3: Premature ventricular contraction
– Sudden broad change in the QRS complex shape
ECG Amplifiers in Real World
• Widely used in the emergency room and
intensive care unit
– Daily recording on patients for diagnostic tracking
• Used for automatic detection of cardiac arrest
– Incorporated into automated external defibrillators
(AED)
• Exercise ECG: Detect for cardiac problems
especially during exercise
• Holter monitor: Long-term ECG recording