1. Electrocardiogram Amplifier Design Using Basic Electronic Parts
Summary Lecture

2. 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?

3. Project Review

4. 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

5. Project Structure Three main stages involved in your project
Instrumentation amplifier design
Design fine-tuning via conversion to single-supply-driven circuit
Multi-lead ECG measurements

6. Learning Outcomes Explain biopotential amplifier circuits to others
Their practical importance and technical details
How they can be used for ECG potential measurements
Develop an ECG amplifier
Implemented on a breadboard
Use only basic parts like op-amp chips, resistors, & capacitors
Address the power-line interference problem
Why they appear as common-mode noise in ECG signals
How to reduce them
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

7. Recap of Methodology

8. 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

9. 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
displacement
current to ground through
3rd contact point (right leg)

10. 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 virtual ground voltage
Involve op-amp voltage follower
and shunt capacitors to
maintain virtual ground voltage
stability

11. 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
Three Augmented
Frontal-Plane Leads
Six Transverse-
Plane Leads

12. 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

13. Key Tradeoffs Observed

14. Tradeoff #1: Power Usage vs. Noise
Key control component: Resistor value in the instrumentation amplifier circuit
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)

15. 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

16. 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

17. 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

18. Concluding Remarks

19. 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

20. 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

21. 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