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 Instrumentation amplifier design Design fine-tuning via conversion to single-supply-driven circuit Multi-lead ECG measurements

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

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

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)

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

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

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

Key Tradeoffs Observed

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)

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

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

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