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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