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Similar to IEEE ECPhysiological origin: Sequential electrical activation of cardiac cells –Electrical excitation propagates
IEEE ECPhysiological origin: Sequential electrical activation of cardiac cells –Electrical excitation propagates
- 1. Electrocardiogram Amplifier Design UsingBasic Electronic Parts Background Lecture
- 2. Outline of Discussion •Project scope: What are you going to do? • Background: What is ECG? – Clinical relevance and importance – Technical challenges in measuring this signal • Project description: How will you do the project? – Overview of project stages – Technical principles related to each stage • Conclusion: What are the learning outcomes?
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- 4. Overview of Project •Topic: Biomedical circuits – Interdisciplinary in nature – Involve technical concepts in three areas: 1) Electric circuits 2) Biomedical instrumentation 3) Human physiology • Project aim: Develop an ECG amplifier circuit from scratch – Learn about technical details behind bio-potential measurement devices – Help build your interest in BME or EE!
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- 6. Background: Where isyour heart? A B C
- 7. Background: Heart Diseases •Statistics from World Health Organization (2005) – Heart disease kills one person in every 5 seconds – 7.6 millions of death worldwide each year • Common types of heart problems – Heart rhythm disorder: Irregular beats – Coronary heart disease: Cannot supply adequate circulation to cardiac muscle cells – Tachycardia: Heart beats very fast even whilst at rest ** Electrocardiogram (ECG) Monitoring ** One common way to help diagnose for heart diseases
- 8. Basic Principles ofECG • Physiological origin: Sequential electrical activation of cardiac cells – Electrical excitation propagates from top to bottom of heart – Starting point: sinoatrial node (at top of heart) – End point: ventricular muscles • Responsible for triggering cardiac contraction – Activation does not rely on brain signals Heart can operate on its own Source: www.healthsystem.virginia.edu
- 9. ECG: A Time-VaryingSignal • Heart can be viewed as a time-varying voltage source – Net voltage amplitude = Sum of cardiac cell potentials – Voltage vary periodically based on cardiac cycle P QRS T SA Node Atrial Muscles AV Node Ventricular Muscles Local Cell Potential Relative Time
- 10. ECG Waveform Characteristics •Waveform usually contains three distinct segments P QRS T P wave (atrial excitation) QRS complex (ventricular excitation + atrial recovery) T wave (ventricular recovery)
- 11. Clinical Importance ofECG • Can provide critical insights on potential abnormalities in the subject’s heart functioning – Commonly used as a first line of monitoring for cardiac problems • Example #1: Heart rhythm disorder – Technically known as arrhythmia – Give rise to aperiodic ECG waveforms
- 12. Clinical Importance ofECG • 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
- 13. How to measureECG? Back in the old days…
- 14. ECG Measurements: BasicPrinciples • General approach: Place electrodes at multiple places on the body surface – Measure potential difference across a lead (i.e. a pair of electrodes) – Exploit the fact that body tissue is a conductive medium that can relay cardiac potentials • Commonly used electrodes: Metal disk surrounded by an adhesive foam pad – Can self-attach to subject during operation Lead + –
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- 16. Challenge in ECGMeasurements • Raw ECG signals often low in amplitude and distorted by noise sources – Magnitude range: 0.1 to 5 mV – Examples of interference sources: 1) muscle contractions, 2) power-line radiations • Problem with having poor signal quality: Hard to obtain physiological insights – Low signal level Difficult to detect – High noise level May mask out useful clinical info
- 17. General Solution: AmplificationCircuit • Aim: To boost the raw ECG signal level – Preferably without boosting the noise at the same time • Approach: Amplify only the potential difference across two contact points – Theoretically allows only AC signals to be amplified ** This is what you will do in this project!! ** Amplifier Amplified Lead Output Contact Node #1 Contact Node #2 V+ V–
- 18. What you willdo in this project… • Objective: Prototype an electronic circuit to amplify the potential difference across a lead – Circuit built on a breadboard – Use only basic circuit components like op-amp chips, resistors, and capacitors – Testing conducted using an ECG signal simulator (MCI-430, MediCal Instruments) • Time required: 12-15 hours • Work in teams of at most three people
- 19. Project Structure • Stage#1: Instrumentation amplifier design – Develop an amplifier with a gain large enough to boost raw ECG signals – Account for common-mode noise • Stage #2: Power source reduction – Fine-tune ECG amplifier circuit by using only one 9V battery to drive it – Involve creating a virtual circuit ground • Stage #3: Multi-lead ECG measurements – Use the completed amplifier to estimate direction of ECG propagation in the simulator – Involve measuring ECG from 12 different leads
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- 21. Stage #1: TechnicalDescription • Aim: Design a circuit that only amplifies the differential voltage – Common-mode voltage level remains unchanged • Method: Build an instrumentation amplifier – Circuit structure involves two main stages Input Conditioner vo Difference Amplifier va vb + – Ensure input impedance approach infinity, and apply a gain Amplify the difference of the conditioned input signal
- 22. Main Design Considerations 1)Amplifier gain – How much amplification is needed given that raw ECG signal is between 0.1 to 5 mV? 2) Circuit noise level – How can we reduce power-line interference? 3) Power consumption – Can we save power and extend the battery life?
- 23. Technical Details: InstrumentationAmp. • Overall differential gain is given by: R4 R3 vo R3 R4 R2 va R2 R1 vb VS+ VS+ VS+ 1 2 2 1 R R G 3 4 R R G 3 4 1 2 2 1 R R R R G G GD Difference Amplifier Input Conditioner VS– VS– VS–
- 24. Problem: Power-Line Interference •Origin of common-mode noise: Radiations from power lines – Emitted radiation induces current Give rise to voltage when connected to circuit load – vcm can be as high as 50 mV!!! • Instrumentation amplifier can reduce common-mode noise, but not completely Power-Line Radiations Displacement Current Displacement Current
- 25. Power-Line Noise Reduction •Approach: Suppress common-mode voltage via shunting the displacement current to ground • Implemented by adding an extra contact node with subject – Usually the at the right leg (RL) Power-Line Radiations Displacement Current Instrumentation Amplifier Circuit Ground + –
- 26. Stage #2: PowerSource Reduction
- 27. Stage #2: TechnicalDescription • Aim: Convert amplifier to a single-supply-driven circuit without affecting its operations – i.e. Use only one battery to power the op-amp chips • Method: Create a virtual circuit ground via voltage divider – Involves creating an extra circuit block Circuit becomes more complicated v+ v– vo VS+ Dual-Supply Op-Amp Single-Supply Op-Amp v+ v– vo VS+ VS– Physical GND
- 28. Main Design Considerations 1)Impact of removing the negative battery – How would this affect the operation of the circuit? 2) Virtual ground voltage – What should the ground voltage value be to sustain normal operation of the amplifier circuit? 3) Suppression of virtual ground fluctuations – How can we ensure that the virtual ground voltage remains the same regardless of circuit load?
- 29. Technical Details: VirtualGround • Virtual ground voltage is given by: – Can adjust it by changing Ra and Rb!! • This voltage can be stabilized via two ways: b a b battery virtual R R R v v Vbattery Physical Ground Ra Vvirtual Rb VS+ Vvirtual Op-amp voltage buffer Shunt capacitors Virtual Ground VS+ C C
- 30. Stage #3: Multi-LeadECG Measurements
- 31. Stage #3: TechnicalDescription • Aim: Estimate the true direction of ECG potential propagation using your completed amplifier • Method: Acquire ECG signal from multiple leads – 12 leads commonly used in clinical diagnoses LL RA LA RL V1 toV6 Wilson’s Central Terminal SimulatedUsing MCI-430 Generator
- 32. Technical Details: LeadAngle • Detected voltage affected by electrode location due to spatial dependence of cardiac electric field – Strongest potential when lead parallel to ECG field – Zero potential when at 90 – General relationship: | Vdetected | = | Vactual | cos q • Makes sense to measure ECG from multiple angles in practice! High voltage detected + – No voltage detected + –
- 33. Clinical Practice: FrontalECG • Useful for examining cardiac electric field along front side of human body – Often regarded as the traditional form of ECG recording • Three basic electrode points placed at the limbs – Locations: 1) Right arm (RA), 2) Left arm (LA), 3) left leg (LL) – Forms three leads with pointing directions at ~60 against each other Forms a triangle known as Einhoven triangle LL RA LA Einhoven Triangle
- 34. Clinical Practice: FrontalECG • Three additional leads sometimes included in clinical ECG systems – Formed from connecting RA, LA, and LL electrodes with a central ref. node (Wilson’s central terminal) – Naming: 1) aVR (right arm); 2) aVL (left arm); 3) aVF (foot) • Total frontal ECG leads = 6 – 3 basic + 3 augmented – Helps to more accurately identify the instantaneous cardiac cycle phase during operation aVF aVR aVL Wilson’s Central Terminal – – + + +
- 35. Technical Details: CentralTerminal • General approach: Voltage summing circuit – Need the resistor R in each branch to avoid short circuit • Connect the RA, LA, and LL nodes to this summing circuit to get Wilson’s central terminal – This node is positioned at center of Einhoven triangle Wilson’s Central Terminal R R R LL RA LA
- 36. Clinical Practice: TransverseECG • Useful for examining cardiac electric field over a cross-section around the heart • Six electrode points placed below chest – Forms six leads with pointing directions at ~30 against each other • Clinical systems usually perform frontal and transverse ECG simultaneously – Involve 12 leads in total (6 frontal + 6 transverse) V1 V2 V3 V4 V5 V6 WithRespect to Wilson’s Central Terminal
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- 38. Conclusion: 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