BIOMEDICAL ELECTRONICS (EC-604 (B)) - Unit 2 Short Notes

UNIT 2: BIOMEDICAL ELECTRONICS – COMPREHENSIVE SHORT NOTES

1. FUNDAMENTALS OF HUMAN PHYSIOLOGY AND BIOELECTRICITY

1.1 Cell Structure and Function

• Cell Membrane: Phospholipid bilayer with embedded proteins; selectively permeable; maintains resting membrane potential.

• Organelles: Nucleus (DNA), mitochondria (ATP), endoplasmic reticulum (protein/lipid synthesis).

• Resting Membrane Potential (RMP): Typically -70 mV (inside negative). Established by Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and leak channels.

1.2 Bioelectric Potentials

• Resting Potential (Nernst Equation):

$$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$$

At 37°C, simplifies to: $$\displaystyle E_{ion} = \frac{61.5}{z} \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right) $$ (mV).

[!TIP] For K⁺ (z=1), typical [K⁺]ₒ/[K⁺]ᵢ ≈ 1/20 → Eₖ ≈ -90 mV.

• Action Potential (AP):

  • Phases:

    0. Depolarization: Na⁺ channels open → rapid rise to +30 mV.

    1. Initial Repolarization: Na⁺ channels inactivate, K⁺ channels open.

    2. Plateau (cardiac only): Ca²⁺ channels open, balance K⁺ efflux.

    3. Repolarization: K⁺ efflux dominates → return to RMP.

    4. Hyperpolarization: K⁺ channels slow to close → membrane potential briefly more negative.

  • Propagation: Local current flow depolarizes adjacent membrane.

  • Refractory Periods:

    • Absolute: No stimulus can trigger AP (Na⁺ channels inactivated).

    • Relative: Strong stimulus can trigger AP (some Na⁺ channels recovered).

1.3 Major Body Systems (Functional Relevance)

• Cardiovascular: Heart generates ECG; blood pressure, flow, volume measurements.

• Respiratory: Lung volume/flow (spirometry), gas exchange (capnography, blood gases).

• Nervous: Brain generates EEG, EMG (muscle), nerve conduction studies.

2. BIOELECTRIC SIGNALS: CHARACTERISTICS AND CLINICAL SIGNIFICANCE

2.1 Electrocardiogram (ECG)

• Waveform Components:

  | Component | Origin | Duration (s) | |-----------|--------|--------------| | P wave | Atrial depolarization | 0.08–0.11 | | PR interval | AV nodal delay | 0.12–0.20 | | QRS complex | Ventricular depolarization | 0.06–0.10 | | ST segment | Ventricular plateau | 0.08–0.12 | | T wave | Ventricular repolarization | 0.16–0.24 | | QT interval | Total ventricular activity | <0.44 (rate-corrected) | | U wave | Possible Purkinje repolarization | 0.02–0.04 |

• Lead Configurations:

  • Limb Leads (I, II, III, aVR, aVL, aVF): Einthoven's triangle.

  • Precordial (V1–V6): Horizontal plane.

• Clinical Interpretation:

  • Arrhythmias: Tachycardia/bradycardia, atrial fibrillation (no P waves, irregular R-R).

  • Ischemia/Infarction: ST elevation/depression, Q waves, T wave inversion.

2.2 Electroencephalogram (EEG)

• Frequency Bands:

  | Band | Frequency (Hz) | State/Condition | |------|----------------|-----------------| | Delta | <4 | Deep sleep, coma, brain injury | | Theta | 4–8 | Drowsiness, meditation, some seizures | | Alpha | 8–13 | Relaxed, eyes closed (occipital) | | Beta | 13–30 | Alert, anxious, active thinking | | Gamma | >30 | Cognitive processing, perception |

• Electrode Placement: 10–20 system (based on skull measurements: nasion-inion, preauricular points).

  [!TIP] Fp = frontal pole, C = central, P = parietal, O = occipital, T = temporal, Z = midline.

2.3 Other Bioelectric Signals

• EMG (Electromyogram): Muscle electrical activity; needle/surface electrodes; motor unit action potentials.

• EOG (Electrooculogram): Corneal-retinal dipole; eye movement/blink detection.

• ERG (Electroretinogram): Retinal response to light; photopic (cones), scotopic (rods).

3. ELECTRODES AND TRANSDUCERS

3.1 Electrode Theory

• Half-Cell Potential: Electrochemical potential at metal-electrolyte interface; causes DC offset.

• Polarization: Build-up of reaction products at electrode surface; increases impedance over time.

• Impedance: $$\displaystyle Z = R + jX $$; low and stable impedance crucial for signal quality.

• Skin Preparation: Abrasion, cleaning, electrode gel to reduce impedance and motion artifact.

• Electrode Paste/Gel: Conductive (Ag/AgCl), high water content, low polarization, neutral pH.

3.2 Types of Electrodes & Selection

Selection Criteria: Invasiveness, frequency response, impedance, comfort, longevity.

3.3 Transducers

• Definition: Device converting one energy form to another (e.g., physiological → electrical).

• Classification:

  • Active (generate own signal): Piezoelectric, thermocouple.

  • Passive (require external power): Strain gauge, capacitive.

  • Primary (direct): Thermocouple.

  • Secondary (indirect): Strain gauge (measures strain → force).

• Piezoelectric: Certain crystals (PZT) generate charge under stress; used in ultrasound transducers, pressure sensors.

• Temperature:

  • Thermocouple: Two dissimilar metals → Seebeck voltage (mV/°C).

  • RTD: Resistance increases with temperature (Pt100: 100 Ω at 0°C).

  • Thermistor: Semiconductor; NTC (resistance ↓ with T) or PTC.

• Optical:

  • Photodiode/Phototransistor: Light → current; used in pulse oximetry (photoplethysmography).

• Resistive/Capacitive:

  • Strain gauge (resistive): ΔR/R ∝ strain.

  • Capacitive pressure sensor: ΔC ∝ 1/distance.

4. SIGNAL CONDITIONING FOR BIOMEDICAL SIGNALS

4.1 Biomedical Amplifiers

• Requirements:

  • High Input Impedance (>100 MΩ): Prevents loading high-impedance sources (e.g., electrodes).

  • Low Noise: <1 µV rms input-referred.

  • High CMRR (>80–100 dB): Rejects common-mode (e.g., 50/60 Hz power line).

  • Safety Isolation: Patient protection (IEC 60601).

• Instrumentation Amplifier (Three-Op-Amp Topology):

  • First stage: Two buffers → high input impedance.

  • Difference amplifier: Rejects common-mode, amplifies differential.

  • Gain set by single resistor: $$\displaystyle G = 1 + \frac{50\text{k}\Omega}{R_G} $$ (typical AD620).

• ECG Front-End: Typically 3-lead or 5-lead; bandwidth 0.05–150 Hz; high CMRR for limb leads.

4.2 Filters

• Low-Pass: Anti-aliasing (cutoff <½ sampling rate); e.g., 150 Hz for ECG.

• High-Pass: Remove baseline wander (e.g., 0.05 Hz for ECG).

• Band-Pass: Combine LPF and HPF; e.g., EEG: 0.5–70 Hz.

• Notch: 50/60 Hz power line interference; active twin-T or digital.

• Active vs Passive: Active (op-amp) provides gain and high input impedance; passive (RC) attenuates.

4.3 Isolation and Safety

• Optical Isolators: LED + phototransistor; electrical isolation via light.

• Transformer Isolation: AC-coupled; barrier for patient leakage.

• IEC 60601 Leakage Current Limits:

  | Type | Application | Max Leakage (µA) | |------|-------------|------------------| | B | Non-patient contact | 100 | | BF | Patient contact (except heart) | 100 | | CF | Direct cardiac connection | 10 |

5. CARDIOVASCULAR SYSTEM MEASUREMENTS

5.1 Blood Pressure Measurement

• Non-Invasive:

  • Auscultatory: Korotkoff sounds (Phase I = systolic, Phase V = diastolic); mercury/aneurid sphygmomanometer.

  • Oscillometric: Cuff pressure oscillations; algorithm detects mean pressure, then systolic/diastolic.

• Invasive: Catheter-tip transducer (strain gauge or capacitive); dynamic response must be critically damped to avoid overshoot.

5.2 Heart Rate and Rhythm Monitoring

• From ECG: QRS detection (amplitude/threshold); RR interval → heart rate.

• From Pulse (PPG): Photoplethysmography; peak detection.

• Heart Rate Variability (HRV): Analysis of RR interval variations; time-domain (SDNN, RMSSD) and frequency-domain (LF, HF) metrics; autonomic nervous system assessment.

5.3 Heart Sounds (Phonocardiography)

• S1: AV valve closure (lub); start of systole.

• S2: Semilunar valve closure (dub); start of diastole.

• S3: Ventricular filling (pathological in adults).

• S4: Atrial kick (pathological).

• Murmurs: Turbulent flow; systolic/diastolic timing indicates valve disorder.

• Electronic Stethoscopes: Amplify, filter (20–2000 Hz), may include digital processing for noise reduction.

5.4 Photoplethysmography (PPG) and Pulse Oximetry

• Principle: Light (660 nm red, 940 nm IR) transmitted/reflected through tissue; blood absorption varies with pulse volume.

• Pulse Oximeter:

  • SpO₂ Calculation: Beer-Lambert Law: $$\displaystyle I = I_0 e^{-\epsilon c d} $$; ratio of ratios:

$$\text{SpO}_2 = \frac{\text{AC}_{\text{red}}/\text{DC}_{\text{red}}}{\text{AC}_{\text{IR}}/\text{DC}_{\text{IR}}}$$

Calibrated against arterial blood gases.

• Limitations: Motion artifact, low perfusion, nail polish, dyshemoglobins (COHb, MetHb).

6. RESPIRATORY SYSTEM MEASUREMENTS

6.1 Spirometry

• Parameters:

  • Vital Capacity (VC): Max volume exhaled after max inhalation.

  • Forced Vital Capacity (FVC): VC exhaled forcefully.

  • Forced Expiratory Volume in 1 s (FEV₁): Volume exhaled in first second of FVC.

  • FEV₁/FVC Ratio: <70% indicates obstructive disease (e.g., asthma).

• Types:

  • Volume-Measuring: Water bellows or dry rolling seal; direct volume.

  • Flow-Measuring (Pneumotachograph): Measures flow (ΔP across resistor); volume = ∫ flow dt.

6.2 Photo Spirometer

• Working: LED/photodiode pair detects vane position in airflow; vane angle ∝ flow; integrated for volume.

• Advantages: No moving seals, compact.

6.3 Additional Measurements

• Capnography: End-tidal CO₂ (EtCO₂) via infrared absorption; monitors ventilation.

• Respiratory Rate: From flow/pressure waveform or impedance pneumography.

7. BLOOD GAS AND CHEMICAL ANALYSIS

7.1 Partial Pressure of Oxygen (PO₂)

• Clark Electrode (Polarographic):

  • Electrochemical Cell: Pt cathode, Ag/AgCl anode, KCl electrolyte, O₂-permeable membrane.

  • Reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻; current ∝ PO₂.

  • Calibration: Two-point (0% and 100% O₂); temperature compensation required.

  • Drift: Membrane degradation, electrolyte depletion.

7.2 pH, PCO₂, Electrolytes

• Ion-Selective Electrodes (ISE):

  • pH: Glass membrane (H⁺-selective); potential vs. reference follows Nernst.

  • PCO₂: Severinghaus electrode (pH-sensitive ISE behind CO₂-permeable membrane; CO₂ hydration changes pH).

  • Na⁺, K⁺, Ca²⁺: Liquid membrane or solid-state ISEs.

• Blood Gas Analyzers: Combine Clark, Severinghaus, ISEs; sample handling (heparinized syringe, temperature control).

8. MEDICAL IMAGING MODALITIES

8.1 Magnetic Resonance Imaging (MRI)

• Principles:

  • Nuclear Magnetic Resonance: Protons align with B₀ field; RF pulse at Larmor frequency $$\displaystyle f_0 = \frac{\gamma B_0}{2\pi} $$ (γ = 42.58 MHz/T for H).

  • Relaxation:

    • T1 (spin-lattice): Longitudinal recovery; fat = short T1 (bright on T1-weighted).

    • T2 (spin-spin): Transverse decay; fluid = long T2 (bright on T2-weighted).

• System Components:

  • Main Magnet: Superconducting (liquid He), 1.5–3 T typical.

  • Gradient Coils: Spatial encoding (slice selection, frequency/phase encoding).

  • RF Transmitter/Receiver: Excite spins, receive signal.

• Safety:

  • Ferromagnetic objects: Projectile hazard.

  • Specific Absorption Rate (SAR): RF energy deposition; limits to prevent heating.

  • Acoustic Noise: Gradient coil vibration; hearing protection required.

8.2 Endoscopy

• Types:

  • Rigid: Metal tubes; laparoscopy, arthroscopy.

  • Flexible:

    • Fiberscope: Image via fiber bundle; lower resolution.

    • Video endoscope: CCD/CMOS at tip; digital signal transmission.

• Image Transmission: Fiber optics (coherent bundle) or electrical (video).

• Applications: GI tract (esophagoscopy, colonoscopy), bronchoscopy, cystoscopy.

8.3 Thermography

• Infrared Imaging: Detects radiant heat (8–14 µm wavelength).

• Applications: Inflammation (arthritis), vascular disorders (deep vein thrombosis), breast cancer screening (as adjunct).

9. THERAPEUTIC EQUIPMENT

9.1 Defibrillators

• Types:

  • DC: Capacitor discharge (monophasic/biphasic); most common.

  • AC: Outdated (60 Hz sine wave).

  • AED: Automated external; voice prompts, automatic rhythm analysis.

• Synchronization: Synchronized shock (R wave detection) for cardioversion; unsynchronized for VF/VT.

• Waveforms: Biphasic (more effective, less myocardial damage) vs. monophasic.

• Energy Dosing: Biphasic: 120–200 J; Monophasic: 360 J. Pediatric: 2–4 J/kg.

9.2 Pacemakers

• Types:

  • Fixed-rate: Constant rate (VOO, AOO).

  • On-demand:

    • VVI: Ventricular pacing, ventricular sensing, inhibition.

    • DDD: Dual-chamber pacing and sensing; maintains AV synchrony.

    • Biventricular (CRT): For heart failure; LV and RV pacing.

• Components: Pulse generator (battery + circuitry), leads (unipolar/bipolar), programmer.

• Modes: NBG code ( chambers paced, sensed, response).

9.3 Heart-Lung Machine (Cardiopulmonary Bypass)

• Components:

  • Blood Pump: Roller or centrifugal; non-pulsatile or pulsatile flow.

  • Oxygenator: Bubble (direct gas contact) or membrane (semipermeable; gas exchange across membrane).

  • Heat Exchanger: Controls blood temperature (hypothermia).

  • Filters: Remove air, debris, clots.

• Operation: Venous blood drained → oxygenated → temperature controlled → returned to arterial system; heart arrested (cardioplegia).

9.4 Hemodialysis (Dialyzers)

• Principle: Diffusion across semipermeable membrane; blood and dialysate flow counter-current.

• Components:

  • Dialyzer ("artificial kidney"): Hollow fibers (polysulfone); high surface area.

  • Blood Pump: Peristaltic.

  • Dialysate System: Concentrate mixed with water; ultrafiltration control.

  • Heparin Pump: Anticoagulation.

• Clearance: Small solutes (urea, creatinine) diffuse; fluid removal via ultrafiltration (pressure gradient).

10. BIOTELEMETRY AND PATIENT MONITORING

10.1 Bio-Telemetry Systems

• Block Diagram: Sensor → Signal Conditioning → Transmitter (modulator + RF) → Antenna → Receiver → Demodulator → Display/Recorder.

• Types:

  • Wireless: RF (MHz–GHz), infrared; implantable (telemetry pill) vs. wearable.

  • Wired: For bed-bound patients (less common).

• Advantages: Mobility, continuous monitoring, reduced infection risk (wired).

• Disadvantages: Noise, interference, security (encryption needed), battery life.

10.2 Biometric Systems

• Identification Methods:

  • Fingerprint: minutiae matching.

  • Iris: Pattern recognition (high accuracy).

  • Face: Feature extraction (geometry, texture).

  • Voice: Spectral analysis.

• Applications: Patient ID, access control to records/areas, attendance.

10.3 Remote Patient Monitoring

• Telemedicine Architecture: Wearable sensors → gateway (smartphone) → cloud/EMR → clinician dashboard.

• Use Cases: Chronic disease (diabetes, hypertension), post-discharge, ICU (early warning scores).

11. EMBEDDED SYSTEMS IN BIOMEDICAL DEVICES

11.1 Embedded System Fundamentals

• Definition: Dedicated computer system within larger device; real-time constraints.

• Characteristics: Single-purpose, low power, high reliability, cost-sensitive.

• Quality Attributes: Safety, maintainability, usability, real-time performance.

• Design Metrics: Cost, performance (MIPS), power (mW), size, time-to-market.

• Classification:

  • By Performance: Microcontroller (low), DSP (medium), ARM (high).

  • By Complexity: Small (8-bit), medium (16-bit), complex (32-bit with OS).

11.2 Processors for Embedded Biomedical Systems

• RISC vs CISC:

  | Feature | RISC | CISC | |---------|------|------| | Instruction Set | Fixed-length, few | Variable, many | | Execution | Single-cycle (pipelined) | Multiple cycles | | Registers | Many (16–32) | Few (8–16) | | Examples | ARM, MIPS, PIC | x86, 8051 |

• Von Neumann vs Harvard:

  • Von Neumann: Single memory for code/data; simpler, slower (bus bottleneck).

  • Harvard: Separate memories/buses for code/data; faster, used in DSPs (e.g., TMS320).

11.3 Microcontroller 8051

• Architecture:

  • ALU: 8-bit; operations on ACC, B.

  • Registers: ACC (A), B (for multiply/divide), PSW (flags), DPTR (16-bit data pointer), SP.

  • Memory: 128 B internal RAM (00–7F: general, 80–FF: SFRs); 4 KB internal ROM (code).

  • I/O Ports: P0–P3; quasi-bidirectional (pull-up resistors when written 1).

• Timers/Counters:

  • Mode 0: 13-bit timer (THx: 8 bits, TLx: 5 bits).

  • Mode 1: 16-bit timer (THx, TLx full).

  • Mode 2: 8-bit auto-reload (TLx reloads THx on overflow).

  • Mode 3: T0 split into two 8-bit timers; T1 stopped.

  • Baud Rate (Serial Mode 1/3): $$\displaystyle BR = \frac{2^{SMOD}}{32} \times \frac{f_{osc}}{12 \times (256 - TH1)} $$.

• Serial Communication:

  • Mode 0: Synchronous, shift register; baud = fₒₛc/12.

  • Mode 1: 8-bit UART; baud from Timer 1.

  • Mode 2: 9-bit UART; baud = fₒₛc/64 or /32 (SMOD).

  • Mode 3: 9-bit UART; baud variable (like Mode 1).

• Interrupts: Sources: IE0 (external 0), TF0 (Timer 0), IE1, TF1, RI/TI (serial). Enable via IE register; priority via IP.

• Interfacing:

  • ADC0808: 8-bit, 8-channel; start conversion on high-to-low pulse; EOC goes high when done; connect P1 for data, P2 for address.

  • DAC0832: 8-bit; two inputs (DAC0, DAC1); current output → op-amp for voltage.

  • Stepper Motor: Unipolar (5/6 wire) or bipolar (4 wire); sequence via port.

  • LCD 16x2: HD44780 controller; 4-bit or 8-bit mode; RS, RW, E signals.

• Embedded C Example (Serial Mode 1):

  
  #include <reg51.h>
  
  void main() {
  
      TMOD = 0x20;       // Timer1, Mode2 (auto-reload)
  
      TH1 = 0xFD;        // 9600 baud @ 11.0592 MHz
  
      SCON = 0x50;       // Mode1, enable receive
  
      TR1 = 1;           // Start Timer1
  
      TI = 1;            // Ready to transmit
  
      while(1) {
  
          SBUF = 'H'; while(!TI); TI=0;
  
          SBUF = 'E'; while(!TI); TI=0;
  
          SBUF = 'L'; while(!TI); TI=0;
  
          SBUF = 'L'; while(!TI); TI=0;
  
          SBUF = 'O'; while(!TI); TI=0;
  
          SBUF = '\n'; while(!TI); TI=0;
  
      }
  
  }
  
  

11.4 Microcontroller 8096

• Functional Block Diagram:

  • CPU: 16-bit; 8-bit data bus, 16-bit address bus.

  • Memory: 4 KB ROM, 232 B RAM (internal); external up to 64 KB.

  • I/O: 4 8-bit ports (P0–P3); P0/P2 multiplexed with address/data.

  • Timers: Timer1 (16-bit, event counter), Timer2 (16-bit, watchdog).

  • ADC: 10-bit, 8-channel; successive approximation.

  • PWM: Pulse width modulation output.

  • Watchdog: Timer2; resets on overflow if not cleared.

  • Interrupt Controller: 5 sources (external, timer1, timer2, serial, ADC); priority levels.

• Addressing Modes (with examples):

  | Mode | Syntax | Example | Description | |------|--------|---------|-------------| | Immediate | LD R1, #05H | LD R1, #05H | Load immediate 05H into R1 | | Direct | LD R1, 0200H | LD R1, 0200H | Load from memory 0200H | | Indirect | LD R1, @R2 | LD R1, @R2 | Load from address in R2 | | Relative | JBC R1, label | JBC R1, LOOP | Jump if R1 bit clear | | Indexed | LD R1, 0200H[R2] | LD R1, 0200H[R2] | Load from 0200H + R2 |

• Instruction Set Classification:

  • Data Transfer: LD, ST, PUSH, POP.

  • Arithmetic: ADD, SUB, MPY (multiply → R1:R0), DIV (divide R1 by R0).

  • Logic: AND, OR, XOR, NOT.

  • Branch: JMP, JBC, DJNZ (decrement and jump if not zero).

• I/O Ports:

  • Structure: Port latch → buffer → pin. Configured as input (write 1 to latch) or output (write 0/1).

  • Modes: Input, output, bidirectional (P0/P2 for external memory).

• Control/Status Registers:

  • CS0, CS1: Chip select for external memory.

  • PWM Control: Duty cycle register.

  • ADC Control: Start conversion, channel select.

• Memory Mapping:

  • Internal: 0000–0FFFH (ROM), 1000–10FFH (RAM), 2000–20FFH (SFRs).

  • External: 8000–FFFFH (if CS0/CS1 configured).

11.5 ARM and DSP Processors

• ARM:

  • RISC Features: Load-store architecture (memory access only via LDR/STR), fixed 32-bit instructions, large register file (R0–R15, CPSR).

  • Thumb: 16-bit compressed instruction set for code density.

  • Use: Mobile/wearable medical devices (low power, high performance).

• DSP:

  • Harvard Architecture: Separate program/data memories/buses.

  • Hardware Multiplier: Single-cycle MAC (multiply-accumulate).

  • Pipelining: Deep pipelines for high throughput.

  • Circular Buffering: Auto-increment/decrement for FIR/IIR filters.

  • Applications: Real-time ECG/EEG filtering, FFT for spectral analysis.

11.6 Peripherals and Interfacing

• Watchdog Timer:

  • Purpose: Recover from software hangs (e.g., infinite loop).

  • Implementation: Timer must be cleared periodically; if overflow, resets system.

  • Timing Diagram: Clear signal before timeout; timeout → reset pulse.

• Interrupt Controller (8259A):

  • Priority Management: Fixed or rotating priority; cascade for >8 interrupts.

  • ISR Addressing: Vector provided by controller or CPU fetches from interrupt vector table.

• Keyboard Controller (8279):

  • Modes:

    • Scanned: Keyboard scanned by CPU; debounce in software.

    • Interrupt: Key press generates interrupt; FIFO stores code.

  • Display Scanning: Multiplexed 7-segment/LED; refresh rate >50 Hz.

• Display Interfacing:

  • 7-segment: Common anode/cathode; multiplexed for multiple digits.

  • LCD (HD44780): Commands (clear, cursor home, entry mode); data write; 4-bit mode saves I/O.

• RS-232:

  • Signals: TXD (transmit), RXD (receive), RTS (request to send), CTS (clear to send), DTR (data terminal ready), DSR (data set ready).

  • Handshaking: Hardware (RTS/CTS) or software (XON/XOFF).

  • Voltage Levels: ±3 to ±15 V; logic 1 = negative, 0 = positive.

11.7 Data Acquisition System Design

• Block Diagram:

  
  Sensor → Signal Conditioning (amp, filter) → Multiplexer → ADC → Microcontroller → (Storage/Display/Transmission)
  
  

• Hardware:

  • Op-amp: Low offset, low noise (e.g., INA128 for IA).

  • Filter: Anti-aliasing LPF (e.g., 150 Hz for ECG).

  • ADC: Resolution (12–16 bit), sampling rate (>2× max signal frequency).

  • Isolation: Opto-isolator or transformer for patient safety.

• Software:

  • Sampling Rate: Nyquist: $$\displaystyle f_s > 2 f_{max} $$.

  • Processing: Digital filtering (FIR/IIR), calibration (offset/gain correction).

  • Example 8051 ECG Monitor:

    • Hardware: AD620 (IA), RC filter (0.05–150 Hz), ADC0808, LCD.

    • Software: Sample at 200 Hz, moving average filter, display heart rate.

12. POWER ELECTRONICS FOR MEDICAL EQUIPMENT

12.1 Power Semiconductor Devices

• Power Diode:

  • Types: Standard (slow), fast recovery (reverse recovery <5 µs), Schottky (low Vf, low reverse voltage).

  • V-I: Forward: $$\displaystyle V_f \approx 0.7–1.2 $$ V; Reverse: blocking until breakdown.

• Thyristor (SCR):

  • Structure: PNPN four-layer; three terminals (anode, cathode, gate).

  • Static Char: Forward blocking (J2 reverse-biased), forward conducting (latching current $$\displaystyle I_L $$ required to stay on), reverse blocking.

  • Dynamic: Turn-on time (delay + rise), turn-off time (reverse recovery + gate recovery).

  • Latching Current ($$\displaystyle I_L $$): Minimum anode current to maintain conduction after gate pulse.

  • Holding Current ($$\displaystyle I_H $$): Minimum anode current to keep SCR on.

• TRIAC: Bidirectional SCR; 5-layer; triggering in any quadrant (I+, I-, III+).

• DIAC: Two-terminal, bidirectional; breaks over at $$\displaystyle V_{BO} $$; used to trigger TRIACs.

• Power MOSFET:

  • Structure: N-channel vertical; gate oxide insulation.

  • Transfer Char: $$\displaystyle I_D = k(V_{GS} - V_{th})^2 $$ (quadratic in saturation).

  • Switching Losses: $$\displaystyle E_{on/off} = \frac{1}{2} V_{DS} I_D t_{r/f} $$.

  • Applications: Low-voltage (<200 V), high-frequency (>100 kHz); DC-DC converters, battery chargers.

• IGBT:

  • Structure: MOSFET gate + BJT output; P⁺ substrate, N⁻ drift, P body, N⁺ source.

  • V-I Char: MOSFET-like turn-on, BJT-like saturation (low $$\displaystyle V_{CE(sat)} $$).

  • Applications: Medium-power (600 V–3 kV); motor drives (MRI gradients), inverters.

12.2 AC-DC Converters (Rectifiers)

• Single-Phase Half-Wave:

  • Output: $$\displaystyle V_{dc} = \frac{V_m}{\pi} $$ (uncontrolled); $$\displaystyle V_{dc} = \frac{V_m}{\pi}(1 + \cos\alpha) $$ (controlled).

  • Ripple frequency: $f$ (50 Hz).

• Single-Phase Full-Wave:

  • Center-Tap: $$\displaystyle V_{dc} = \frac{2V_m}{\pi} $$ (uncontrolled); $$\displaystyle V_{dc} = \frac{2V_m}{\pi}\cos\alpha $$ (controlled).

  • Bridge (more common): $$\displaystyle V_{dc} = \frac{2V_m}{\pi} $$ (diodes); $$\displaystyle V_{dc} = \frac{2V_m}{\pi}\cos\alpha $$ (SCRs).

• Three-Phase Full-Wave Bridge:

  • Diode: $$\displaystyle V_{dc} = \frac{3\sqrt{6}}{\pi} V_{LL} \approx 2.34 V_{LL} $$.

  • SCR: $$\displaystyle V_{dc} = \frac{3\sqrt{6}}{\pi} V_{LL} \cos\alpha $$.

• RMS Output for Half-Controlled (Example):

  Q: Single-phase full-wave half-controlled rectifier, $$\displaystyle V_m=300 $$ V, $$\displaystyle \alpha=30° $$. Find $$\displaystyle V_{rms} $$.

  A: Output consists of positive half-sines from 30° to 180° and zero from 180° to 360°.

$$V_{rms} = \sqrt{\frac{1}{2\pi} \int_{30°}^{180°} (V_m \sin\theta)^2 d\theta} = V_m \sqrt{\frac{1}{2\pi} \left( \pi - \frac{\pi}{6} - \frac{\sin 60°}{2} \right)}$$

$$= 300 \sqrt{\frac{1}{2\pi} \left( \frac{5\pi}{6} - \frac{\sqrt{3}}{4} \right)} \approx 300 \times 0.577 = 173.1\ \text{V}$$

\boxed{V_{rms} = 173.1\ \text{V}}

12.3 DC-AC Converters (Inverters)

• Single-Phase Bridge (180° Conduction):

  • Switches: T1/T4 on → +Vdc; T2/T3 on → -Vdc.

  • Resistive Load: Square wave output; $$\displaystyle V_{rms} = V_{dc} $$.

  • Inductive Load: Current continuous; output current sinusoidal (if filtered).

• Three-Phase Bridge (120° Conduction):

  • Each switch conducts 120°; output line voltage: 6-step waveform.

  • $$\displaystyle V_{L,rms} = \sqrt{\frac{2}{3}} V_{dc} $$ (ideal).

• PWM Inverter:

  • Principle: High-frequency carrier (e.g., 5 kHz) compared with sine reference; switches turned on/off to approximate sine.

  • Modulation Index $$\displaystyle m = V_{ref}/V_{car} $$; amplitude control.

  • Advantages: Voltage control without extra stage, harmonic reduction (carrier frequency harmonics).

• Harmonic Reduction:

  • PWM: Shift carrier phase (phase-shifted PWM) to cancel low-order harmonics.

  • Multi-level: More voltage levels (e.g., NPC); lower dv/dt, THD.

  • Selective Harmonic Elimination: Solve nonlinear equations to eliminate specific harmonics.

12.4 AC Voltage Controllers

• On-Off Control (Integral Cycle):

  • Whole cycles on/off; output $$\displaystyle V_{rms} = V_s \sqrt{k} $$ where $k$ = duty cycle (on cycles/total).

  • Low harmonic distortion, but discontinuous power.

• Phase Control:

  • Firing angle $\alpha$ controls conduction angle; $$\displaystyle V_{rms} = V_s \sqrt{\frac{1}{2\pi} \int_{\alpha}^{\pi} \sin^2\theta d\theta} $$.

  • For RL load, must consider $$\displaystyle \phi = \tan^{-1}(\omega L/R) $$; conduction starts at $\alpha + \phi$ if $$\displaystyle \alpha < \pi - \phi $$.

  • Expression (RL load, continuous conduction):

$$V_{rms} = V_s \sqrt{\frac{1}{\pi} \left[ (\pi - \alpha) + \frac{\sin 2\alpha}{2} \right]}$$

• Applications: Light dimmers, heater control.

12.5 DC-DC Converters (Choppers)

• Step-Down (Buck):

  • Switch (MOSFET) on: inductor charges (di/dt = (V_in - V_out)/L); diode reverse-biased.

  • Switch off: inductor discharges through diode to load (di/dt = -V_out/L).

  • V_out = D V_in (D = duty cycle = T_on/T); continuous if $$\displaystyle L > \frac{(1-D)R}{2f} $$.

• Step-Up (Boost):

  • Switch on: inductor charges (di/dt = V_in/L); diode blocks.

  • Switch off: inductor voltage adds to V_in → V_out = V_in/(1-D).

• Buck-Boost:

  • Inverting: $$\displaystyle V_{out} = -\frac{D}{1-D} V_{in} $$.

  • Non-inverting ( SEPIC ): $$\displaystyle V_{out} = \frac{D}{1-D} V_{in} $$.

• Applications: Battery-powered devices (infusion pumps, portable monitors); input from battery (3.7 V Li-ion), output 5 V/3.3 V.

12.6 Cycloconverters

• Principle: Direct AC-AC conversion; no DC link; output frequency $$\displaystyle f_o < f_i $$.

• Single-Phase to Single-Phase:

  • Midpoint: Two antiparallel thyristor pairs; each half-cycle one pair conducts.

  • Bridge: Four thyristors; more flexible.

  • Operation: Firing angles $$\displaystyle \alpha_1 $$ (positive) and $$\displaystyle \alpha_2 $$ (negative) control output voltage.

• Three-Phase: Used for large slow-speed motors (e.g., rolling mills, surgical tables).

• Advantages: Regenerative (can feed back power), no intermediate DC link.

• Limitations: Low output frequency (<½ input), complex control, poor power factor.

12.7 Switched-Mode Power Supplies (SMPS)

• Principle: Switch at high frequency (20–500 kHz); transformer/inductor small; feedback loop regulates output.

• Topologies:

  • Flyback (isolated):

    • Switch on: energy stored in transformer (primary current ramps up); secondary diode reverse-biased.

    • Switch off: energy transferred to secondary (diode conducts); output capacitor supplies load.

    • V_out = \frac{N_s}{N_p} \frac{D}{1-D} V_{in}.

  • Forward: Energy transferred while switch on; requires reset winding or tertiary winding.

  • Push-Pull: Two switches alternate; transformer flux bidirectional.

  • Half-Bridge: Two switches + capacitors; voltage across primary = V_in/2.

  • Full-Bridge: Four switches; full V_in applied.

• Advantages over Linear: Efficiency >80% (vs. ~50%), smaller size, wide input range (85–265 VAC).

• Applications: Universal input power supplies for all medical electronics (monitors, imaging systems).

12.8 Special Topics in Power Electronics

• Commutation:

  • Natural (Line): AC source voltage reverses; SCR turns off when current crosses zero.

  • Forced:

    • External Pulse: Auxiliary SCR discharges capacitor into main SCR cathode.

    • Resonant: LC circuit creates zero current/voltage.

    • Complementary: Two SCRs in parallel with opposite polarity; one turns on to commutate other.

• Series/Parallel Operation of Thyristors:

  • Series: Voltage sharing: use snubber (RC across each); derating factor (e.g., 2 in series → each rated for >½ total voltage).

  • Parallel: Current sharing: use small ballast resistors; match V-I characteristics.

  • String Efficiency:

$$\eta = \frac{\text{Total voltage/current rating}}{\text{Sum of individual ratings}} \times 100\%$$

> **Example**: 7.5 kV, 1 kA system; each SCR: 500 V, 75 A; derating 14% → effective rating: 430 V, 64.5 A.

> Series: $$\displaystyle N_s = \lceil 7500/430 \rceil = 18 $$; Parallel: $$\displaystyle N_p = \lceil 1000/64.5 \rceil = 16 $$; Total SCRs = 288; String efficiency = $$\displaystyle \frac{7500 \times 1000}{18 \times 500 \times 16 \times 75} \times 100\% \approx 55.6\% $$.

• Effect of Source Inductance on Rectifiers:

  • Overlap Angle $\mu$: During commutation, both incoming and outgoing SCRs conduct.

  • Output Voltage Reduction: $$\displaystyle V_d = V_0 \cos(\alpha + \mu) $$ for three-phase full converter.

  • $$\displaystyle \mu \propto \frac{\omega L_s}{V_d} $$ (source inductance $$\displaystyle L_s $$).

• Harmonic Control in Inverters:

  • Space Vector PWM: Better DC bus utilization, lower THD.

  • Selective Harmonic Elimination: Solve Fourier equations to set switching angles to eliminate specific harmonics (e.g., 5th, 7th).

13. SYSTEM INTEGRATION, SAFETY, AND STANDARDS

13.1 Medical Device Standards

• IEC 60601-1: General requirements for basic safety and essential performance.

  • Protection Against Electric Shock:

    • Type B: Body (non-patient) contact; ≤100 µA leakage.

    • Type BF: Patient contact (except heart); ≤100 µA.

    • Type CF: Direct cardiac contact; ≤10 µA.

  • EMC: Emissions (CISPR 11), Immunity (IEC 61000-4 series).

• IEC 62304: Medical device software life cycle processes.

13.2 Design for Reliability and Safety

• Fault Tolerance:

  • Watchdog Timers: Reset on software hang.

  • Error Detection: Parity, CRC, checksums.

  • Redundancy: Triple modular redundancy (voting) for critical systems.

• FMEA/FTA: Systematic analysis of failure modes and their effects; fault tree for top-event probability.

• Software Safety: Coding standards (MISRA C), validation/verification, change control.

13.3 Testing and Validation

• Electrical Safety: Ground resistance (<0.2 Ω), leakage current (per IEC 60601), dielectric strength (hi-pot test).

• Performance: Accuracy (calibration against reference), repeatability, linearity.

• Clinical Evaluation: Usability studies (human factors), clinical trials (efficacy/safety).

END OF UNIT 2 NOTES
Focus on past paper patterns: ECG/EEG, electrodes, 8051/8096 programming & architecture, power device characteristics, rectifier/inverter calculations, SMPS topologies, safety standards.