DIY Electrocardiography Circuit Using Op-Amps | ECG Amplifier Design

The AD8232 ECG module, manufactured by SparkFun, interfaces with Arduino by connecting its ground, 3.3V, output (to analog pin), LO- (to digital pin 10), and LO+ (to digital pin 11) pins, with ECG electrodes placed on the right arm (red), left arm (yellow), and right leg (green) following standard color conventions; the Arduino code initializes serial communication at 9600 baud, checks LO- and LO+ pins for high state, and reads ECG data from the analog pin to display in the serial plotter.

ADCs convert analog voltages to digital numbers, enabling microcontrollers to process real-world signals. Two main architectures exist: parallel comparator ADCs (fast, high-speed applications up to GHz, but poor scalability requiring 2^n comparators for n-bit resolution) and successive approximation ADCs (slower but more practical with single comparator and integrated DAC). Resolution defines step size (reference/2^n), while accuracy depends on component precision. These concepts form the foundation for understanding all subsequent ADC discussions.

This comprehensive lesson covers the complete EKG signal acquisition system. The system consists of three main components: (1) an EKG amplifier plate that receives bioelectric signals from electrodes attached to the extremities and amplifies them, (2) an Arduino microcontroller that digitizes the analog signal through its ADC (Analog-to-Digital Converter), and (3) a computer that receives the digital data via serial communication for visualization and further processing. A potentiometer functions as a variable voltage divider with three terminals, allowing any voltage between 0V and the supply voltage to be obtained. The Arduino Uno has analog input pins A0 through A5, and the Nano has A0 through A7. The analogRead() function reads ADC values (0-1023), while serial.begin() and serial.println() enable data transmission. To convert ADC values to voltage, use: Voltage = ADC_Value × (Reference_Voltage / 1023). The voltage resolution is Reference_Voltage / 1023 (approximately 4.89mV for 5V reference). The ADC can only convert voltages within its reference range (5V for Uno, 3.3V for Nano), which is why EKG signals (typically in millivolt range) must be amplified before ADC conversion to avoid signal distortion or loss of information.

The ADC supports data acquisition from defibrillators by placing paddles on discharge pads and performing discharge, automatically capturing waveforms and displaying total energy, peak voltage, and peak current. Results can be saved with an 'M' indicator. The ECG simulator allows frequency adjustment from 30-300 bpm and amplitude from 0.5-4 mV. Multiple simulation modes include sinusoidal (10-100 Hz, 1-8 mV), triangular (2-2.5 Hz, 1-8 mV), and square wave (0.5-2 Hz, 1-8 mV) for filter testing.

A portable ECG measurement system requires a multi-stage signal conditioning chain: an instrumentation amplifier with 38dB gain and high CMRR to amplify the weak 1mV ECG signal while rejecting common-mode noise and 60Hz interference, followed by a bandpass filter (0.1-150 Hz) to remove high-frequency noise and DC offset, then a programmable gain amplifier to adjust signal levels before digitization, and finally a 10-bit SAR ADC operating at 10Ksps with a bootstrap sampler and binary-weighted capacitor array to convert the amplified signal into digital form for microcontroller processing and display.

A notch filter is a digital filter designed to remove a specific frequency component from a signal; in this MATLAB tutorial, a 60 Hz notch filter is created using the filter design tool with a sampling frequency of 1000 Hz and notch frequencies set to 55, 59, 61, and 69 Hz, then applied to a 60 Hz signal through convolution to demonstrate effective attenuation of the target frequency.

A notch filter (also called a band-stop filter) is an electronic circuit that rejects a specific frequency while allowing all other frequencies to pass through, making it ideal for eliminating power line interference (typically 60 Hz) from electronic circuits; the basic design uses operational amplifiers with resistors and capacitors configured according to the formulas Rf = R/2, R = 1/(2π×C×fc), and Csf = 2×C, where fc is the cutoff frequency.

A notch filter is a signal processing filter that creates one or more deep notches (perfect nulls where magnitude becomes zero) in its frequency response, useful for eliminating specific unwanted frequencies such as power line interference (50 Hz or 60 Hz) and their harmonics in instrumentation and communication systems; the transfer function for a notch filter with a single notch at frequency Ω₀ is H(z) = B₀(1 - 2cos(Ω₀)z⁻¹ + z⁻²), which is derived by placing a pair of complex conjugate zeros on the unit circle at angles +Ω₀ and -Ω₀.

A practical notch filter design example: reject 50 Hz with Q=10. Normalized values: C1=C2=1F, R3=1MΩ, R1=1/(2×10)=0.05Ω, R2=2×10=20Ω, R4=2×10²=200Ω. Normalization uses impedance scaling factor of 10³ and frequency scaling factor of 100/π. The resulting capacitor value is approximately 3.18 μF. The design demonstrates how to translate normalized values to practical component values.

A notch filter (also called a band-elimination filter) is designed to remove a very narrow range of frequencies, specifically targeting 50Hz or 60Hz power line interference that commonly affects bio-sensor measurements. This interference occurs because the human body acts as an antenna, picking up electromagnetic noise from power lines. The filter uses specific resistor and capacitor values to create a notch at the target frequency, effectively eliminating this interference while allowing other frequencies to pass through.

IEC 60601-1 is a base standard with collateral and particular standards for specific device types. Section 8.6.4 covers ground bond test requirements (25A current, 100mΩ/200mΩ limits). Section 8.8.3 with Table 6 specifies dielectric strength requirements based on rated voltage and means of protection. Table 7 addresses products exceeding 200/240V. Test voltage depends on peak working voltage (DC equivalent of AC input) and protection classification. Section 8.7 covers leakage current testing with different measuring device placements for earth leakage, touch current, and patient auxiliary current. Advanced testing includes Mains on Applied Part (MOAP) testing applying line voltage to applied parts through isolation transformers with current limiting resistors, simulating worst-case energization scenarios. Signal input/output voltage testing checks leakage through communication ports. Earth leakage measures current from earthed parts to earth; touch current measures leakage from accessible metal parts to earth. For Type BF applied parts, maximum allowed leakage is 500μA under single fault conditions; for Type CF applied parts, maximum is 50μA.

Electrical isolation is essential for patient safety in medical circuits, achieved through optocouplers, transformers, or capacitive couplers; the ISO 124 isolation amplifier provides a single integrated circuit solution that separates power supplies into two independent ground systems, preventing dangerous current paths while maintaining signal integrity for ECG applications.

IEC 60601-1 requires medical electrical devices to have two independent means of protection against electrical hazards: operator protection and patient protection, with patient protection requiring higher safety standards; insulation can be achieved through solid insulation, creepage distance, and air clearance, with specific testing requirements including dielectric strength testing for voltages above 71V and minimum insulation thickness of 0.4mm; secondary circuits require at least one means of protection and can be powered by batteries; different applied part classifications (F-type and B-type) determine specific insulation requirements based on patient connection characteristics and working voltage levels.

IEC 60601 serves as the overarching international standard umbrella for medical equipment electrical safety, containing minimum performance and safety requirements, testing methods, and limits established by IEC committees. Specific category standards exist for patient monitors (separate standards for each vital sign), defibrillators, electrosurgery units, infusion devices, ventilators, and anesthesia systems. Electric shock effects depend on current magnitude, path through the body (direct heart path is most dangerous), and duration. High-voltage devices like ECG transesophageal probes (400-800V) pose fibrillation risks. Testing occurs at design validation, type testing, acceptance testing, routine maintenance, and after repairs breaching isolation barriers. Medical devices are classified by patient applied part type with corresponding leakage current limits: Type B (grounded applied parts), Type BF (body float - isolated but body-connected parts with tighter limits), and Type CF (cardiac float - most stringent for heart-contacting devices). IEC 60601-1 third edition renamed 'enclosure leakage' to 'touch current' to better describe current that could flow if patient or clinician touches the device while connected. For plastic-enclosed devices with no conductive test points, the aluminum foil method measures capacitively coupled current - the plastic enclosure acts as an insulator between conductor (clinician/patient) and internal circuitry, creating a capacitor that produces measurable current flow requiring measurement and minimization.

Isolation transformers make unsafe amplifier circuits (those without power transformers) relatively safe to work on. In transformerless amplifier designs, AC passes directly through all circuitry, creating potential leakage paths to the chassis. Without a power transformer, the entire circuit remains earth-ground referenced, making it extremely dangerous. Plugging such an amplifier into an isolation transformer removes earth-ground referencing from the circuit, preventing electrons from taking shortcuts through a technician's body to ground.