ECG/Heart Rate Monitor: AD8232

Working Principle

The AD8232 from Analog Devices is a single-lead heart rate monitor front-end that amplifies and filters the tiny electrical signals generated by the heart (<1 mV on the skin surface). It uses a three-electrode configuration (RA, LA, RL) with an instrumentation amplifier, a two-pole high-pass filter (0.5 Hz to reject DC offset and motion artifacts), and a two-pole low-pass filter (40 Hz to reject muscle noise). The output is a clean analog ECG waveform centred around V_ref/2.

Note: The AD8232 is for educational, fitness, and research purposes only — not for clinical diagnosis. Medical-grade ECG requires certified multi-lead systems (12-lead) with patient isolation.

Electrical Characteristics

Parameter	Value
Supply Voltage	2.0–3.5 V
Gain	~100 (instrumentation amp)
Bandwidth	0.5–40 Hz (cardiac band)
CMRR	80 dB (common mode rejection)
Output	Analog voltage (~1.5 V centre, 0–3.3 V swing)
Leads-Off Detection	AC or DC selectable (LO+/LO- outputs)
Electrodes	3-electrode (RA, LA, RL) snap connectors
Current Draw	~170 µA

Interfacing with an MCU

Connect the AD8232 breakout: 3.3 V, GND, OUTPUT → ADC pin, LO+ → digital pin, LO- → digital pin. When either LO+ or LO- goes HIGH, a lead has disconnected — discard readings until both are LOW. Attach three Ag/AgCl electrodes: RA (right arm/chest), LA (left arm/chest), RL (right leg/lower abdomen as reference).

Calibration

• Electrode placement: Clean and dry skin; place RA and LA on opposite sides of the heart, RL on the lower torso (ground reference)
• Gel electrodes: Use Ag/AgCl electrode pads with conductive gel for best signal quality; dry electrodes give noisy signals
• Baseline settling: Wait 5–10 seconds after electrode placement for the high-pass filter to settle
• BPM calculation: Detect R-peaks (highest spike in QRS complex) and calculate inter-beat interval (IBI) → BPM = 60000 / IBI

Code Example

/*
 * AD8232 ECG/Heart Rate Monitor — Arduino
 * Wiring: 3.3V, GND, OUTPUT → A0, LO+ → Pin 5, LO- → Pin 6
 * View in Arduino Serial Plotter for ECG waveform
 */
#define ECG_PIN A0
#define LO_POS  5   /* Leads-off detection + */
#define LO_NEG  6   /* Leads-off detection - */

void setup() {
    Serial.begin(9600);
    pinMode(LO_POS, INPUT);
    pinMode(LO_NEG, INPUT);
}

void loop() {
    if (digitalRead(LO_POS) == HIGH ||
        digitalRead(LO_NEG) == HIGH) {
        Serial.println("!");  /* Leads off — no valid signal */
    } else {
        int ecg = analogRead(ECG_PIN);
        Serial.println(ecg);  /* Plot in Serial Plotter */
    }
    delay(2);  /* ~500 Hz sample rate */
}

Real-World Applications

Limitations

• Not medical-grade: Single-lead ECG cannot replace clinical 12-lead systems for diagnosis.
• Motion artifacts: Movement causes baseline wander and noise; requires stationary recording for clean signals.
• Electrode quality: Signal quality depends heavily on electrode adhesion and skin preparation.
• No isolation: Requires battery operation or USB isolator for safety.