How it works
The ECG (electrocardiogram) signal is 0.5–4 mV peak, bandwidth 0.05–150 Hz; EMG (electromyogram) is 50 μV–5 mV, 20 Hz–2 kHz; EEG (electroencephalogram) is 5–300 μV, 0.5–100 Hz. Ag/AgCl electrodes are preferred because their half-cell potential is stable at +0.222 V, minimising offset at the amplifier input. A bio-amplifier (cardiac) must have: input impedance > 10 MΩ, CMRR > 80 dB, gain ≈ 1000, noise < 1 μVrms referred to input, and leakage current < 10 μA (IEC 60601-1 safety limit). Patient isolation is achieved using an isolation amplifier (ISO124) or optical coupling, keeping the patient circuit floating from mains earth — a fault current of > 50 μA through the heart can induce ventricular fibrillation. Pacemakers deliver 1–10 V pulses of 0.5–2 ms duration at 60–100 ppm.
Key points to remember
The 50 Hz interference on ECG is a power-line common-mode signal rejected by the CMRR of the differential amplifier — improving electrode contact quality (reducing skin-electrode impedance below 10 kΩ) also reduces the converted interference at the amplifier input. ECG lead configuration: Lead I = VLA − VRA, Lead II = VLL − VRA, Lead III = VLL − VLA; these obey Einthoven's triangle: Lead I + Lead III = Lead II. IEC 60601-1 specifies that patient leakage current must not exceed 10 μA under normal conditions and 50 μA in single-fault conditions. The notch filter (50 Hz, Q ≈ 30) using a twin-T network removes power-line interference without distorting the ECG P and T waves significantly. Cardiac output monitors, pulse oximeters (SpO₂), and blood pressure transducers also fall under biomedical instrumentation and may appear in exam questions alongside ECG.
Exam tip
The examiner always asks you to list the specifications of a bio-amplifier for ECG measurement — always include input impedance (> 10 MΩ), CMRR (> 80 dB), gain, bandwidth (0.05–150 Hz), leakage current limit (< 10 μA), and the reason for patient isolation in your answer.