biopotential

A biopotential is a tiny voltage your own body generates as living cells do their work. Every heartbeat, every thought, every muscle twitch is, at bottom, an electrical event: ions (charged sodium, potassium, calcium atoms) rush across cell membranes, and that movement of charge shows up as a faint voltage on the skin. Stick two electrodes on your chest and you can read the heart's electricity (the ECG); on your scalp, the brain's (the EEG); over a muscle, its firing (the EMG). The body is, quite literally, a battery-powered signal generator that you can eavesdrop on without ever breaking the skin.

What makes biopotentials hard is how small and slow they are. An ECG is roughly 1 mV peak — a thousandth of a flashlight battery — and an EEG is just 10–100 µV, smaller than the electrical hum leaking from the mains wiring in your walls. They also live at low frequencies: the useful information sits below about 1 kHz (ECG mostly 0.05–150 Hz, EEG 0.5–40 Hz), down where 1/f flicker noise and slow electrode drift are worst. So the entire art of biomedical front-end design is pulling a whisper out of a noisy room: huge amplification, careful filtering, and clever rejection of the interference both common to and larger than the signal you actually want.