ECG analog front-end

The analog front-end (AFE) is the signal chain between the electrodes on your skin and the digital number a microcontroller finally records — the gauntlet a fragile 1 mV heartbeat must survive. A textbook ECG AFE runs: electrodes → instrumentation amplifier (reject mains hum, amplify ~×100) → high-pass filter (strip out the slow electrode DC offset and breathing baseline wander) → low-pass / anti-alias filter (chop off everything above the band of interest and above half the sampling rate) → analog-to-digital converter. Modern chips fold this entire chain, for eight or more leads at once, into one tiny integrated AFE.

Two design battles define it. The first is dynamic range: the electrode offset can be hundreds of millivolts of DC while the heartbeat is 1 mV of AC, so you cannot just slap on a gain of 1000 — you'd saturate the amplifier on the offset before the signal even arrived; the high-pass filter (cutoff ~0.05 Hz) exists precisely to remove that offset first. The second is the driven-right-leg circuit: instead of passively grounding the patient, the AFE measures the common-mode hum on the body, inverts it, and feeds it back through a third electrode — actively cancelling the interference and boosting effective CMRR by 20–40 dB. The result is a clean P-QRS-T waveform a cardiologist can read.