Biomedical Instrumentation · Signals & Systems

The Four Filters of Biosignal Conditioning

Every filter is really a statement about frequency: which components of a signal survive and which are attenuated. Below, each filter's magnitude response |H(f)| is drawn on a shared log-frequency axis, laid over the bands where real physiological signals and their contaminants actually live. Drag the cutoffs and watch the passband move across the biology.

Where the signals live

Occupancy of the frequency axis — the physiological bands worth keeping, and the interference worth removing.

Model. First-order roll-offs for high-pass |H|=r/√(1+r²), r=f/f_c and low-pass |H|=1/√(1+(f/f_c)²); the band-pass is their cascade; the notch is a 2nd-order band-stop |H|=|f₀²−f²| / √((f₀²−f²)²+(f·f₀/Q)²). Physiological band edges are nominal teaching values (ECG monitoring 0.5–40 Hz, diagnostic to ~150 Hz; EMG surface 20–500 Hz; EEG 0.5–40 Hz), not device specs. −3 dB (0.707) marks the conventional passband edge.