Notch Filter for PPG Interference Removal
Notch (band-stop) filters suppress narrowband interference at specific frequencies in PPG signals, primarily targeting 50 Hz (Europe/Asia) or 60 Hz (Americas) powerline noise and their harmonics that couple into the photodetector electronics through ambient light modulation or electrical interference.
A second-order IIR notch filter places conjugate zeros on the unit circle at the interference frequency and conjugate poles slightly inside, creating a deep null at the target frequency. The quality factor Q = f₀/BW controls the notch width: Q = 30–50 provides narrow rejection (1–2 Hz bandwidth) that minimally affects the PPG signal while providing 40–60 dB attenuation at the powerline frequency. Higher Q values reduce signal distortion but require more precise frequency targeting.
Adaptive notch filters automatically track the interference frequency, accommodating variations from nominal 50/60 Hz that occur in practice (49.5–50.5 Hz typical variation). The LMS-based adaptive notch filter uses a reference sinusoid at the estimated interference frequency and adapts its amplitude and phase to cancel the interference component. This approach requires no a priori knowledge of the interference frequency and tracks frequency variations in real-time.
For PPG sampled at 25–100 Hz, powerline interference at 50/60 Hz may be aliased to lower frequencies. At 25 Hz sampling, 50 Hz interference aliases to 25-50=25 Hz (Nyquist, folded), and at 100 Hz sampling, 50 Hz appears directly. Anti-aliasing analog filters in the PPG analog front-end should suppress powerline interference before digitization when feasible.
Frequently Asked Questions
Is powerline interference a significant problem for PPG?
In clinical settings with fluorescent lighting, powerline interference can be significant. In consumer wearables with LED-based sensors and ambient light rejection, powerline interference is typically minimal. It becomes problematic in research setups with external light exposure.
Can a simple bandpass filter replace a notch filter?
If the bandpass filter's upper cutoff is below 50 Hz (e.g., 5 Hz for HR), it inherently rejects powerline interference. Dedicated notch filters are only needed when the analysis bandwidth extends to 50/60 Hz, as in high-frequency morphological analysis.
How do notch filters affect nearby signal frequencies?
A Q=50 notch at 50 Hz attenuates frequencies within ±0.5 Hz. This has negligible effect on PPG cardiac content (0.5–8 Hz) but could affect very high harmonic content if the analysis bandwidth extends to the notch region.