Signal processing
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.
How it works
Schematic summary of this guide. Waveforms are illustrative, not measured data.
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 controls the notch width: = 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 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.