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AC/DC Ratio in PPG: Pulsatile vs. Static Components
In a PPG signal, the AC component represents the pulsatile variation in optical absorption caused by the cardiac-driven blood volume pulse, while the DC component represents the relatively static absorption from tissues, venous blood, and non-pulsatile arterial blood. Their ratio is the basis for perfusion index and SpO2 calculation.
One signal. Two components.
Detected intensity contains a large baseline and a smaller pulse-related variation.
The DC component accounts for most of the detected PPG signal intensity and arises from steady-state absorption by skin, fat, bone, venous blood, and non-pulsatile arterial blood. Amplifying the AC component while suppressing DC is the fundamental pre-processing step in every PPG analysis pipeline.
AC/DC separation is typically achieved through high-pass filtering (cutoff 0.5–1 Hz) to isolate AC, with DC estimated as a slowly varying baseline using moving average filters, polynomial fitting, or morphological operators. The choice of DC estimation method significantly impacts the reliability of downstream metrics: aggressive high-pass filtering that removes respiratory modulation (0.1–0.3 Hz) will also remove the low-frequency HRV components needed for LF/HF analysis.
Multi-wavelength PPG systems use the AC/DC ratios at each wavelength to compute SpO2 through the ratio of ratios
This ratio is theoretically independent of tissue scattering and absorption properties that are common to both wavelengths, making it robust to inter-individual variability in skin thickness and pigmentation.
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Frequently Asked Questions
Why is the DC component so much larger than the AC component?
Most optical absorption in tissue is from static structures (bone, fat, venous blood). Only a small fraction of detected light is modulated by the pulsatile arterial volume change, making the AC component inherently small.
How does motion affect the AC and DC components?
Motion artifacts affect both AC and DC components. They introduce large, non-physiological oscillations in the AC range that overlap cardiac frequencies, and slow drift in the DC baseline from changing skin pressure and optical coupling.
Can you extract SpO2 from a single wavelength using AC/DC ratio?
No. The AC/DC ratio at a single wavelength gives perfusion index but not SpO2. At least two wavelengths are required to compute the ratio of ratios that distinguishes HbO2 from Hb.