What’s in store for optical biosensors? Part 2
Part 1 of this series provided an overview on using reflectometry for a pulse plethysmograph (PPG) waveform and described the physical and physiological principles at work.What does a PPG sensor capture?
Much attention has been focused on using PPG in a clinical setting; for example, a pulse oximeter on a finger clip. However, even in the most sterile clinical settings, an optical sensor would capture all environmental changes that affect its optical path(s) and confound the plethysmography information. Situations become even more challenging in less controlled wearable configurations.
Generally, environmental confounders, or noises, fall into two major categories: optical and physiological. Optical noise refers to changing characteristics of the optical path as seen by the sensor and unrelated to light absorption by the volume of blood observed. For example, an optical sensor can pick up ambient light. This can be particularly troublesome as indoor lighting commonly contains a flicker which can periodically affect the offset of the sensed optical signal and interfere with the PPG signal. Likewise, a physiological change could alter blood flow and volume in the tissue, which in turn changes the PPG signal.
These challenges exist in every setting and become more pronounced in less controlled environments found in wearable applications. However, PPG remains popular in wearables because it is a proven, reliable way to monitor key vital signs of the wearers.
To meet these challenges, advanced PPG ICs now feature intelligent signal paths to alleviate the impact of some of these artifacts. Along with improved algorithms, they have made it possible for designers to include PPG in many form factors, including earbuds, rings, necklaces, head and arm bands, bracelets, watches, and smartphones.
The following sections will discuss the optical noises in more detail.PPG circuits and noises
Figure 1 Noise sources in a PPG circuit
Before delving into optical noises, it is useful to understand the overall performance considerations of a PPG sensing system (Figure 1). The primary mission of a wearable PPG circuit is to maximize the signal-to-noise ratio (SNR) while conserving expended power.
The value PI, which stands for perfusion index, is the ratio of the pulsatile blood flow to the static (non-pulsatile) blood in the tissue. Mathematically, it is the AC portion of the PPG signal as a fraction of the overall signal.
LED drivers, which control the magnitude, transients, and rise and fall times of the LED currents, are key contributors to noise and power on the transmit path. On the receive path, PPG circuits handle anti-aliasing, sampling, and ambient light rejection. These circuits also maintain power efficiency and, increasingly, signal linearity over a wide sensing range.
An integrated PPG sensor front-end circuit, such as the MAX30112, simplifies the implementation considerations for PPG by combining these functions into a single, cost-effective IC. It drives the LED light sources and samples the resulting output of a photodetector. Depending on the selection of LEDs and photodetectors, the photocurrent involved ranges from sub-nA to tens of µAs.