MEDICAL u ANALOG DEVICES Need clinical-grade PPG from your wearable?
Heart rate (HR) and blood oxygen saturation (SpO2) are quickly moving from the “desirable” to the “expected” phase in the list of features available in
health and fitness wearables, says Andrew Burt, executive business manager, Analog Devices
A
consequence of this transition, however, has been a reduction in the quality of readings caused by some sensor
manufacturers making questionable claims about the accuracy of their products in the rush to meet the market demand for these features. While the accuracy of readings may not be critical in everyday wearables, the quality and integrity of measurements must be unquestionable in clinical-grade wearables. A key challenge for designers is how to make
high quality HR and SpO2 measurements in a way that doesn’t place a heavy drain on the battery of the device. In this design solution, we show why the conventional approach to optical readings wastes power before presenting a sensor IC that uses a novel architecture to substantially reduce power consumption while making clinical-grade measurements.
Figure 3: Measuring SpO2 and HR in a clinical setting However, these devices operate in conditions
very different to those encountered in everyday life. First, the patient is relatively immobile, and the measurement is performed using a sensor that is securely fastened to a fingertip. Lighting conditions are relatively constant, which simplify light detection for the PD and power consumption is not a concern since these devices are usually mains powered. By contrast, a wearable device is typically wrist-
Figure 1: Measuring HR and SpO2 using a wrist-worn device
PHOTOPLETHYSMOGRAPHY (PPG)
HR and SpO2 are measured using an optical technique called photoplethysmography or PPG (Figure 1). A PPG signal is obtained by illuminating skin using a light-emitting diode (LED) and detecting changes in the intensity of light reflected from blood vessels below the surface (Figure 2) using a photodiode (PD) that generates a current proportional to the amount of received light.
worn, meaning the level of skin contact varies, depending on the personal preferences (strap tightness) and the motion of the wearer. Lighting conditions can vary considerably depending on location and time of day and since these devices are battery-powered, it is important for the current consumption of the sensor to be as low as possible.
PPG AFE WITH SINGLE ADC CHANNEL Increasing LED current, or using two LEDs, is an intuitive way to achieve a higher degree of skin illumination (Figure 4) since it can illuminate a greater skin area. However, this is a power-hungry approach because LED current accounts for at least 50% of the power consumed in a PPG system that, depending on the skin perfusion index of the wearer, can be up to an average of 1mW. Overall, this approach is inefficient and detrimental to battery life.
Figure 2: PPG measurement using an LED and a PD
18 May 2024 Irish Manufacturing
PPG AFE WITH DUAL ADC CHANNELS A better way to increase skin illumination is to use a single LED with two PDs that can be used to detect a greater amount of reflected light (Figure 5).
Figure 5: Using one LED with two PDs for better light detection
be attained. For example, applying the same LED current as in the previous arrangement delivers a 100% increase in PD current, thus delivering higher overall sensitivity, albeit at the cost of increased power consumption.
PPG AFE WITH QUAD ADC CHANNELS Using four PDs (which require a quad-channel ADC) to detect reflected light saves yet more power (Figure 6) as the LED can be run for a lot less power (Table 1). Table 1 summarises the relative power consumption of each of the architectures previously
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The current signal is conditioned by the PPG analogue front end (AFE) before being converted by an ADC for processing by an optical algorithm running on the system’s microcontroller. In principle, a single LED-PD pair is sufficient to make a PPG measurement and this architecture is common in equipment used in clinical settings
Figure 4: Using two LEDs to achieve better skin illumination
The advantage here is that the standard 20mA
LED current can be reduced to 10mA to achieve the same level of total PD current when compared to using a single PD. In challenging operating conditions (low skin perfusion and/or when the wearer is moving) where the system algorithm determines that a higher LED current is required, a proportional increase in system sensitivity can
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