MEDICAL u ANALOG DEVICES
Figure 6: PPG using one LED and four PDs
considered, assuming a 1.6V typical supply voltage. This architecture delivers higher quality readings because blood vessels and bones are distributed asymmetrically in the wrist and four PDs help mitigate the effects of motion and how tight the wearer fastens the device. Four PD receivers also increase the probability of detecting light reflected from illuminated blood vessels. The graph in Figure 7 shows HR measured using four photodiodes (configured as two independent pairs: LEDC1 and LEDC2) with respect to a reference measurement (polar). The wearable needs to ensure that a good skin contact is maintained while this measurement is being performed. Initially, the wearer is at rest, then after 5 minutes (300 seconds) they begin exercising causing their HR to increase. It is clear that the signals on LEDC1 and LEDC2 deviate differently from the reference measurement and the benefit of using two pairs of PDs to capture and combine all of these deviations is apparent.
PRACTICAL QUAD ADC SOLUTION The MAX86177 (Figure 8) is an ultra low power quad-channel optical data acquisition system with both transmit and receive channels that is ideal for use in clinical-grade (as well as general purpose) portable and wearable devices. On the transmitter side, it has two high current 8-bit programmable LED drivers that support up to six LEDs. On the receiver side, it has four low noise charge integrating front ends that each include independent 20-bit ADCs that can multiplex input signals from eight PDs (configured as four independent pairs). It achieves a dynamic range of 118dB and provides ambient light cancellation (ALC) up to 90dB at
Figure 7: HR readings using two pairs of independant PDs
Figure 8: A MAX86177 quad-channel optical AFE block diagram
120Hz. It operates on a 1.8V main supply voltage with a 3.1V to 5.5V LED driver supply voltage. The device provides fully autonomous support for both I2C and SPI compatible interfaces. The MAX86177 is available in a 7 × 4, 28-ball wafer-level package (WLP) with dimensions of 2.83mm × 1.89mm and that operates over –40ºC to +85ºC temperature range. Lab-tested samples of this AFE exhibited an
overall root-mean-square error for hypoxia measurement of 3.12%, well inside the 3.5% limit set by the FDA for clinical-grade monitors.
CONCLUSION A major headache for designers of clinical- grade wearables is how to make optical PPG
measurements of HR and SpO2 without placing a significant drain on the battery life of the device. In this design solution, we have shown that a four-channel ADC architecture can deliver power savings of up to 60% when compared to the basic architecture that uses a single LED and PD. The MAX86177’s four-channel architecture in a small form factor package is ideal for use in finger, wrist and ear-worn wearables to measure
clinical-grade HR and SpO2. It can also be used to measure body hydration, muscle, and tissue oxygen
Table 1: Comparison of the Typical Power Consumption for 1-, 2-, and 4-Channel ADC Architectures 20 May 2024 Irish Manufacturing
saturation (SmO2 and StO2) and maximum oxygen consumption (VO2 Max).
www.irish-manufacturing.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
