Comparative Assessment of PPG-Derived HRV Using MAX30102 Sensor and Analog Circuitry with ADS1115 ADC

Figures

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  Figure 1. PPG acquisition front-end: 940 nm LED/photodiode illumination and biasing, passive AC coupling (2.2 μF) with 560 kΩ bleed to ground, an LM358 active stage with frequency-dependent feedback (100 kΩ ‖ 220 nF) and 4.7 kΩ reference, followed by a buffering stage and digital back-ends (ADS1115 and MAX30102) interfaced to an ESP32-S3 via I2C. Discrete LEDs and components were chosen over fully integrated optical modules to enable customized placement and intensity control, reduce cost, and leverage their widespread commercial availability.

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  Figure 2. Experimental configuration for the bilateral comparison of the two PPG acquisition systems.

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  Figure 3. PPG signal acquisition at rest using both index fingers (Experiment 1). The GUI displays three channels: IR (880 nm) and RED (660 nm) signals from the MAX30102 sensor, and an IR (940 nm) channel acquired via the ADS1115. The PPG signal from the MAX30102 appears inverted relative to the ADS1115 signal; the systolic peak corresponds to the highest point of the PPG waveform [21,22].

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  Figure 4. PPG signals from Experiment 1 (Subject 2) for maxir1, maxred1, and ads1. The red box highlights a segment affected by motion artifact or pressure disturbance, showing waveform distortion and peak alteration.

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  Figure 5. PPG signals from Experiment 2 (Subject 2) for maxir1, maxred1, and ads1. The red box highlights a segment affected by motion artifact or pressure disturbance, showing waveform distortion and peak alteration.

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  Figure 6. Bland–Altman plot for Experiment 1 comparing BPM estimates derived from the MAX (maxir1 and maxred1) and ADS1115 (ads1) sensors.

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  Figure 7. Bland–Altman plot for Experiment 2 comparing BPM estimates derived from the MAX (maxir2 and maxred2) and ADS1115 (ads2) sensors.

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  Figure 8. ICC values per subject for two experimental conditions, comparing measurements obtained from the MAX30102 (IR/RED) and the ADS1115 (IR) acquisition.

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  Figure 9. Comparison of the mean and standard deviation of the SNR for MAX30102 (IR and RED) and ADS1115-based measurements across Experiments 1 and 2.