What is the role of diodes in heart rate monitoring of smart wristbands?

1, PPG technology principle: the core mechanism of diodes
The heart rate monitoring of smart wristbands is mainly based on PPG technology, and its core process includes four steps: light source emission, light signal penetration and reflection, light signal reception, and signal processing. In this process, diodes play a dual role of light source emission and light signal reception:

Light source emission: The LED diode (usually green light, wavelength 530nm) built into the wristband regularly emits light that penetrates the skin. Green light is efficiently absorbed by hemoglobin in the blood, while unabsorbed light is reflected back to the surface of the skin. This wavelength selection is based on the strong absorption characteristics of blood towards green light, which can maximize signal contrast.
Optical signal reception: Photodiodes (such as Vishay's VEMD2704) are responsible for receiving reflected optical signals and converting them into electrical signals. When the heart pumps blood, the arterial vessel volume changes periodically, causing fluctuations in reflected light intensity. Photodiodes generate electrical signals synchronized with heart rate by capturing these subtle changes in light intensity.
Signal processing: The built-in chip of the wristband filters, amplifies, and analyzes the electrical signal using algorithms, ultimately calculating the heart rate value. For example, the Xiaomi Mi Band 7 achieves electrostatic protection through the TVS diode (ESD5641D12) packaged in DFN2x2-3L, ensuring signal transmission stability.

2, Device selection: adaptation to performance and scenarios
The performance of photodiodes directly determines the accuracy and reliability of heart rate monitoring. In practical applications, the following key parameters need to be comprehensively considered:

Wavelength response range: Heart rate monitoring needs to cover the green light (530nm) band, while blood oxygen monitoring needs to support both red light (660nm) and infrared light (940nm). Some high-end wristbands, such as the Apple Watch, use multi wavelength photodiodes to achieve synchronous monitoring of heart rate and blood oxygen by analyzing the differences in light absorption at different wavelengths.
Sensitivity and signal-to-noise ratio: Under motion, the intensity of light reflection on the skin surface fluctuates dramatically, requiring photodiodes to have high sensitivity to capture weak signals. Vishay's VEMD2704 improves the signal-to-noise ratio to more than twice that of traditional devices by optimizing the infrared light response curve, effectively reducing motion artifact interference.
Package size and power consumption: Smart wristbands have strict requirements for device miniaturization and low power consumption. For example, VEMD2704 adopts a 1.8mm × 2.0mm micro package with a power consumption of only 0.1mW, which can meet the battery life requirement of the wristband for up to 7 days.

3, Application Challenge: Interference from Environmental and Physiological Factors
Despite the high maturity of PPG technology, it still faces multiple challenges in practical use, which need to be addressed through a combination of diode selection and algorithm optimization

Environmental light interference: Strong light (such as sunlight) may cause saturation of photodiodes, leading to signal distortion. The solution includes:
Using narrowband filters, only the target wavelength (such as 530nm) is allowed to pass through;
Dynamically adjust the LED driving current to balance signal strength and environmental light effects.
Motion artifact: Arm swinging or muscle contraction can alter the light path on the skin surface, causing signal fluctuations. For example, the heart rate data of the wristband may be temporarily abnormal during running. Optimization directions include:
Choose photodiodes with strong anti-interference ability (such as low junction capacitors);
Combining acceleration sensor data, eliminate motion interference through algorithms.
Skin differences: Dark skin or tattoos can reduce light reflection efficiency and affect monitoring accuracy. Some wristbands alleviate this problem by increasing LED driving power or using multi wavelength light sources (such as red and green light).

4, Technical Optimization: Innovation from Devices to Systems
To improve the medical grade accuracy of heart rate monitoring, the industry is promoting technological upgrades from two levels: device design and system integration

Device innovation:
Multi wavelength integration: For example, ROHM's BPW34 photodiode integrates green, red, and infrared sensors on a single chip to achieve synchronous monitoring of heart rate, blood oxygen, and blood pressure.
Flexible substrate: The 3D liquid diode (3D LD) technology proposed by City University of Hong Kong achieves unidirectional sweat transmission and improved breathability by constructing heterogeneous wetting microstructures, solving the signal drift problem caused by sweat accumulation in traditional wristbands.
Algorithm optimization:
Deep learning model: Huawei GT series wristbands use convolutional neural networks (CNN) to analyze the waveform characteristics of PPG signals, distinguish between real heart rate and motion artifacts, and reduce static heart rate errors to within ± 1bpm.
Multi sensor fusion: Combining ECG electrodes with PPG sensors, calibrating heart rate data by comparing the time delay of ECG signals and optical signals, and improving accuracy in dynamic scenes.