How to ensure accurate measurement of diodes in oximeter circuits?

1, Dual wavelength LED: the cornerstone of precise signal generation
The oximeter adopts a dual wavelength LED with 660nm red light and 940nm infrared light, and its design is based on the difference in absorption characteristics of hemoglobin (Hb) and oxygenated hemoglobin (HbO ₂) for different wavelengths of light. Specifically:

660nm red light: HbO ₂ absorption rate is low, Hb absorption rate is high, and signal intensity is negatively correlated with arterial oxygen content;
940nm infrared light: HbO ₂ absorption rate is significantly higher than Hb, and signal intensity is positively correlated with arterial oxygen content.
Key points of technical implementation:

Timing control: Drive the LED to flash alternately (usually at a frequency of 100-500Hz) through an H-bridge circuit to avoid mutual interference between the two light signals. For example, a certain model of oximeter uses the MSP430 microcontroller's PWM signal to control the LED driver chip, achieving alternating lighting of red and infrared light at intervals of 0.5ms.
Constant current drive: using a constant current source circuit to ensure stable LED luminous intensity and eliminate the interference of power supply fluctuations on light intensity. A clinical grade oximeter uses a precision resistor (such as 0.1% accuracy) and an operational amplifier to form a feedback loop, controlling LED current fluctuations within ± 0.5%.
Light intensity calibration: In the production process, the LED output light intensity is adjusted through optical filters to match the signal amplitudes of two wavelengths and improve the dynamic range of subsequent signal processing. For example, a portable oximeter uses an integrating sphere calibration system to control the intensity ratio of red and infrared light at 1:1.2 ± 0.05 before leaving the factory.
2, Photodiode: the core of high-sensitivity photoelectric conversion
Photodiodes are responsible for converting light signals transmitted through fingers into electrical signals, and their performance directly affects the signal-to-noise ratio (SNR). The key technical parameters include:

Response wavelength range: It needs to cover 400-1050nm to respond to both red and infrared light simultaneously;
Response speed: The rise time should be less than 1 μ s to capture small changes in pulse waves;
Dark current: It needs to be lower than 0.1nA to reduce environmental light interference.
Typical application cases:
A certain medical grade oximeter uses OSRAM SFH 2701 photodiode. When the reverse bias is 5V, the dark current is only 0.05nA, and the responsivity reaches 0.55A/W at 940nm. The device significantly improves its high-frequency response capability by optimizing the PN junction structure and reducing the junction capacitance to 1.7pF.

Key points of circuit design:

Trans impedance amplifier (TIA): converts the weak current signal (usually 0.1-10 μ A) of a photodiode into a voltage signal. For example, a certain design uses AD8065 operational amplifier to construct TIA, with a feedback resistance of 1M Ω, achieving a conversion gain of 0.1V/μ A.
Environmental light suppression: Dual suppression of environmental light interference is achieved through optical filters (such as 660nm and 940nm bandpass filters) and circuit filters (such as RC low-pass filters). Experimental data shows that this scheme can reduce 50Hz power frequency interference by 40dB.
Temperature compensation: An NTC thermistor is integrated next to the photodiode, and the TIA gain is adjusted in real-time through a microcontroller to compensate for temperature drift. For example, a certain design controls the output voltage fluctuation within ± 0.5% within the range of -20 ℃ to 50 ℃.
3, Noise Suppression: Full Link Optimization from Hardware to Algorithm
The signal of the oximeter contains multiple sources of noise, which need to be suppressed through hardware and algorithm coordination:

Hardware filtering:
Preamplification: A low-noise operational amplifier (such as OPA2333, with an input voltage noise density of only 3.5nV/√ Hz) is used to construct a TIA and reduce thermal noise;
Bandpass filtering: Extract pulse wave signals of 0.7-3Hz through a second-order low-pass filter (cut-off frequency 11.25Hz) and a first-order high pass filter (cut-off frequency 0.0159Hz);
50Hz notch: using a dual T network or active filtering circuit to suppress power frequency interference.
Digital filtering:
FIR filter: used to remove high-frequency noise and preserve pulse wave features;
Adaptive filtering: dynamically adjusting filter coefficients through LMS algorithm to suppress motion artifacts. A certain experimental data shows that this scheme can reduce the measurement error caused by motion interference from ± 5% to ± 1.5%.
4, Dynamic compensation: adapt to different physiological and usage scenarios
To improve the universality of measurement, the oximeter needs to dynamically compensate for the following scenarios:

Skin color difference: Dark skin has stronger absorption of light and needs to be compensated for signal attenuation by adjusting the LED driving current (such as increasing from 5mA to 10mA) or TIA gain. A certain design uses a microcontroller to monitor the output voltage of photodiodes in real time and automatically adjust the gain coefficient.
Low perfusion state: Shock or hypothermia leads to a decrease in pulse wave amplitude, and the signal-to-noise ratio needs to be improved by increasing the sampling rate (such as from 100Hz to 500Hz) and prolonging the integration time (such as from 100ms to 500ms). A clinical study showed that this approach can increase the measurement success rate of low perfusion patients from 75% to 92%.
Probe displacement: By monitoring changes in signal amplitude (such as a decrease of more than 30%), an alarm is triggered to prompt the user to re fix the probe. A portable oximeter integrates an acceleration sensor and further suppresses displacement interference through motion detection algorithms.
5, Clinical validation and standard compliance
Medical grade oximeters require strict clinical validation and standard compliance:

Clinical data fitting: Establish a mapping curve between R value (red light to infrared light AC/DC signal ratio) and SpO ₂ based on a large amount of volunteer data. For example, the calibration curve of a certain model of oximeter covers the range of SpO ₂ 70% -100%, with a maximum error of ≤ 2%.
IEC 60601-2-20 standard: requires LED light intensity not to exceed 10mW/cm ² to avoid skin burns; At the same time, it is stipulated that the measurement error shall not exceed ± 3% within the range of SpO ₂ 70% -100%.