What is the role of diodes in signal conditioning of medical device sensors?

1, Photodiode: the "optical signal translator" for vital sign monitoring
Photodiodes convert optical signals into electrical signals through the photoelectric effect of PN junctions, and are widely used in medical devices for monitoring blood oxygen saturation, pulse wave detection, endoscopic imaging, and other scenarios. Its core advantages are reflected in three aspects:

High sensitivity spectral response
Taking silicon-based photodiodes as an example, they have linear response characteristics in the wavelength range of 400-1100nm, covering exactly the hemoglobin absorption peak (940nm) and the oxygenated hemoglobin absorption peak (660nm). Through dual wavelength detection technology, blood oxygen saturation (SpO ₂) can be accurately calculated with an error rate controlled within ± 2%. A certain brand of portable oximeter adopts PIN type photodiodes, which can maintain a response of 0.1 μ A/lux under low light conditions, ensuring the accuracy of night monitoring.
Rapid dynamic response capability
In pulse wave detection, photodiodes need to capture millisecond level changes in heartbeat signals. Avalanche photodiodes (APDs) shorten the response time to nanoseconds through internal avalanche multiplication effect, and with the help of a transimpedance amplifier circuit, can clearly reproduce the rising edge and characteristics of pulse waves and heavy pulse waves. A certain dynamic blood pressure monitor uses an APD array to achieve waveform distortion free acquisition at a sampling rate of 200Hz.
Anti interference structure design
Fluorescent lamps and LED light sources in medical environments may generate 50Hz power frequency interference. By integrating optical filters within the photodiode package, non target wavelength optical signals can be effectively suppressed. A certain type of medical endoscope adopts multi-layer coating technology, which increases the visible light transmittance to 95% while suppressing infrared interference to below -40dB.
2, Zener diode: the "safety valve" of medical power supply
Medical equipment has strict requirements for power stability. The precision voltage regulator circuit constructed by the reverse breakdown characteristic of the voltage regulator diode plays a key role in the following scenarios:

Lithium battery protection system
When portable medical devices such as defibrillators and portable ultrasound are powered by lithium batteries, it is necessary to prevent battery damage caused by overcharging/overdischarging. The MMSZ5231B series voltage regulator diode, with precise voltage regulation characteristics of 3.3V ± 0.05V and a charging management chip, controls battery voltage fluctuations within ± 0.1%. After adopting this solution, the battery cycle life of a certain AED device has been extended to over 2000 times.
High precision analog circuit power supply
In devices such as electrocardiographs and electroencephalographs, analog front-end circuits are sensitive to power supply noise. The BZX85C series low-noise voltage regulator uses temperature coefficient control technology of 0.01 μ A/℃ to suppress power ripple to below 50 μ V within the working range of -20 ℃ to+85 ℃, ensuring a signal-to-noise ratio (SNR) of ≥ 60dB for bioelectric signal acquisition.
Design of medical grade isolated power supply
In strong electromagnetic interference environments such as operating rooms, the output terminal of the isolation transformer needs to be equipped with a clamp circuit constructed by a voltage regulator diode. A certain medical isolation power supply adopts a bidirectional voltage regulator diode array to limit the common mode interference voltage within ± 60V, meeting the requirements of IEC 60601-1 medical electrical safety standard.
3, Innovative Applications in Signal Conditioning Circuits
Limiting protection circuit
In the ultrasonic probe drive circuit, the 1N4148 high-speed switch diode forms a bidirectional limiting network, which clamps the transient overvoltage within ± 5.7V to protect the piezoelectric ceramic transducer from pulse impact. After adopting this solution, the probe failure rate of a portable ultrasound device decreased by 80%.
Logarithmic amplification circuit
In a fluorescence immunoassay analyzer, the output signal of the photodiode is logarithmically related to the fluorescence intensity. The logarithmic amplifier constructed with BAS70 Schottky diode extends the dynamic range to 10 ⁶, achieving pg/mL level trace substance detection. After adopting this technology, the detection limit of a certain type of chemiluminescence detector exceeded 0.1 pg/mL.
Temperature compensation network
In an infrared thermometer, a thermistor and a 1N4728A type voltage regulator diode form a temperature compensation circuit, which cancels out the influence of ambient temperature on the infrared sensor through nonlinear characteristics. After adopting this scheme, a certain ear temperature gun can control the measurement error within ± 0.2 ℃ at an ambient temperature of 10 ℃ -40 ℃.