How to use diodes in medical instruments to reduce circuit noise?

1, Sources and Effects of Medical Circuit Noise
The noise of medical equipment is mainly divided into two categories:

High frequency electromagnetic interference (EMI): generated by switching power supplies, wireless communication modules, or external devices, with a frequency range typically between 100kHz and 1GHz. For example, if an electrocardiograph (ECG) does not effectively suppress high-frequency noise, it may cause distortion of the QRS complex and affect the diagnosis of arrhythmia.
Power ripple noise: caused by insufficient rectification circuit or capacitor filtering, manifested as low-frequency fluctuations (50Hz/60Hz power frequency interference). In portable devices such as blood glucose meters, power supply noise may mask weak current signals, resulting in measurement errors exceeding ± 10%.
The harm of noise is not limited to signal distortion, but may also cause equipment failure. For example, in defibrillators, if power noise is not suppressed, the high-voltage discharge module may damage the circuit due to accidental triggering, endangering patient safety.

2, The core mechanism and selection principles of diode noise reduction
1. Nonlinear rectification characteristics: suppress high-frequency noise
A diode exhibits high impedance when reverse biased and conducts when forward biased, making it a "one-way valve" for high-frequency noise. When the noise signal passes through the diode, the forward component is absorbed by the conduction path, and the reverse component is blocked by high impedance, thereby converting AC noise into DC component and consuming it in the circuit. For example, in the ECG front-end circuit, the use of Schottky diodes (such as BAT54S) can effectively suppress high-frequency interference caused by antenna coupling, and improve the signal-to-noise ratio (SNR) by about 15dB.

Key parameters for selection:

Reverse Recovery Time (TRR): It should be less than 1/10 of the noise frequency cycle. For example, for 1MHz noise, TRR should be ≤ 100ns, and it is recommended to use ultrafast recovery diodes (such as UF4007, TRR=50ns).
Junction capacitance (Cj): Low junction capacitance can reduce high-frequency signal coupling. At the input of the bioelectric amplifier, diodes with Cj<2pF (such as the HSMS-286x series) should be selected to avoid signal attenuation.
2. Zener diode: clamp power supply ripple
Zener diodes maintain voltage stability through their reverse breakdown characteristics, effectively clamping power supply ripple. For example, in the low-voltage power supply (5V) of portable ultrasound equipment, using 1N4733A (with a voltage regulation value of 5.1V) can suppress the ripple voltage from ± 200mV to within ± 50mV, meeting the ADC sampling accuracy requirements.

Key parameters for selection:

Dynamic resistance (Zz): reflects the accuracy of voltage regulation. The smaller the Zz, the better the ripple suppression effect. It is recommended to choose models with Zz<10 Ω for medical grade equipment (such as BZT52C5V1).
Temperature coefficient (TC): Medical equipment needs to operate in an environment of -20 ℃ to 60 ℃, and a voltage regulator with TC<2mV/℃ should be selected to avoid temperature drift affecting performance.
3. Suppressing diode: dedicated high-frequency noise absorption
Suppressing diodes (such as 1N5711) form low capacitance PN junctions through special doping processes, which can absorb GHz level noise. In the RF front-end of magnetic resonance imaging (MRI) equipment, using 1N5711 can attenuate noise from 100MHz to 1GHz by more than 40dB, protecting the low-noise amplifier (LNA) from interference.

Key parameters for selection:

Reverse leakage current (Ir):<1 μ A (25 ℃) is required to avoid introducing additional noise in low-power circuits.
Rated power (Pd): It should be selected based on the noise power. For example, in MRI equipment, models with Pd ≥ 1W should be selected to withstand high-energy pulse interference.
3, Noise reduction practice in typical medical application scenarios
1. ECG signal acquisition: front-end circuit protection
The amplitude of ECG signal is only 1mV to 5mV, which is easily masked by high-frequency noise. When designing, a bidirectional suppression diode (such as BAV99) should be connected in parallel at the input end to form a ± 10V clamp protection, and a 0.1 μ F capacitor should be connected in series to filter out high-frequency interference. Tests have shown that this scheme can suppress 50Hz power frequency interference by 60dB and improve the accuracy of QRS complex detection to 99.5%.

2. Portable blood glucose meter: power supply noise suppression
The blood glucose meter is powered by a single lithium battery, and power ripple may affect the detection of enzyme electrode current. By paralleling Schottky diodes (such as SS14F) at the input of the LDO regulator, the ripple voltage can be reduced from ± 50mV to ± 10mV, and the measurement repeatability (CV%) can be optimized from 8% to within 3%.

3. Endoscopic imaging system: RF interference isolation
The camera module of wireless endoscope is susceptible to 2.4GHz Wi Fi signal interference, resulting in horizontal noise in the image. By connecting a suppression diode (such as HSMS-2850) in series between the antenna and the RF front-end, the interference signal can be attenuated by 30dB, and the image signal-to-noise ratio (PSNR) can be improved by 12dB, meeting clinical diagnostic needs.