The portable finger clip oximeter for home use has high flexibility and can quickly measure the blood oxygen saturation percentage (SpO2) and pulse rate of the human body on a non-invasive basis. It has been widely praised by the market and continues to sell well.

The portable finger clip oximeter for home use has high flexibility and can quickly measure the blood oxygen saturation percentage (SpO2) and pulse rate of the human body on a non-invasive basis. It has been widely praised by the market and continues to sell well.

Compared with reduced hemoglobin (Hb), oxygenated hemoglobin (HbO2) absorbs more red light and less infrared light because hemoglobin in blood has different absorption capabilities for red light and infrared light. HbO2, on the other hand, absorbs less red light and more infrared light. The red and infrared LED lights are structurally next to each other, which facilitates light transmission at a single location on the finger. The light penetrates from one side of the finger to the other side, and after being received by the photodiode, a corresponding voltage is generated.

As a company with high-performance analog signal chain design capabilities, Runshi Technology’s product line has a very high degree of matching with consumer medical products such as finger clip oximeters, including operational amplifiers, comparators, analog switches, LDOs and other products. .

The block diagram of a typical portable finger clip oximeter is as follows:

An article to understand the principle and parameters of portable finger clip oximeter

In the above picture, the analog switch RS2105 is used to switch the red LED and infrared LED lights. They are time-multiplexed and modulated with a certain frequency. The op amp RS321 cooperates with the DAC inside the MCU to generate a constant current output signal, because the LED light has a constant current A relatively stable wavelength can be obtained by working, and the emission current often reaches tens of mA, so the on-resistance of the analog switch is required to be preferably less than 1Ω, and the on-resistance of RS2105 is 0.6Ω, which meets the application requirements.

On the photosensitive receiving tube side, the op amp RS622 is used to realize photoelectric conversion, which requires the input bias current Ib of the op amp to be pA level, and the bandwidth of the op amp needs to be more than 5MHz to meet the fast response, low noise series of op amps Can better improve the resolution of the system. In terms of power supply topology, one is to use two dry batteries (3V), then boost to 5V and then step down for use in each circuit of the system, and the other is to use lithium batteries, which can be recharged and used. Low-power LDO RS3236 is suitable for this. Two topological systems.

The main parameters of the low noise op amp RS622 are as follows:

◆ Wide working voltage range: 2.5V~5.5V;

◆ Low offset voltage: typical value is 0.7mV, maximum value is 3mV;

◆ Low temperature drift: typical 2uV/°C;

◆ Low input bias current: 1pA;

◆ Unity gain bandwidth: 7MHz;

◆ Quiescent current: 600uA per channel;

◆ Noise figure: 11nV/√Hz.

The main parameters of the analog switch RS2105 are as follows:

◆ Wide input voltage range: 1.8V~5.5V;

◆ On-resistance value: 0.6Ω, maximum 1Ω;

◆ Continuous output current: 500mA;

◆ -3dB bandwidth: 30MHz;

◆ Quiescent current: less than 1uA;

◆ Available in MSOP-10 and TDFN-3*3-10L packages.

The main parameters of the low power LDO RS3236 are as follows:

◆ Wide input voltage range: 1.7V~7.5V, support 4 dry battery input;

◆ Internal maximum output current limiting point: >500mA;

◆ Ultra-low quiescent current: 20uA;

◆ Ultra-low dropout: 1mV/mA;

◆ Ultra-high PSRR:[email protected]