TIDUF16 December   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Photodiode, TIA, and ADC
      2. 2.2.2 LED Driving
      3. 2.2.3 Power
      4. 2.2.4 Display, Orientation, and Communication Features
      5. 2.2.5 Software
        1. 2.2.5.1 Timing Structure
        2. 2.2.5.2 Oversampling and Digital Filtering to Increase Dynamic Range
        3. 2.2.5.3 Calculating Vitals
    3. 2.3 Highlighted Products
      1. 2.3.1 MSPM0L1306
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Software Requirements
      1. 3.2.1 TI GUI
      2. 3.2.2 CCS Project
      3. 3.2.3 Analog Engineers Calculator
    3. 3.3 Test Setup
    4. 3.4 Test Results
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks

1 System Description

The pulse oximeter is a medical instrument for monitoring the saturation of oxygen in the blood and the pulse rate. The oxygen level and heart rate measured by the instrument can monitor the health of the user, and can also help the doctor to quickly diagnose the cause and condition of the disease. Thus, this instrument is widely used in hospitals and homes.

A pulse oximeter is a non-invasive device used to monitor the pulse rate and peripheral oxygen saturation (% SpO2) of blood.

In a pulse oximeter, the calculation of the level of oxygenation of blood (SpO2) is based on measuring the intensity of light that has been attenuated by body tissue. SpO2 is defined as the ratio of the level oxygenated hemoglobin (HbO2) over the total hemoglobin level (oxygenated hemoglobin and de-oxygenated hemoglobin (Hb)), see Equation 1.

Equation 1. S p O 2 = H b O 2 H b O 2 + H b

In principle, the HbO2 and Hb respond differently to different wavelengths of light. Hb absorbs more red light compared to infrared (IR) light, whereas, HbO2 absorbs more infrared light. As shown in Figure 1-1, when Red and IR Light Emitting Diodes (LEDs) are driven alternately through a finger, the amount of unabsorbed light that travels through the finger (where a photodiode is used as the sensing element) is related to the concentration of Hb and HbO2 in the blood.

GUID-20231204-SS0I-QNH3-TXWR-1N2HTSSDXTQV-low.svg Figure 1-1 Block Diagram of Pulse Oximeter

Two different wavelengths of light are used; each is turned on and measured alternately. By using two different wavelengths, the mathematical complexity of measurement can be reduced.

Equation 2. R = log ⁡ ( I α c ) λ 1 log ⁡ ( I α c ) λ 2           S p O 2   α     R

where

  • λ1 and λ2 represent the two different wavelengths of light used

There is a DC and an AC component to the measurements. It is assumed that the DC component is a result of the absorption and scattering by body tissue, blood in the veins and capillaries, and non-pulsatile (without periodic variations) blood in the arteries. The AC component is the result of the absorption by pulsatile (with periodic variations) blood in the arteries.

In practice, the relationship between SpO2 and R is not as linear as indicated by Equation 2. For this reason, a look up table is used to provide a correct reading.

The reliability of the R, and therefore the SpO2, is dependent on the ability to achieve good dynamic range on the signal input. Dynamic range (DR) is calculated from the effective number of bits (ENOB) using Equation 3.

Equation 3. DR=20×log10⁡(2ENOB)