Pulse oximetry measurement solution design

Hemoglobin is an important component of blood cells and is responsible for transporting oxygen from the lungs to other tissues in the body. The amount of oxygen contained in hemoglobin at any one time is called blood oxygen saturation (SpO2).

Blood oxygen saturation is an important physiological parameter that reflects whether the human body's respiratory function and oxygen content are normal. It is also an important physiological parameter that shows whether our body's tissues are healthy. Severe hypoxia can directly lead to tragedies such as suffocation, shock, and death.

In the lungs, oxygen is attached to a protein bound to red blood cells, called hemoglobin (symbol Hb). There are two forms of hemoglobin in the blood: oxygenated hemoglobin (HbO2) and reduced hemoglobin (Hb).

Blood oxygen saturation SpO2 = (HbO2x100)/(HbO2+Hb)x100%

The testing principle of the oximeter is: oxygenated hemoglobin and reduced hemoglobin have different absorption characteristics in the spectrum range of visible light and near infrared light. Reduced hemoglobin absorbs more red frequency light and less infrared frequency light, while oxygenated hemoglobin absorbs less red frequency light and more infrared frequency light. This difference is the most basic basis of the SpO2 measurement system.

To measure the absorption of red and infrared light by the human body. Red and infrared light emitting diodes are positioned as close to each other as possible, and the emitted light can pass through a single tissue point in the human body. The light is first received by a single photodiode that responds to the red and infrared light, and then a transimpedance amplifier generates a voltage proportional to the intensity of the received light. The red and infrared LEDs are usually time-multiplexed so that they do not interfere with each other. Ambient light is estimated and subtracted from each red and infrared light. Measurement points include fingers, toes, and earlobes.

Pulse oximeters provide a non-invasive method of measuring blood oxygen saturation or arterial hemoglobin saturation. The pulse oximeter works on the basis of the variation in light absorption during arterial pulsations. Two light sources, one in the visible red light spectrum (660 nanometers) and one in the infrared spectrum (940 nanometers), alternately illuminate the area being tested (usually a fingertip or earlobe). The amount of light absorbed during these pulsations is related to the oxygen content in the blood. A microprocessor calculates the ratio of the two absorbed spectra and compares the result with a table of saturation values ​​stored in memory, giving the blood oxygen saturation.

A typical oximeter sensor has a pair of LEDs that face a photodiode through a translucent part of the patient's body (usually a fingertip or earlobe). One of the LEDs is red light with a wavelength of 660nm; the other is infrared light with a wavelength of 940nm. The percentage of blood oxygen is calculated by measuring the two wavelengths of light with different absorption rates after passing through the body.

Figure 1: Block diagram of a blood oximeter circuit based on ADI’s ADuC7024.

The figure above shows the circuit block diagram of the oximeter based on ADI's ADuC7024. ADuC7024 oximeter chip. The microcontroller core of this precision analog microcontroller is ARM7TDMI, with 8KB of SRAM and 62KB of non-volatile flash/EE memory integrated on the chip. The ADuC7024 integrates a MSPS, 12-bit, multi-channel high-performance ADC data acquisition system, 16-bit/32-bit MCU and Flash/EE memory on a single chip. The ADC has up to 12 single-ended input channels, and another 4 ADC input channels can also be multiplexed with the output pins of the 4 DACs. The ADC can operate in single-ended mode or differential input mode, with an input voltage of 0 V to VREF. A low-drift bandgap reference voltage source, temperature sensor and voltage comparator complete the ADC peripheral settings.

This solution has low cost, small size, excellent low perfusion and spontaneous anti-interference performance, and high flexibility. The cost of this oximeter chip and some analog components is lower than the cost of a complete oximeter OEM module. The firmware can be customized to meet user application requirements. By changing the firmware, any type of communication, display and operation interface can be handled. The parameters of the oximeter algorithm can also be changed to meet special application requirements, such as sleep studies, home telemetry, etc. The solution is a single chip and requires only a small amount of front-end regulation circuits, so the entire device will be very small.

The picture below is the ADI SpO2 demonstration system.

Figure 2: ADI SpO2 demonstration system.