Oximetry is the analysis of oxygen saturation in the blood and is usually given as a percentage. A pulse oximeter is a non-invasive product that measures the level of oxygen in a person’s blood and heart rate. The devices can be easily recognized by their clamp, which is attached to the finger of a patient. The devices are used by nurses, patients at home, fitness enthusiasts and even pilots in aircraft without a pressurized cabin.
Blood oxygen saturation
A hemoglobin test measures the oxygen content of the blood. It is the oxygen-carrying pigment of the red blood cells that gives them the red color and serves to channel oxygen into the tissue. Hemoglobin can be found in two forms: once as oxidized hemoglobin (HBO 2, i.e., enriched with oxygen), and as hemoglobin without oxygen (Hb, i.e., low in oxygen). The oxygen saturation in the blood (S P O 2 ) is the ratio of oxygen-enriched and oxygen-poor hemoglobin:
- S P O 2 = HBO 2 / (Hb + HBO 2 )
The value is given as a percentage and should usually be 97% or higher. What’s interesting about hemoglobin is how it reflects and absorbs light: Hb absorbs more visible red light and therefore reflects it less. HBO 2 absorbs more infrared light and reflects it less. Since the oxygen saturation in the blood by the comparison between Hb and HbO 2can determine by a red LED and by an infrared LED, which shines through a body part such as a finger or a wrist, the corresponding light intensity. Figure 1 illustrates the two methods of measurement that are used: First, the measurement of the proportion of light through which the tissue penetrates (transmissive oximetry). Second, the measurement of the proportion of light that reflects the tissue (reflection oximetry).
An example of transmissive oximetry can be found in hospitals. Most patient monitoring systems have an integrated transmissive pulse oximeter there. Many of the new high-end wearables, on the other hand, use reflection pulse oximetry.
Heart rate and heart rate
When the heart beats blood is pumped through the body and pressed into capillaries, whose volume increases slightly. Between the heartbeats, the volume decreases again. This volume transformation affects the amount of light (red light or infrared light) that passes through the tissue. Although these fluctuations are tiny, a pulse oximeter can measure them. The same measuring setup is used as for the blood oxygen content.
Pulse oximeters monitor the oxygen saturation (S P O 2 ) in the blood by absorbing the red light (wavelength 600 … 750 nm) and infrared light (850 … 1000 nm) in oxygen-enriched hemoglobin (HBO 2 ) and deoxygenated hemoglobin (Hb). In this case, red light and infrared light are alternately transmitted through a body part (for example fingers) to a photodiode.
Measuring technology
The photodiode receives the non-absorbed light of each LED, and an inverting operational amplifier processes this signal. The result represents the amount of light that the finger has absorbed (Figure 2). The pulse amplitudes (V PP ) of the red light and infrared signals are measured and converted into the effective voltage V RMS to be able to form a ratio:
- Ratio = (V RMS (red light AC ) / red light DC ) / (V RMS (IR AC ) / IR DC )
The S P O 2 value can be determined by the ratio and a lookup table based on empirical formulas. The heart rate is calculated based on the A / D converter samples and the sample rate. The look-up table is an important part of a pulse oximeter. It is designed specifically for each device and is based on calibration curves obtained, among other things, from numerous measurements of patients with different SP O 2 values. Figure 3 shows an example of such a calibration curve.
Circuit diagram
The design in Figure 4 shows the measurement of heart rate and oxygen content in the blood. The S P O 2 probe used is a common finger clip that includes a red light LED, an IR LED and a photodiode. The LEDs are controlled via the LED driver circuit. They transmit red light and IR light through the finger, the signal conditioning circuit detects the signals and feeds them into the 12-bit ADC module, which is integrated into the DSC (Digital Signal Controller). The DSC calculates the percentage for S P O 2.
A dual single-pole analog switch, controlled by two PWM signals from the DSC, alternately switches the red light and IR LEDs on and off. To get the correct number of ADC samples and allow enough time for data processing before the next LED turns on, the instrument turns the LEDs on and off according to the flowchart in Figure 5. The LED current (light intensity) is controlled by a 12-bit DAC (A / D converter) controlled by the DSC.
Signal conditioning
The signal conditioning circuit consists of two stages. The first is the transimpedance amplifier; the second is the gain amplifier. Between both stages, there is a high pass filter. The transimpedance amplifier converts the few microamps of current generated by the photodiode into a few millivolts. The signal received by the first-stage amplifier will then pass through a high-pass filter that eliminates background light interference.
The output of the high pass filter goes to the second stage amplifier – with a gain of 22 and a DC offset voltage of 220 mV. These values are accurately determined to relocate the output of the gain amplifier into the ADC area of the MCU. The output of the analog signal transforming circuit is connected to the integrated 12-bit ADC module of the DSC. In this example, a dsPIC from Microchip is used. The dsPIC33FJ128GP802 offers not only DSP capabilities but also Microchip’s digital filter design tool.
During each LED turn-on period, an ADC scan occurs, and another scan during each LED turn-off period. One of the challenges of human-based light-based measurements is the fit of a 513th order digital FIR bandpass filter, which can be easily implemented via the filter design tool. The filter affects the ADC data. The result is then used to calculate the pulse amplitude (Figure 6). The specifications of the FIR bandpass filter used here are:
- Sampling frequency: 500 Hz
- Passband Frequency: 1 and 5 Hz
- Barrier frequency: 0.05 and 25 Hz
- FIR window: Emperor
- Passband ripple -0.1 dB
- Barrier ripple: 50 dB
- Filter length: 513
Growing market
The market for home medical technology and fitness wearables is proliferating. The demand for devices that measure heart rate and blood oxygen levels will continue to rise over the next few years. Pulse oximeter reference designs, such as the one described here, help developers make their designs faster.
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