How to quickly design a pulse oximeter? TI offers tips!

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How to quickly design a pulse oximeter? TI offers tips! [Copy link]

Pulse oximeters (shown in Figure 1) monitor heart rate and blood oxygen saturation non-invasively by measuring changes in infrared and red light absorption by oxygenated and deoxygenated blood in the finger. Normally, the blood oxygen saturation of a healthy person is in the range of 95% to 100% . Measuring blood oxygen saturation using an oximeter like the one shown in Figure 1 is becoming increasingly popular as it helps monitor our health.

Figure 1: Finger-clip pulse oximeter

TI provides a single-chip pulse oximeter system solution based on the MSP430FR235x microcontroller and its internally integrated 4x Smart Analog Combo (SAC ). The Smart Analog Combo (SAC) is a peripheral unique to TI MSP430FR235x that can be configured into different application modes by user software for analog signal conditioning of input or output paths. The specific application mode is shown in Figure 2:

Figure 2 Smart Analog Combo (SAC) block diagram and modes

The system block diagram is shown in Figure 3:

Figure 3 Pulse oximeter system block diagram example

The MSP430FR2355 microcontroller, as the main MCU of the solution , can provide the following functions and features for the pulse oximeter system:

1. The MSP430FR2355 single-chip solution effectively eliminates the need for external op amps and DACs, reducing system complexity and cost .

2. The MSP430FR2355 integrates four smart analog combos (SACs) on-chip. Each SAC can be configured as an operational amplifier (OPA), a programmable gain amplifier (PGA), or a 12-bit D AC .

   In the above pulse oximeter solution:

   a) Two SACs are configured as two 12-bit resolution current DACs to achieve adjustable constant current drive for IR/RED LEDs ;

   b) One SAC is configured as a transimpedance amplifier (TIA) with only 50pA bias current to achieve photodiode current-to-voltage conversion ;

   c) One SAC is configured as a 32-times gain programmable gain amplifier (PGA), which cooperates with the DAC to achieve DC compensation and AC component amplification of the signal ;

3. The chip integrates a 12-bit ADC and a 1.5/2.0/2.5V adjustable reference voltage source. The ADC channel is internally connected to the SAC output, effectively reducing the complexity of PCB wiring and noise interference ;

4. I2C and SPI interfaces can meet the communication requirements between the microcontroller and the direction sensor and LCD/OLED screen;

5. The on-chip integrated timer module can output multiple PWM signals to drive devices such as buzzers, LCD backlight charge pumps, etc.

6. Ultra-low standby current as low as 42nA and GPIO wake-up function in standby mode can provide longer standby time and fast key wake-up for battery-powered oximeter systems .

The MSP430FR235x series of microcontrollers are ultra-low power, low-cost, and rich analog signal chain resources 16-bit RISC MCUs launched by TI . Its low power consumption and VQFN32 (4*4mm) package are very suitable for portable devices. Figure 4 shows the rich on-chip analog and digital resources of the MSP430FR235x.

Figure 4 MSP430FR235x on-chip resources

The TI devices described in detail in this article, as well as the rich online software and hardware resources, will help engineers quickly design a blood oximeter. If you have any questions about how to design a blood oximeter, our team is always there to provide you with strong technical support.