Currently, medical appliance OEMs are developing more specialized personal care devices for the treatment and monitoring of common diseases. These products are reasonably priced and greatly improve the quality of health care. MCUs play an important role in portable medical devices such as home sphygmomanometers, spirometers, pulse oximeters, and heart rate monitors. The actual physiological signal in most of these products is an analog signal that needs to be amplified, filtered, etc. before being measured, monitored or displayed.
Embedding high-performance analog peripherals into ultra-low-power MCUs not only enables on-chip systemization of portable medical electronics, but also extends battery life. This article describes various ways to simplify the analog front-end design of portable battery-powered medical devices, such as combining high-performance peripherals such as op amps, ADCs, and DACs with low-power MCUs. The MCU has digital filtering and processing functions, and can also display physiological data such as blood pressure, vital capacity, heart rate and blood oxygen content. Combining these peripherals with the MCU not only performs all of the above functions, but also satisfies the power requirements by turning off the peripherals into standby mode (current consumption is only a few mA).
A good example is the MSP430FG4619. Its 16-bit RISC CPU not only provides the required signal processing power, but also has an ultra-low operating current, allowing the battery to last for years in such applications. The MCU integrates peripherals such as op amps, 12-bit multi-channel ADCs, and dual 12-bit DACs as part of the analog signal processing circuitry. In addition to embedding high-performance analog peripherals, the device features 120KB of on-chip flash and Universal Serial Communication Interface (USCI). The following is a detailed introduction to a single-chip solution for medical products with integrated analog peripherals.
sphygmomanometer
Figure 1 is a functional block diagram of a sphygmomanometer. This application typically uses a bridge pressure transmitter as a sensor and is connected to an inflatable cuff. The transmitter can be activated via a port pin and can save significant power since it is only activated during pressure measurement. The mV output of the sensor is proportional to the pressure. This signal needs to be amplified before digitization and then measured by the ADC. The amplified signal detects the Korotkoff tone and determines the systolic and diastolic pressure readings. The three op amps in the MCU do the job well. A high gain differential amplifier block of several amplifiers eliminates common mode noise in the application. The differential amplifier function block using 3 amplifiers is shown in Figure 2. The amplified signal is input internally to the 12-bit ADC. The DMA peripherals in the device enable efficient data processing, enable fast execution of Korotkoff tone detection algorithms, and filter out noise that affects measurement results. The 16-bit CPU handles the above algorithm with lower MIPS processing power. The device also integrates a 160-segment LCD driver with a regulated charge pump to provide stable contrast, further complementing this single-chip solution. The 120KB low-power flash memory in the MCU can be upgraded in the field. Because flash memory has in-system programmability, it can be used as a data logger. The USCI serial port in the device can communicate with a PC or PDA to download recorded data. Due to the ultra-low power architecture of the MCU, the solution operates at less than 3mA in blood pressure measurement mode. In Idle mode, the device is operating normally and shows that the real-time clock consumes less than 3mA.
Figure 1 sphygmomanometer functional structure
The DC motor controlled by the PWM output in the MCU charges/deflate the cuff. This is the only place where the sphygmomanometer uses a 6V power drive motor. If the power requirements are not met, the entire sphygmomanometer can be powered by a 3V lithium-ion button battery. However, only a few motors can be driven by this high-impedance button battery, so this example can provide 3.3V power to the MCU using four common low-cost AAA alkaline batteries and a low-dropout regulator (LDO). Assuming two measurements of blood pressure per day, these batteries can be used for two years. The MCU can work in the active display timing mode for a long time because the current consumption of this mode is very small. In addition, the user does not increase the current consumption when viewing the stored blood pressure readings. In addition, the integrated dual-channel DAC is capable of producing a sine wave with a phase shift of 180°, which improves transmitter performance.
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