simpler and cheaper
As we all know, portable medical electronics have been greatly developed and widely used in recent years. More and more new products have appeared on the market. The solutions with good effectiveness and mass production are those with simple design and superior performance, so as to ensure the reduction of equipment costs. To do this, designers need to consider power consumption, cost, size, FDA approval of the device, and other factors.
A typical portable medical Electronic system consists of the following parts: analog front end (for data acquisition), amplifiers and filters (for signal conditioning), analog-to-digital converters and sensors (for signal acquisition), buttons (for accept user feedback), microcontroller (execute the algorithm), and various interfaces such as LCD Display, USB port, etc. Traditional designs require placing all required components on the PCB. This approach increases the overall system BOM, PCB complexity and design cycle. Using discrete devices is also not good for IP protection because it can be easily copied.
Portable medical device design and production are also regulated by the Food and Drug Administration (FDA). This means that their design and production must follow precise process documentation, their performance must be rigorously documented, and they must pass development testing, mass production testing, and field application. A State Food and Drug Administration requires devices used in medical devices to be available within the next 5 years. This motivates developers to reduce the overall device count to make FDA certification easier.
Figure 1 Traditional design of blood pressure monitor
Figures 1 and 2 show the architecture of a typical blood pressure monitor (BPM) and a non-contact digital thermometer using conventional designs.
Figure 2 Traditional design scheme of non-contact digital thermometer
A typical blood pressure monitor uses a differential pressure sensor to measure pressure in the cuff or arm. This sensor is used as an output, only a few tens of microvolts (30µV~50µV), and the output pressure signal needs to be amplified by a high gain amplifier to achieve a good common mode rejection ratio (CMRR). Typically gain and common-mode rejection ratios need to be around 150 and 100 dB, respectively. The frequency of the pulse vibration of the pressure signal is between 0.3Hz and 11Hz, and the amplitude is about several hundreds of microvolts. Such oscillations can be extracted by a bandpass filter with a gain of 200 and a cutoff frequency between 0.3Hz and 11Hz. A 10-bit ADC with a speed of 50 Hz is used to digitize the pressure sensor and vibration signals. Two timers are used to calculate the heartbeat and implement the safety timer function. A safety timer is used to regulate the pressure to remain on the target arm for a certain period of time. Safety timers are part of the safety rules in the AAMI standard. A microcontroller core uses various algorithms to calculate its systolic and diastolic pressure values. A PWM motor drives the expansion and contraction of the cuff.
A typical non-contact digital thermometer uses a sensor (or a thermocouple thermocouple), which consists of a thermocouple (used to measure the temperature of the thermocouple) and a thermistor (used to measure the ambient temperature). film. Thermocouples generate a DC voltage based on the voltage difference between nodes. The output of the thermocouple is held at a few microvolts. The thermocouple signal is amplified by a low noise precision amplifier. A voltage divider is formed by a thermistor and an external precision voltage regulator. This voltage divider is used to convert changes in thermistor voltage to changes in temperature. The thermocouple and thermistor voltages are used to calculate the thermocouple and ambient temperature. The voltage is obtained through the formula given by the sensor manufacturer or the pre-stored look-up table to obtain the temperature. The ambient temperature plus the thermocouple temperature is the final temperature measurement.
Of course, in the above application, other peripheral circuits are also required, such as field LCD driver, real-time clock RTC, button, EEPROM and USB.
Devices external to the microcontroller, such as sensors, analog-to-digital converters, LCD drivers/controllers, USB controllers, filters, and amplifiers are all peripheral devices. These devices can be connected to the microcontroller through the general purpose input and output ports GPIO or dedicated pins. The more externally discrete components, the more limitations and constraints the designer must consider, such as component inventory management, the complexity of multilayer PCBs, achieving FDA certification for each device, increasing design/development time, and detrimental to analog IP protection etc.
The design of today’s system-on-chip (SoC) architecture provides a new way of thinking for portable medical electronic devices. Using a system-on-chip design can bring a lot of added value. The designs of Figures 3 and 4 depict implementations of blood pressure monitoring and non-contact digital thermometers using a system-on-chip architecture.
Figure 3 Blood pressure monitoring scheme using system-on-chip
Using a blood pressure monitoring device based on a system-on-chip can simplify the design and achieve better results. The system-on-chip can integrate the high-gain amplifiers required by the design. Oscillating pulses can be extracted by an integrated analog/digital filter. The analog-to-digital converter ADC in the SoC can be used to digitize the data. The integrated CPU core provides the processing power to process the required advanced process algorithms. This device can also integrate field LCD driver (for display), EERPROM (data logging), real-time clock (time stamping), full-speed USB (as PC interface), direct memory access DMA (for data exchange with CPU) and capacitive sensing buttons (Can replace mechanical keys). A timer in the system-on-chip can be used to calculate heart rate and handle safety functions, and an integrated pulse width modulator PWM can be used to control the motor. The SoC can also operate at a unique wide operating voltage and achieve low power consumption, ideal for battery-operated devices.
Figure 4 Non-contact digital thermometer scheme using system-on-chip
For infrared thermometers, the system-on-chip also integrates the required amplifiers as well as an analog-to-digital converter (to detect microvolt changes). A precision voltage reference inside the system-on-chip provides a stable, accurate reference for the sensor. The SoC also integrates other functions, including field LCD driver, EEPROM, real-time clock RTC, USB interface, capacitive sensing, and more.
As mentioned above, the system-on-chip integrates most of the peripheral devices required for portable medical electronics applications. This not only reduces external components, but also protects the analog IP because it integrates most of the analog components into the chip. And, fewer components also means that the PCB can be simplified, reducing design time and bringing it to market faster. The power supply of different peripheral devices in the chip can be individually set to different modes, so the system power management becomes simpler and more efficient. SoCs can also be dynamically reconfigured, which also reduces cost and time and facilitates redesigns or changes to designs. Most importantly, the use of a system-on-chip architecture simplifies the device listing, which makes FDA certification simpler. All types of portable medical electronic devices – blood glucose meters, blood pressure monitors, portable ECG devices, etc., can be implemented using this method.
For example, Cypress’s PSoC products (Programmable System-on-Chip) can be used to tailor handheld applications such as blood pressure monitors, blood glucose meters and pulse oximeters. PSoC3/5 integrates 8051/ARM cortex M3 core (can work at 33 MIPS and 100 DMIPS), amplifier, dedicated digital filter block, configurable Delta Sigma analog-to-digital converter, can drive 736 fields LCD driver, capacitive Touch-sensitive buttons and proximity detection, 2KB of EEPROM, full-speed USB 2.0 and many other features make it a true single-chip solution. All of the above, combined with the PSoC Creator integrated development environment (programmable IP blocks for each function are pre-built, providing all the design tools needed), this allows product designers to program their products however they want, allowing them to Shorten the design cycle.
Overall, the use of SoCs can make portable medical electronics design simpler, protect intellectual property, provide novel and unique ways to personalize functionality, and make FDA certification simpler.