Using SoCs for portable medical equipment
Portable medical electronics has seen tremendous growth and adoption in the recent years. More equipment variants are being introduced in the market by an increasing number of companies. The need of the hour is better mass producible designs which are low in complexity and provide acceptable performance so as to keep the cost of the device low. To achieve this, designers need to consider power efficiency, cost, form factor, and FDA certification of components, among other factors.
A typical portable medical electronic system comprises components like analog front-ends for data acquisition, amplifiers and filters for signal conditioning, analog-to-digital converters (ADCs) for signal and sensor data acquisition, buttons to accept user feedback, an MCU to execute algorithms, and a variety of interfaces such as an LCD display, USB port and so on. Traditional design methodologies bring together all of the needed functionality onto a PCB by way of individual components. This method increases the overall system BOM, PCB complexity, and design cycle. Using individual analog components also reduces analog IP protection as the system can be reverse engineered easily.
Portable medical equipment design and manufacture is also regulated by the Food and Drug Administration (FDA). This means that their design and construction must follow precisely documented processes, and performance must meet stringent documentation, development testing, production testing, and field maintenance requirements. One FDA regulation requires that the components used in a medical device have to be guaranteed to be available in production for the next five years. This provides an incentive for developers to reduce the overall number of components used to make FDA certification simpler.
Figures 1 and 2 show a typical blood pressure monitor (BPM) and a non-contact digital thermometer built using traditional approach.
Figure 1: Blood pressure monitor in traditional design approach
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Figure 2: Non contact digital thermometer in traditional design approach
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A typical BPM uses a differential pressure sensor to measure cuff or arm pressure. As the output of this sensor lies within a few micro volts (30-50µV), the output pressure signal has to be amplified using a high-gain instrumentation amplifier with a good common mode rejection ratio (CMRR). Usually the gain and CMRR need to be around 150 and 100 dB respectively. The frequency of oscillatory pulses in the pressure signal lies between 0.3-11Hz with an amplitude of a few hundred microvolts. These oscillations are extracted using band-pass filters with gain around 200 and cutoff frequency at 0.3-11Hz. A 10-bit ADC with a speed of 50 Hz is used to digitize the pressure sensor and oscillatory signal. Two timers are used to calculate the heart rate and implement safety timer functionality. A safety timer regulates the pressure kept on a subject’s arm for a certain period of time. This safety timer is a part safety regulation in AAMI standards. A microcontroller core calculates the systolic and diastolic pressures values using an oscillometric algorithm. The cuff is inflated and deflated using motors driven by PWMs.
A typical non-contact digital thermometer uses a transducer, also called a thermopile, consisting of a micro machine embedded membrane with thermocouples to measure thermocouple temperature and a thermistor to measure ambient temperature. The thermocouple generates a DC voltage corresponding to the temperature difference in its junctions. The output of the thermocouple is on the order of a few µV. The signal from the thermocouple is amplified using a low-noise precision amplifier. A voltage divider is constructed with the thermistor and external precision voltage reference. This voltage divider converts the change in thermistor resistance with respect to temperature to change in voltage. Voltages from the thermocouple and thermistor are used to calculate the thermocouple and ambient temperatures. The temperature is obtained from voltages using a polynomial function given by the sensor manufacturer or through a look-up table with pre-stored readings. The ambient temperature is added to the thermocouple temperature to get the final temperature measurement.
A segment LCD driver, RTC, push buttons, EEPROM and USB are the other peripherals needed in both of the above applications.
The components which are external to microcontroller like the transducer, ADC, LCD driver/controller, USB controller, filter, and amplifiers are the peripheral components. These components interface to the microcontroller through either a GPIO or a dedicated pin. The more external components there are, the more limitations and constraints developers have to account for, such as managing the bill of materials, higher PCB complexity, achieving FDA certification for each and every component, increased design/development time, and reduced analog IP protection. System-on-chip based approach
Today’s system-on-chip (SoC) architectures provide a new way of designing portable medical electronic devices. Designing with SoC-based devices brings in numerous value additions. Figures 3 and 4 depict the designs of a blood pressure monitor and non-contact digital thermometer using SoC architectures.
Figure 3: Blood pressure monitor using SoC
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Figure 3: Non contact digital thermometer using system-on-chip
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For an infrared thermometer, SoCs also integrate the amplifiers and ADC needed to detect microvolt variations. The SoC’s internal precision voltage reference provides a stable and accurate reference for sensors. Other functionality SoCs integrate a segment LCD driver, EEPROM, RTC, USB interface, capacitive touch sensing, etc.
As discussed above, SoCs integrate most of the peripheral components required by portable medical electronics applications. This not only reduces the number of external components required, it protects analog IP as well since all the analog components are integrated into the chip. Fewer components mean simpler PCBs, shorter design time, and faster time to market. The power of different peripherals inside the chip can be managed individually in different modes so system power management is made simpler and more efficient. Reconfigurability of SoCs chips also reduces the cost and time of redesigning or changing a design over time. More than anything, using SoC architectures makes FDA certification simpler by reducing the bill of materials. Portable medical electronics equipment of all types – glucose meters, pulseoximeters, portable ECG devices, etc. – can be implemented using SoCs.
As an example, Cypress’s PSoC 3/5 products (Programmable System on Chip) are tailor-made for portable handheld applications like Blood Pressure Monitors, Blood Glucose Meters and Pulse Oximeters. The PSoC3/5 integrates an 8051/ARM cortex M3 core running at 33 MIPS and 100 DMIPS, amplifiers, dedicated digital filter blocks, configurable Delta Sigma ADC, integrated LCD driver that can drive a maximum of 736 segments, capacitive sensing for touch buttons and proximity detection, 2KB of EEPROM, Full Speed USB 2.0, and various other functions, thereby enabling true single-chip solutions. This, combined with the PSoC Creator IDE which has pre-programmed configurable IP modules for each function, gives product designers all the tools they need to design a small form factor, highly programmable end product with a very short design cycle.
Overall, using SoCs in portable medical electronics applications makes design simpler, protects IP, and provides novel and unique methods to implement product-differentiating functionality and make FDA certification simpler.
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