Advanced SoC technology enables simple yet highly accurate measuring of glucose concentrations in the blood
BY JAKOB NIELSEN, SR.
Senior Manager, Consumer Health
ON Semiconductor
www.onsemi.com
Advances in self-administered blood glucose metering (BGM) are revolutionizing the lives of people living with diabetes. The ability to collect, log, and track personal data, coupled with meters that are smaller, easier-to-use and have standardised communications capabilities, means that diabetics receive improved and more personalised treatment. Simultaneously, regulatory agencies are driving for increased accuracy and improved test strip chemistries that address drug interactions that can sometimes provide misleading results.
Fig. 1: Typical blood glucose meter.
Such meters demand power-efficient processing, and semiconductor vendors are rising to the challenge with new generations of ultra-low-power, system-on-chip (SoC) technologies. These technologies integrate sensor interfacing and 32-bit cores powerful enough to handle new user interfaces, capacitive touch inputs, and full-speed communications within a power budget that meets the demanding three-year battery lifetime of a typical strip-based blood glucose meter.
A focus on precision performance and a flexible, software-reconfigurable sensor interface, means that a wide range of strip chemistries can be addressed through software. This also ensures that refinements in strip chemistries to address improved accuracy requirements and unexpected issues like drug interactions can be quickly addressed. A wide range of interfaces including USB and integrated LCD drivers ensure that the solution can be quickly integrated into a wide range of cost-effective applications with minimal off-chip components.
Taking an SoC approachAdopting an SoC approach for microcontrollers to be used in portable self-administered blood glucose testing equipment results in multiple benefits for both the equipment manufacturers and the end users.
For manufacturers, the integration of a wide range of analog and digital functionality in a single chip reduces overall component count and the number of vendors required to realize the design. It can also lead to economies in total system cost. Furthermore, an SoC solution will simplify the system design and assembly as well as saving board space to allow an overall smaller form factor and therefore enhance the portability of the design. The reduction in size is important to end users who need to carry the product with them and want it to impinge on their day-to-day lives as little as possible.
Having “hard-wired” interconnects between the functional blocks within the SoC rather than soldered connections and tracks on a PCB, as is the case with a discrete component solution, enhances system reliability and robustness and can reduce electromagnetic interference (EMI) susceptibility. Interconnects made inside the chip are also well protected against moisture and vibration effects that can be common and unpredictable in portable applications.
Combining analog and digital circuitry in close proximity in a single package requires advanced mixed-signal know-how and technology and is the key to a fully integrated approach. Among the functionality that can be included in addition to the central microcontroller when utilising mixed-signal technology are flash memory and SRAM for storage of user and program data, power management circuitry to maximise efficiency, the sensor interface and analog front-end, on-chip pulse width modulation (PWM), clocks, and I/O including comprehensive data and display interfacing.
With pressure to get the latest products to market quickly and with minimal upfront costs and maximum flexibility, industry-standard programmable cores have become preferred to ASICs for applications such as microcontrollers for BGMs. Processors such as the ARM Cortex-M3 core used in ON Semiconductor’s Q32M210, that has been designed with the needs of portable medical applications in mind, allow straightforward code portability making re-spins of existing products fast and economical.
Delivering high accuracy, predictable operationThe consequences of inaccurate metering in patient monitoring applications such as blood glucose meters can be serious and even life threatening. To support precise sensing as well as accuracy and repeatability in glucose level measurements, a measurement engine is needed that is based on 16-bit ADCs with low noise and excellent linearity specifications.
An accurate, factory-calibrated voltage reference with a low temperature drift is a further prerequisite to help ensure the precision operation of a mixed-signal microcontroller for BGM applications. It establishes a “solid under all conditions” reference when performing the auto calibration that is required before each glucose level measurement.
Another key factor for the operation of a glucose meter is the need for a high level of reliability under any operating condition. A glucose meter is powered by one or more batteries where variations in the supply voltage can have an impact on the quality and reliability of the measurements.
To ensure that the integrity of the application running in the glucose meter is always preserved, integrated flash memory error checking and correction circuitry should be an embedded part of the SoC. Using such a scheme single bit errors occurring at run-time can be corrected and alerts triggered if multi-bit errors occur. This approach alleviates the need to implement less robust error checking and correction through software.
Further, SoC microcontrollers can include system functions such as ‘brown-out detection’ to protect the application against sags in supply voltage that could compromise reliable operation, and low battery detection circuitry to warn the user of the need to charge or change the battery. A watchdog timer, also often known as a ‘computer operating properly timer’, ensures that the system always returns to a deterministic state in the event of a battery supply or other fault condition.
Portability demands power efficiencyGetting the maximum life from batteries is an ongoing challenge that occupies designers in many market sectors. In portable BGM applications a three-year battery life is a desired but demanding requirement.
Ensuring that the main elements of the mixed-signal microcontroller SoC are ultra power efficient is the first step to achieving a design that meets the battery life demands placed on portable medical metering applications. These elements include the processor itself and the sensor interface that incorporates at least one 16-bit ADC and one or more 10-bit digital-to-analog (DAC) converters as well as a number of operational amplifiers and voltage references.
Multiple operating modes, and intelligent power supply monitoring are used to enhance the overall power efficiency further. A typical portable BGM application is only required to be in a full ‘on’ state for approximately five minutes per day, in the case of ON Semiconductor’s Q32M210 (see Fig. 2 ), this means that the ARM Cortex-M3 core is running at 2 MHz and the overall power consumption is approximately 1.6 mA including the current consumption of the ADCs, DACs, voltage references and operational amplifiers. Inactivity prompts the microcontroller to go into “sleep mode,” with a current consumption of 700 nA in that mode. This approach maximizes the operational life of the BGM.
Fig. 2: Functional block diagram of Q32M210 for portable medical devices Multiple strip chemistriesPortable BGM equipment tests the concentration of glucose in the blood (glycaemia) by analysing a drop of blood on a test strip inserted into the meter; most systems measure an electrical characteristic representing the chemical reaction in the test strip to determine the glucose level. As the chemistry of strips varies from manufacturer to manufacturer and technology advances, it is important that the BGM can be quickly and easily re-configured. SoC microcontrollers such as the Q32M210 achieve this through the use of a fully programmable core and a configurable analog front-end and DACs that allow the specific needs of different strip chemistries to be met through software.
Advanced SoC technology is enabling simple yet highly accurate and dependable measuring of glucose concentrations in the blood of diabetes patients. For users, the use of small-form-factor accurate meters can help make diabetes less of a burden, while medical professionals can be confident that their patients are receiving appropriate care. ■
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