Blood Glucose Meters
Blood glucose meters are handheld instruments that detect glucose levels in blood samples. The devices are used primarily by diabetics.
There are continuous and discrete (single-test) meters on the market today. In addition, continuous meters are available by prescription. These use a subcutaneous electrochemical sensor to measure at a programmed interval.
Single-test meters use electrochemical or optical reflectometry to measure the glucose level in units of mg/dL or mmol/L. The majority of blood glucose meters are electrochemical. Electrochemical test strips have electrodes to which a precise bias voltage is applied. The applied voltage causes an electrochemical reaction on the test strip with the resulting current being proportional to the glucose in the blood. The current is then converted to a glucose scale for display.
Each packet of test strips contains a calibration code that must be entered into the meter for calibration purposes prior to use, although some newer test strip designs have eliminated the calibration step.
A blood glucose meter is a handheld instrument that consists of an electrochemical sensor and a battery-powered embedded system with a display that reads the sensor, processes the sensor reading, displays it, and optionally stores it.
These meters require very precise analog processing in order to determine the glucose level of the sample. Both optical-reflectometry and electrochemical meters need to resolve currents in the single-digit nano-amp range. The devices are generally accurate within a narrow temperature range, so accurate temperature measurement is also required.
Many new designs store readings and can communicate the readings to a PC or smartphone for record keeping.
Electrochemical Test-Strip Configurations
Most test strips are proprietary and vary by meter manufacturer. The variations include the reagent formulation, the number of electrodes, the number of channels, and biasing method of the reagent. The simplest configuration is a self-biased test strip (Figure 1) that has two electrodes with current measured at the working electrode and the common electrode grounded.
Figure 1. Electrochemical test strip in a self-biased configuration
There can be multiple channels on a single test strip; the additional channels are used for a reference measurement, initial blood detection, or to ensure that the blood has saturated the reaction site. An alternate configuration actively drives both electrodes and measures at the common electrode. Another more advanced design is a counter configuration (Figure 2).
Figure 2. Electrochemical test strip in a counter configuration
Here there are three electrodes with current measured at the working electrode, and a force-sense circuit drives the common and reference electrodes. There is an important advantage to this configuration: the bias voltage at the reaction site on the test strip is set and maintained more accurately throughout the measurement. The disadvantage of this design is its additional complexity and the larger headroom required to allow the force-sense amplifier to swing negative to maintain the bias voltage during current flow.
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Sensor Analog Signal Chain
Both optical-reflectometry and electrochemical meters need to resolve currents in the single-digit nano-amp range. To meet the error budget for a meter, components must have extremely low leakage and drift over supply voltage, temperature, and time once the meter has been calibrated during manufacture. An operational amplifier's key specifications are ultra-low input bias current (< 1nA), high linearity, and stability when connected to a capacitive electrochemical test strip. The operational amplifier is typically configured as a TIA for both types of meters. A voltage reference's key specifications include a temperature coefficient less than 50ppm/°C, low drift over time, and good line and load regulation. A 10- or 12-bit DAC is used to set the bias voltage for an electrochemical test strip and to set the LED current for an optical-reflectometry test strip. Sometimes a comparator is employed with electrochemical test strips to detect when blood has been applied to the test strip. This saves power while waiting for blood to be applied to the test strip, and ensures that the reaction site is fully saturated with blood. The ADC requirements vary depending on the type of meter, but most require ≥ 14-bit resolution and low noise for repeatable results. Sometimes 12-bit resolution is used when there is a programmable gain stage before the ADC to extend the dynamic range.
Maxim offers precision single-chip data acquisition systems (DAS) that integrate all the functionality discussed in the previous sections. Our recommended DAS ICs are designed to meet the specifications and performance required in blood glucose meters. These DAS ICs are also suitable for similar applications such as coagulation and cholesterol meters. Click the DAS block in the block diagrams to view recommended products.
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Ideally, the temperature of the blood on the test strip should be measured, but usually the ambient temperature near the test strip is measured. Temperature measurement accuracy varies by test-strip type and chemistry, but is typically in the ±1°C to ±2°C range. This measurement can be accomplished with stand-alone temperature-sensor ICs, or with a remote thermistor or PN junction together with an ADC. Using a thermistor in a half-bridge configuration driven by the same reference as the ADC provides more accurate results because this design eliminates any voltage-reference errors. Remote or internal PN junctions can be measured with highly precise integrated analog front-ends (AFEs).
Maxim's single-chip DAS ICs offer integrated temperature sensors that provide the required level of accuracy for this application.
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User Interface: Display, Audio
Most blood glucose meters use a simple liquid-crystal display (LCD) with approximately 100 segments that can be driven with an LCD driver integrated in the microcontroller. Color displays require additional and higher voltages than both the segment LCDs. Backlighting can be added by using one or two white LEDs.
Audible indicators range from simple buzzers to more advanced talking meters for the vision impaired. A simple buzzer can be driven by one or two microcontroller port pins with pulse-width modulation (PWM) capability. More advanced voice indicators and even voice recording for test result notes can be achieved by adding an audio codec along with speaker and microphone amplifiers.
Maxim offers step-up switching regulators that might be required to power the LCD display and high-brightness LED drivers for display backlighting applications. Click the block diagram to view recommended products.
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All meters must pass IEC 61000-4-2 electrostatic discharge (ESD) requirements. Using electronics with built-in ESD protection or adding ESD line protectors to exposed traces can help meet this requirement.
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Power and Battery Management
Meters with simple displays can run directly off of a single lithium coin cell or two alkaline AAA primary batteries. To maximize battery life, this meter requires electronics capable of running from 3.6V down to 2.2V for the lithium coin cell or 1.8V for the alkaline AAAs. If the electronics require a higher or regulated supply voltage a step-up switching regulator can be used. Powering down the switching regulator during sleep mode and running directly off the batteries extends battery life, as long as the sleep circuitry can run from the lower battery voltages. Adding a backlit or a more advanced display will require higher and sometimes additional voltages. A more advanced power management scheme may be required at this point. Rechargeable batteries such as single-cell lithium ion (Li+) can be used by adding a battery charger and fuel-gauge circuitry. Charging with USB is a convenient option for the user, if USB is available in the meter.
Maxim offers several power regulation and battery management circuits for this application. The exact products for a design will depend on the size and type of battery that is selected.
Please see the block diagram and click on the power supply and battery symbols to view recommended Maxim ICs for this application.
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4.5 to 60V, 5A, High-Efficiency Step-Down ConverterMAX17506
Includes integrated high-side MOSFET with output from 0.9V up to 0.9 x VIN in a 5mm x 5mm TQFN
High-Speed, Half-Bridge MOSFET DriversMAX5064
125V half-bridge, n-channel MOSFET drivers drive high-and low-side MOSFETs in high-voltage applications.
12 Bit & 10 Bit Multichannel Low-Power, High Speed, SAR ADCsMAX11135
Family of 12-/10-/8-bit 500 ksps SAR ADC's with industry-leading 1.5MHz , full linear bandwidth, low-power, serial output, and SampleSet channel sequencing.
Stand-Alone OCV-Based Fuel GaugeDS2786B
This IC estimates available capacity for rechargeable Li-ion (Li+) and Li+ polymer batteries based on the cell voltage in the open-circuit state following a relaxation period.