Maxim > Design=20 support > App = notes=20 > 1-Wire=C2=AE=20 Devices > APP 4659
Maxim > Design = support >=20 App = notes=20 > A/D and=20 D/A Conversion/Sampling Circuits > APP=20 4659
Maxim > Design = support >=20 App = notes=20 > Interface=20 Circuits > APP 4659

May 10, = 2010

Keywords: blood glucose meter, portable = medical,=20 home medical, test-strip, analog front end, blood = sugar,=20 electrochemical, optical-reflectometry, counter = configuration,=20 self biased configuration, blood sample, alternate = site=20 testing
Related=20 parts

Download,=20 PDF format (88kB)

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APPLICATION NOTE 4659

Blood glucose meters

By: John=20 DiCristina

Abstract: This=20 tutorial introduces the different types of glucose = meters. It=20 explains the different types of calibration used for=20 test-strips, and discusses other variables that = designers must=20 consider when selecting products.

Overview

Blood glucose meters and other home = medical=20 devices today are small, portable, and easy to use. = The mark=20 of a good meter is one that the patient will use = regularly and=20 that returns accurate and precise results. Over the = past few=20 years the trend with blood glucose meters has been to = maximize=20 patient comfort and convenience by reducing the volume = of the=20 blood sample required. The blood sample size is now = small=20 enough that alternate-site testing is possible. This=20 eliminates the need to obtain blood from the fingers = and=20 greatly reduces the pain associated with daily = testing.=20 Accurate and precise results have been increased by = using=20 better test strips, electronics, and advanced = measurement=20 algorithms. Other conveniences include speedy results, = edge=20 fill strips, and illuminated test strip ports, to name = just a=20 few.

Meter types

There are continuous and discrete=20 (single-test) meters on the market today, and = implantable and=20 noninvasive meters are in development. Continuous = meters are=20 by prescription only and use a subcutaneous = electrochemical=20 sensor to measure at a programmed interval. = Single-test meters=20 use electrochemical or optical reflectometry to = measure the=20 glucose level in units of mg/dL or mmol/L.

The = majority=20 of blood glucose meters are electrochemical. = Electrochemical=20 test strips have electrodes where a precise bias voltage=20 is applied with a digital-to-analog=20 converter (DAC),=20 and a current=20 proportional to the glucose in the blood is measured = as a=20 result of the electrochemical reaction on the test = strip.=20 There can be one or more channels, and the current is = usually=20 converted to a voltage by a transimpedance=20 amplifier (TIA)=20 for measurement with an analog-to-digital converter = (ADC).=20 The full-scale current measurement of the test strip = is in the=20 range of 10=C2=B5A to 50=C2=B5A with a resolution of = less than 10nA. Ambient=20 temperature needs to be measured because the test = strips=20 are temperature dependent.

Electrochemical=20 blood glucose meter.

Optical-reflectometry = test=20 strips use color to determine the glucose = concentration in the=20 blood. Typically, a calibrated current passes through = two=20 light-emitting diodes (LEDs) that alternately flash = onto the=20 colored test strip. A photodiode senses the = reflectedlight=20 intensity, which is dependent on the color of the test = strip,=20 which, in turn, is dependent on the amount of glucose = in the=20 blood. The photodiode current is usually converted to = a=20 voltage by a TIA for measurement with an ADC. The = full-scale=20 current from the photodiode ranges from 1=C2=B5A to = 5=C2=B5A with a=20 resolution of less than 5nA. Ambient temperature is = required=20 for this method.

Optical-reflectometry=20 blood glucose meter.

Test-strip calibration

The test strips usually = need to=20 be calibrated to the meter to account for = manufacturing=20 variations in the test strips. Calibration is done by = entering=20 a code manually or by inserting a memory device from = the=20 package of test strips. An EPROM or EEPROM=20 memory device enables additional information to be=20 transferred, which is a significant advantage over = manually=20 entering a code. A 1-Wire=C2=AE memory device provides = an=20 additional benefit, because the unique serial=20 identification number in each 1-Wire=20 device ensures that the proper test strip is = used.

Some=20 meters use test strips that do not require any coding = by the=20 user. This "self-calibration" can be accomplished = three ways:=20 with tight manufacturing controls, built-in = calibration on=20 each test strip, or built-in calibration on a pack of = test=20 strips loaded into the meter. A pack of test strips = inside the=20 meter also facilitates testing because these often = small=20 strips do not need to be handled and inserted by the=20 user.

Functional=20 block diagram of a blood glucose meter. For a list of = Maxim's=20 recommended glucose-meter solutions, please go to: http://www.maxi= m-ic.com/solutions/glucose_meter/.

Glucose meter solutions

Accuracy and precision

Both = optical-reflectometry and=20 electrochemical meters need to resolve currents in the = single-digit nano-amp range. To meet the error budget = for a=20 meter, components must have extremely low leakage and = drift=20 over supply voltage, temperature, and time once the = meter has=20 been calibrated during manufacture. An operational = amplifier's=20 key specifications are ultra-low input bias current = (<=20 1nA), high linearity, and stability when connected to = a=20 capacitive electrochemical test strip. The operational=20 amplifier is typically configured as a TIA for = both types=20 of meters. A voltage reference's key specifications = include a=20 temperature coefficient less than 50ppm/=C2=B0C, low = drift over=20 time, and good line and load=20 regulation. A 10- or 12-bit DAC is used to set the = bias=20 voltage for an electrochemical test strip and to set = the LED=20 current for an optical-reflectometry test strip. = Sometimes a=20 comparator=20 is employed with electrochemical test strips to detect = when=20 blood has been applied to the test strip. This saves = power=20 while waiting for blood to be applied to the test = strip, and=20 ensures that the reaction site is fully saturated with = blood.=20 The ADC requirements vary depending on the type of = meter, but=20 most require =E2=89=A5 14-bit resolution and low noise = for repeatable=20 results. Sometimes 12-bit resolution is used when = there is a=20 programmable gain stage before the ADC to extend the = dynamic=20 range.

Temperature measurement

Ideally, the = temperature of=20 the blood on the test strip should be measured, but = usually=20 the ambient temperature near the test strip is = measured.=20 Temperature measurement accuracy varies by test-strip = type and=20 chemistry, but is typically in the =C2=B11=C2=B0C to = =C2=B12=C2=B0C range. This=20 measurement can be accomplished with stand-alone=20 temperature-sensor ICs, or with a remote thermistor=20 or PN junction together with an ADC. Using a = thermistor in a=20 half-bridge configuration driven by the same reference = as the=20 ADC provides more accurate results because this design = eliminates any voltage-reference errors. Remote or = internal PN=20 junctions can be measured with highly precise = integrated=20 analog front-ends (AFEs).

Electrochemical test-strip configurations

Most = test=20 strips are proprietary and vary by meter manufacturer. = The=20 variations include the reagent formulation, the number = of=20 electrodes, the number of channels, and biasing method = of the=20 reagent. The simplest configuration is a self-biased = test=20 strip (Figure 1) which has two electrodes with = current=20 measured at the working electrode and the common = electrode=20 grounded. There can be multiple channels on a single = test=20 strip; the additional channels are used for a = reference=20 measurement, initial blood detection, or to ensure = that the=20 blood has saturated the reaction site.

Figure=20 1. Electrochemical test strip in a self-biased=20 configuration.

An alternate configuration = actively=20 drives both electrodes and measures at the common=20 electrode.

Another more advanced design is a = counter=20 configuration (Figure 2). Here there are three=20 electrodes with current measured at the working = electrode and=20 a force-sense=20 circuit drives the common and reference electrodes. = There is=20 an important advantage to this configuration: the bias = voltage=20 at the reaction site on the test strip is set and = maintained=20 more accurately throughout the measurement. The = disadvantage=20 of this design is its additional complexity and the = larger=20 headroom required to allow the force-sense amplifier = to swing=20 negative to maintain the bias voltage during current=20 flow.

Figure=20 2. Electrochemical test strip in a counter=20 configuration.

Integrated AFE

Maxim's precision AFEs = integrate all=20 the functionality discussed in the previous sections, = and are=20 designed for the specifications and performance = required in=20 blood glucose meters. The AFEs are also suitable for = similar=20 applications such as coagulation and cholesterol=20 meters.

Display and backlighting

Most blood glucose = meters use=20 a simple liquid-crystal display (LCD)=20 with about 100 segments that can be driven with an LCD = driver=20 integrated in the microcontroller. Some meters feature = a more=20 complicated dotmatrix LCD which usually requires using = a=20 module with the glass, bias voltages, and drivers = assembled=20 together. The dot-matrix display also requires = additional=20 memory to store the messages to be displayed. There = are also=20 color displays that require additional and higher = voltages=20 than both the segment or dot-matrix LCDs. Backlighting = can be=20 added by using one or two white LEDs (WLEDs) or an=20 electroluminescent source.

Data interface

The ability to upload test = results to a=20 computer has existed for many years, but utilization = of this=20 data interface has been low. Initially to keep the = cost of the=20 meter down, the incremental cost for this = functionality was=20 designed into a proprietary cable.=20 Today meters are moving from proprietary data = interfaces to=20 industry-standard interfaces such as USB=20 and Bluetooth=C2=AE. The added cost of these open = interfaces is now=20 moving into the meters, a movement driven, in part, by = the=20 Continua Health Alliance=C2=AE and the push to = conveniently upload=20 patient data to your health-care provider.

Audio

Audible indicators range from simple = buzzers to=20 more advanced talking meters for the vision impaired. = A simple=20 buzzer can be driven by one or two microcontroller = port pins=20 with pulse-width modulation (PWM)=20 capability. More advanced voice indicators and even = voice=20 recording for test result notes can be achieved by = adding an=20 audio codec=20 along with speaker and microphone amplifiers.

Power and battery management

Meters with = simple=20 displays can run directly off of a single lithium coin = cell or=20 two alkaline AAA primary batteries. To maximize = battery life,=20 this meter requires electronics capable of running = from 3.6V=20 down to 2.2V for the lithium coin cell or 1.8V for the = alkaline AAAs. If the electronics require a higher or=20 regulated supply voltage, then a step-up switching=20 regulator can be used. Powering down the switching = regulator during sleep mode and running directly off = the=20 batteries extends battery life, as long as the sleep = circuitry=20 can run from the lower battery voltages. Adding a = backlit or a=20 more advanced display will require higher and = sometimes=20 additional voltages. A more advanced power-management = scheme=20 may be required at this point. Rechargeable batteries = such as=20 single-cell lithium=20 ion (Li+) can be used by adding a battery charger = and=20 fuel-gauge circuitry. Charging with USB is certainly a = convenient option for the user, if USB is available in = the=20 meter. If the battery is removable, then = authentication may be=20 required for safety and aftermarket control.

Electrostatic discharge

All meters must pass = 61000-4-2=20 electrostatic=20 discharge (ESD) requirements. Using electronics = with=20 built-in ESD=20 protection or adding ESD line protectors to = exposed traces=20 can help meet this requirement.

Functional scalability

Once the core meter = design is=20 complete using a precision, integrated AFE,=20 the goal is not to redesign that portion of the meter = when=20 another feature is needed later. Instead, standard = parts with=20 a singular function targeted for portable medical = devices can=20 be used to add a feature with minimal disruption. That = minimal=20 disruption translates into lower risk, easier FDA = approvals,=20 and faster time to market. It also means that more = meters will=20 be available with the features that patients want and = need.=20 Blood glucose testing will be more frequent with the=20 predictable result of increased compliance to = acceptable=20 glucose levels and better individual health. =

1-Wire=20 is a registered trademark of Maxim Integrated = Products,=20 Inc.

Continua Health Alliance is a = registered service=20 mark of Continua Health Alliance.

IrDA is a registered service mark of = Infrared=20 Data Association Corporation.

The Bluetooth word mark and logos are = registered=20 trademarks owned by Bluetooth SIG, Inc. and any use of = such=20 marks by Maxim is under license.

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Automatic updates
Would you = like to be=20 automatically notified when new application notes are=20 published in your areas of interest? S= ign=20 up for EE-Mail=E2=84=A2.

Download,=20 PDF format (88kB)
APP 4659: May 10, = 2010=20
AN4659, AN 4659, = APP4659,=20 Appnote4659, Appnote 4659=20 =