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4659
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> Interface
Circuits > APP 4659

May 10, 2010

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

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APPLICATION NOTE 4659

Blood glucose meters

By: John
DiCristina

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

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

Meter types
There are continuous and discrete
(single-test) meters on the market today, and implantable and
noninvasive meters are in development. Continuous meters are
by prescription only and 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 where a precise bias voltage
is applied with a digital-to-analog
converter (DAC),
and a current
proportional to the glucose in the blood is measured as a
result of the electrochemical reaction on the test strip.
There can be one or more channels, and the current is usually
converted to a voltage by a transimpedance
amplifier (TIA)
for measurement with an analog-to-digital converter (ADC).
The full-scale current measurement of the test strip is in the
range of 10µA to 50µA with a resolution of less than 10nA. Ambient
temperature needs to be measured because the test strips
are temperature dependent.

Electrochemical
blood glucose meter.

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

Optical-reflectometry
blood glucose meter.

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

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

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

Glucose meter solutions

Accuracy and precision
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.

Temperature measurement
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).

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) which has two electrodes with current
measured at the working electrode and the common electrode
grounded. 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.

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

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

Another more advanced design is a counter
configuration (Figure 2). 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.

Figure
2. Electrochemical test strip in a counter
configuration.

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

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

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

Audio
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.

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, then 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 certainly a
convenient option for the user, if USB is available in the
meter. If the battery is removable, then authentication may be
required for safety and aftermarket control.

Electrostatic discharge
All meters must pass 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.

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

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

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

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

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

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Download,
PDF format (88kB)
APP 4659: May 10, 2010

AN4659, AN 4659, APP4659,
Appnote4659, Appnote 4659

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