Medical Design Briefs: 2021 Technology Leaders - 66

TECHNOLOGY LEADERS Electronics
Developing a Customized Glucometer Using
GreenPAK
G
lucometers play an important
role in managing a diabetic
patient's health issues. Typically,
the patient inserts a disposable
test strip into the meter, pricks their finger,
loads a droplet of blood onto the
test strip, waits a couple of seconds, and
then receives a reading of the current
blood glucose level.
The designs available on the market
today are costly, large, and power inefficient.
The design proposed in this article
uses the analog voltage signal generated
by a blood drop on a customized
glucometer strip to measure glucose levels.
The signal is amplified and fed into
a Dialog GreenPAK™ SLG46580V IC.
The GreenPAK decodes the voltage signal
and compares it with preset thresholds
to determine the glucose level.
The glucometer blood test strip used
in this design is a typical one available on
the market. The GreenPAK design can
be easily modified to adjust the threshold
levels of different glucose ranges. This
example also includes reset and start functions
to make the design more reliable.
Different glucometer blood test strips
may have different connections, so be
sure to check the connections before
implementing this design.
Hardware Schematic
The hardware schematic is shown in
Figure 1. The external 7805 voltage regulator
is used to generate +5 V from the
+12 VDC Adapter. When a blood droplet
is placed on the glucometer strip, a short
circuit is created between the reference
electrode and the counter electrode.
This allows U1 (SLG88104V) to act as a
unity gain amplifier. The output voltage
is equal to the input voltage applied at
the " + " terminal, i.e., 2.1 V.
This short results in current flow
between the counter electrode (now at
2.1 V) and the working electrode. The
magnitude of the current flow is proportional
to the glucose level of the blood
drop.
The op-amp U2 (SLG88104V) acts as
transimpedance amplifier. The output
66
5V
2.1V
U1
+
−
SLG 88104
Counter
Electrode
Glucose
Test Strip
Working
Electrode
C1
5V
100n
R1
10k
U2
2.5V
SLG88104
SLG88104
START
U3
1
2
3
4
5
6
VOUT 3
VIN1
SLG46580
VOUT 2
VOUT 1
VIN0
VOUT 0
C2
4.7uF
2.1V
5V
12V
7805
12V
Input
5V
Output
5V
C3
4.7uF
2.5V
5V
Fig. 1 - Hardware schematic.
voltage of this amplifier can be calculated
with the following formula:
VOUT = [Current flow between the
Counter and Working electrodes] * R1
The 100 nF capacitor C1 is used to
maintain smooth voltage. Op-amp U3 is
another unity gain amplifier that maintains
a constant voltage at its output.
The purpose of using 2.5 and 2.1 V in
this circuit is to maintain a voltage difference
of 400 mV across two of the test
strip electrodes. This is necessary for the
glucometer to function properly. The
output voltage of op-amp U3 is then fed
to the SLG46580V's Pin6, which is configured
as an analog input/output.
Glucometer Blood Test Strip
A typical glucometer strip is shown in
Figure 2. The connections may vary for
different vendor strips; however, there
should always be a reference, counter, and
working electrode. In each test strip, there
is an enzyme called glucose oxidase. This
enzyme reacts with the glucose in the
blood sample and creates gluconic acid.
The gluconic acid then reacts with
another chemical in the testing strip,
called ferricyanide. The ferricyanide and
the gluconic acid combine to create ferrocyanide.
Once the ferrocyanide has
been created, the current moves
through the blood sample on the strip.
The level of current allows the Green -
PAK to read the level of ferrocyanide
and determine how much glucose is in
the sample of blood.
Fig. 2 - Glucometer blood test strip. The connections
of the test strip are: 1) reference electrode, 2) counter
electrode, 3) working electrode, 4) test electrode, and
5) test electrodes. The two electrodes (4 and 5) are
used to detect that the strip is inserted in the slot
reserved for the strip. These two electrodes are not
necessary for the functioning of the glucometer.
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The output voltage from the circuit
used above will range between 0 and 350
mV for typical glucose levels.
GreenPAK Design
The four analog comparators (ACMPs)
are used to determine the voltage of the
Medical Design Briefs 2021 Technology Leaders
Reference
Electrode
VDD
SCL
SDA
10
VDD
GND
19
18
AGND
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