Page 424 - Handbook of Biomechatronics

The Artificial Pancreas 419

Fig. 5 Photos of colorimetric and electrochemical glucose monitors.

from 1 to 5 μA with a resolution of <5nA. The temperature of the strip is
again required.
Examples of colorimetric and electrochemical glucose monitors are
shown in Fig. 5.
Calibration is achieved by entering a code manually or by inserting a
memory device from the package of test strips. An EPROM enables addi-
tional information to be transferred to the monitor, which can be useful.
Some meters are “self-calibrating” either by applying tight manufacturing
control, built-in calibration on each strip, or built-in calibration on a pack
of test strips loaded into the meter.

3.3.1 Accuracy and Precision
Both reflective and electrochemical meters need to accurately measure cur-
rents in the nA range. This requires components with low leakage and min-
imal drift with changes in supply voltage, temperature, and time. For these
applications, an opamp needs an ultralow bias current (typically <1nA) as
well as maintaining good linearity and stability when connected to a capac-
itive test strip (for the electrochemical option). The reference voltage needs a
temperature coefficient of <50ppm/°C, low drift with time and excellent
line and load regulation. A 10- or 12-bit DAC is required to set the bias volt-
age for electrochemical test strips and to set the LED current in reflectance
meters. A 14- or a 12-bit ADC with a programmable gain stage is required to
provide the required dynamic range for reliable operation.

3.3.2 Temperature Measurement
The temperature of the blood on the test strip should be measured, but that is
impractical so the ambient temperature near the strip is measured instead.
This should be accurate to 1°C and can be achieved using a thermometer
IC or a thermistor and one of the ADC channels.