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AN-395
APPLICATION NOTE
ONE TECHNOLOGY WAY
•
P.O. BOX 9106
•
NORWOOD, MASSACHUSETTS 02062-9106
•
617/329-4700
Interfacing the AD22100 Temperature Sensor to a Low Cost
Single-Chip Microcontroller
by Norm Bernstein
The AD22100 is a new monolithic silicon temperature
sensor with several unique attributes. These attributes
make it easier to interface the AD22100 to microcontrol-
lers than other temperature sensor ICs. Available in
both TO-92 and SOIC packages, the AD22100 features
single-supply operation, the ability to measure negative
temperatures without requiring a negative power sup-
ply, and a voltage output that is ratiometric to its power
supply, rather than to an absolute voltage.
Other IC temperature sensors rely on the physics of cer-
tain silicon circuit topologies to measure temperature. In
particular, the AD590, and its successors, make use of
the temperature coefficient of the differential voltage
between two silicon junctions (operating at different
current densities) to produce an output that is propor-
tional to absolute temperature. This kind of circuit, often
called a PTAT circuit, is the basis of all IC temperature
sensors prior to the AD22100.
The PTAT circuit, however, suffers from several liabili-
ties. Since the circuit “pivots” at absolute zero, it is nec-
essary to suppress an offset term of 273 °C in order to
make measurements at 0 °C. More importantly, PTAT
circuits are calibrated in
absolute
units, and produce
output voltages or currents that require a precision
measurement with respect to a calibrated reference.
The AD22100 does not use a PTAT circuit as the basis of
its measurement technique. Instead, the AD22100 uses
the temperature coefficient of a carefully deposited thin
film resistance, much like an RTD (Resistance Tempera-
ture Detector), as the measuring element. By using this
technique, the output of the circuit can be arranged such
that there is no need to suppress a large offset term to
measure temperatures in the device’s operating range.
More importantly, the output is ratiometric to the power
supply, which eliminates the requirement for a precision
calibrated reference voltage in the measurement chain.
A simplified schematic of the AD22100 is shown in Fig-
ure 1. The temperature sensing resistor is excited by a
stable current source; since the temperature coefficient
of the sensing resistor is linear, the output is therefore a
voltage that is linear with respect to temperature.
V+
Ι
V
OUT
R
T
Figure 1. Simplified Schematic
The other resistors in the circuit serve to establish a
particular output offset at a specified temperature and
power supply voltage (+1.375 V at 0 °C with V+ = 5 V),
and provide for a specified gain versus temperature at
the output (22.5 mV per °C). Most importantly, the circuit
configuration provides for ratiometricity; as the power
supply voltage changes, the output will remain at the
same ratio to the power supply voltage (assuming tem-
perature is constant). This important attribute will be uti-
lized in the interface example described in this
applications note.
