Specifications

Sensors
Temperature sensing
70 Maxim Industrial Solutions
voltage. The example circuit in Figure
3 shows one implementation, which
includes a number of precision
components.
In addition to all of the compo-
nents shown in Figure 3, Maxim
manufactures the MAX6674 and
MAX6675 which perform the
signal-conditioning functions
for K-type thermocouples. These
devices simplify the design task and
significantly reduce the number of
components required to amplify,
cold-junction compensate, and
digitize the thermocouple’s output.
Temperature-sensor ICs
Temperature-sensor ICs take
advantage of the linear and predict-
able thermal characteristics of silicon
PN junctions. Because they are
active circuits built using conven-
tional semiconductor processes,
these ICs take a variety of forms.
They include many features such as
digital interfaces, ADC inputs, and
fan-control functions that are not
available in other technologies. The
operating temperature range for
temperature-sensor ICs extends as
low as -55°C and as high as +125°C,
with a few products operating to
an upper limit of around +150°C.
Descriptions of common types of
temperature-sensor ICs follow.
Analog temperature sensors
Analog temperature-sensor ICs
convert temperature to voltage
or, in some cases, to current. The
simplest voltage-output analog
temperature sensors have just three
active connections: ground, power-
supply voltage input, and output.
Other analog sensors with enhanced
features have additional inputs or
outputs, for example, comparator or
voltage-reference outputs.
Analog temperature sensors use the
thermal characteristics of bipolar
transistors to develop an output
voltage that is proportional to
temperature. Gain and offset are
applied to this voltage to provide a
convenient relationship between the
sensor’s output voltage and the die
temperature. Temperature accuracy
can be excellent. The DS600, for
example, is the industry’s most
accurate analog temperature sensor,
with guaranteed error less than
±0.5°C from -20°C to +100°C.
Local digital temperature sensors
Integrating an analog temperature
sensor with an ADC is an obvious
way to create a temperature sensor
with a direct digital interface. Such
a device is normally called a digital
temperature sensor or a local
digital temperature sensor. “Local”
indicates that the sensor measures
its own temperature. This operation
contrasts with a remote sensor that
measures the temperature of an
external IC or a discrete transistor.
Basic digital temperature sensors
simply measure temperature
and allow the temperature data
to be read by a number of inter-
faces including 1-Wire®, I
2
C, PWM,
and 3-wire. More complex digital
sensors offer other features, such as
over-/undertemperature outputs,
registers to set trip thresholds for
these outputs, and EEPROM. Maxim
manufactures several local digital
temperature sensors, including the
DS7505 and DS18B20 that guarantee
accuracy of ±0.5°C over a wide
temperature range.
Remote digital temperature
sensors
A remote digital temperature sensor
is also called a remote sensor or a
thermal diode sensor. The remote
sensor measures the temperature
of an external transistor, either a
discrete transistor or one that is inte-
grated on the die of another IC, as
shown in Figure 4. Microprocessors,
field-programmable gate arrays
(FPGAs), and ASICs often include one
or more sensing transistors, usually
called thermal diodes, similar to the
one shown in Figure 4.
THERMOCOUPLE
TEMPERATURE
SENSOR
ADC
(12 BITS TO 24 BITS)
VOLTAGE
REFERENCE
IN1
TO
MICROCONTROLLER
IN2
PRECISION
AMPLIFIER
PRECISION RESISTORS
Figure 3. Example of a thermocouple signal-conditioning circuit.