Specifications

59Sensors
In addition to all the components shown
in Figure 3, Maxim manufactures the
MAX31855 family of thermocouple-to-
digital converters, which perform the
signal-conditioning functions for J-, K-,
R-, S-, T-, and E-type thermocouples.
These devices simplify the design task
and signicantly reduce the number
of components required to amplify,
cold-junction compensate, and digitize
the thermocouples output. For more
information, refer to Reference Design
5032 at: www.maxim-ic.com/AN5032.
Temperature-Sensor ICs
Temperature-sensor ICs take advantage
of the linear and predictable thermal
characteristics of silicon PN junctions.
Because they are active circuits built
using conventional 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 oset are applied to this voltage
to provide a convenient relationship
between the sensors output voltage
and the die temperature. Temperature
accuracy can be excellent. The
DS600, for example, is the industrys
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 interfaces including SPI/
3-wire, I
2
C, 1-Wire, and PWM. More
complex digital sensors oer 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
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.