Specifications

Sensors
Temperature sensing
www.digikey.com/maxim-industrial 69
Thermistors
Thermistors are temperature-
dependent resistors, usually made
from semiconducting materials like
metal-oxide ceramics or polymers.
The most widely used thermistors
have a negative temperature co-
efficient of resistance and, therefore,
are often referred to as NTCs. There
are also positive temperature co-
efficient (PTC) thermistors.
Thermistor characteristics include
a moderate temperature range
generally up to +150°C, although
some are capable of much higher
temperatures; low-to-moderate cost
depending on accuracy; and poor,
but predictable linearity. Thermistors
are available in probes, in surface-
mount packages, with bare leads,
and in a variety of specialized
packages. Maxim also manufactures
ICs like the MAX6682 and MAX6698
that convert thermistor resistance to
a digital format.
A thermistor is often connected to
one or more fixed-value resistors to
create a voltage-divider. The output
of the divider is typically digitized by
an ADC. The thermistor’s nonlinearity
can be corrected either by a lookup
table or by calculation.
RTDs
Resistance temperature detectors
(RTDs) are resistors whose resistance
varies with temperature. Platinum
is the most common, most accurate
wire material; platinum RTDs are
referred to as Pt-RTDs. Nickel, copper,
and other metals can also be used to
make RTDs.
RTD characteristics include a wide
temperature range up to +750°C,
excellent accuracy and repeatability,
and reasonable linearity. For Pt-RTDs,
the most common values for nominal
resistance at 0°C are 100Ω and 1kΩ,
although other values are available.
Signal conditioning for an RTD can
be as simple as combining the RTD
with a precision, fixed resistor to
create a voltage-divider, or it can be
more complex, especially for wide-
range temperature measurements.
A common approach consists of a
precision current source, a voltage
reference, and a high-resolution ADC,
as shown in Figure 1. Linearization
can be performed with a lookup
table, calculation, or external linear
circuits.
Thermocouples
Thermocouples are made by joining
two wires of dissimilar metals. The
point of contact between the wires
generates a voltage that is approxi-
mately proportional to temperature.
There are several thermocouple
types which are designated by
letters. The most popular is the
K type.
Thermocouple characteristics
include a wide temperature range
up to +1800°C; low cost, depending
on package; very-low-output
voltage of about 40µV per °C for a
K-type device; reasonable linearity;
and moderately complex signal
conditioning, i.e., cold-junction
compensation and amplification.
Measuring temperature with a ther-
mocouple is somewhat difficult
because the thermocouple’s output
is low. Measurement is further
complicated because additional
thermocouples are created at the
point where the thermocouple
wires contact the copper wires (or
traces) that connect to the signal-
conditioning circuitry. This contact
point is called the cold junction (see
Figure 2). To accurately measure
temperature with a thermocouple, a
second temperature sensor must be
added at the cold junction, as shown
in Figure 3. Then the temperature
measured at the cold junction is
added to the value indicated by the
measurement of the thermocouple
PRECISION
CURRENT
SOURCE
ADC
(12 BITS TO 16 BITS)
INPUT
TO MICROCONTROLLER
VOLTAGE
REFERENCE
RTD
THERMOCOUPLE
COLD JUNCTION
METAL 1
METAL 2
COPPER
WIRE
COPPER
WIRE
V
OUT
Figure 1. Simplified RTD signal-conditioning circuit.
Figure 2. Simple thermocouple circuit. The junction between metal 1 and metal 2 is the main
thermocouple junction. Other thermocouples are present where the metal 1 and metal 2 wires join
with the measuring device’s copper wires or PC-board (PCB) traces.