Specifications

58 Control and Automation Solutions Guide
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 coecient of resistance
and, therefore, are often referred to as
NTCs. There are also positive temperature
coecient (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 that convert thermistor
resistance to a digital format.
A thermistor is often connected to
one or more xed-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 100 and 1k,
although other values are available.
Signal conditioning for an RTD can be
as simple as combining the RTD with
a precision, xed 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, through calculation,
or by 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 approximately
proportional to temperature. There
are several thermocouple types
that 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 the 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 amplication.
Measuring temperature with a
thermocouple is somewhat dicult
because the thermocouple’s output is
low. Measurement is further complicated
because additional thermocouples
are created 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 voltage. The
example circuit in Figure 3 shows one
implementation, which includes a
number of precision components.
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 PCB traces.
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