LTC2984
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For more information www.linear.com/LTC2984
OVERVIEW
The LTC2984 measures temperature using the most com-
mon sensors (thermocouples, RTDs, thermistors, and
diodes). It includes all necessary active circuitry, switches,
measurement algorithms, and mathematical conversions
to determine the temperature for each sensor type.
Thermocouples can measure temperatures from as low as
–265°C to over 1800°C. Thermocouples generate a voltage
as a function of the temperature difference between the tip
(thermocouple temperature) and the electrical connection
on the circuit board (cold junction temperature). In order
to determine the thermocouple temperature, an accurate
measurement of the cold junction temperature is required;
this is known as cold junction compensation. The cold
junction temperature is usually determined by placing a
separate (non-thermocouple) temperature sensor at the
cold junction. The LTC2984 allows diodes, RTDs, and
thermistors to be used as cold junction sensors. In order
to convert the voltage output from the thermocouple into
a temperature result, a high order polynomial equation (up
to 14th order) must be solved. The LTC2984 has these
polynomials built in for virtually all standard thermocouples
(J, K, N, E, R, S, T, and B). Additionally, inverse polyno-
mials must be solved for the cold junction temperature.
The LTC2984 simultaneously measures the thermocouple
output and the cold junction temperature and performs
all required calculations to report the thermocouple tem-
perature in °C or °F. It directly digitizes both positive and
negative voltages (down to 50mV below ground) from a
single ground referenced supply, includes sensor burn-
out detection, and allows external protection/anti-aliasing
circuits without the need of buffer circuits.
Diodes are convenient low cost sensor elements and
are often used to measure cold junction temperatures in
thermocouple applications. Diodes are typically used to
measure temperatures from –60°C to 130°C, which is
suitable for most cold junction applications. Diodes gen-
erate an output voltage that is a function of temperature
and excitation current. When the difference of two diode
output voltages are taken at two different excitation current
levels, the result (∆VBE) is proportional to temperature.
The LTC2984 accurately generates excitation currents,
measures the diode voltages, and calculates the tempera-
ture in °C or °F.
RTDs and thermistors are resistors that change value as a
function of temperature. RTDs can measure temperatures
over a wide temperature range, from as low as –200°C to
850°C while thermistors typically operate from –40°C to
150°C. In order to measure one of these devices a precision
sense resistor is tied in series with the sensor. An excitation
current is applied to the network and a ratiometric mea-
surement is made. The value, in Ω, of the RTD/thermistor
can be determined from this ratio. This resistance is used
to determine the temperature of the sensor element using
a table lookup (RTDs) or solving Steinhart-Hart equations
(thermistors). The LTC2984 automatically generates the
excitation current, simultaneously measures the sense
resistor and thermistor/RTD voltage, calculates the sensor
resistance and reports the result in °C. The LTC2984 can
digitize most RTD types (PT-10, PT-50, PT-100, PT-200,
PT-500, PT-1000, and NI-120), has built in coefficients
for many curves (American, European, Japanese, and
ITS-90), and accommodates 2-wire, 3-wire, and 4-wire
configurations. It also includes coefficients for calculat-
ing the temperature of standard 2.252k, 3k, 5k, 10k , and
30k thermistors. It can be configured to share one sense
resistor among multiple RTDs/thermistors and to rotate
excitation current sources to remove parasitic thermal
effects. In addition to built-in linearization coefficients,
the LTC2984 provides the means of inserting custom
coefficients for both RTDs and thermistors.