LTC2984 datasheet(13/78 Pages) AD

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LTC2984

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OVERVIEW

The LTC2984 measures temperature using the most com-

mon sensors (thermocouples, RTDs, thermistors, and

diodes).Itincludesallnecessaryactivecircuitry,switches,

measurement algorithms, and mathematical conversions

to determine the temperature for each sensor type.

Thermocouples can measure temperatures from as low as

–265°Ctoover1800°C.Thermocouplesgenerateavoltage

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

polynomialsbuiltinforvirtuallyallstandardthermocouples

(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

outputvoltagesaretakenattwodifferentexcitationcurrent

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.Inordertomeasureoneofthesedevicesaprecision

senseresistoristiedinserieswiththesensor.Anexcitation

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

resistorandthermistor/RTDvoltage,calculatesthesensor

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.

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