LTC2995 datasheet(14/90 Pages) AD

14 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

OVERVIEW

The LTM2985 measures the temperature of the most

common sensors (thermocouples, RTDs, thermistors,

activeanalogtemperaturesensorsanddiodes).Itincludes

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°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 LTM2985 allows diodes, active analog

temperature sensors, 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 LTM2985 has these polynomials built in

for virtually all standard thermocouples (J, K, N, E, R,

S, T and B). Additionally, inverse polynomials must be

solved for the cold junction temperature. The LTM2985

simultaneously measures the thermocouple output and

the cold junction temperature and performs all required

calculations to report the thermocouple temperature in °C

or °F. It directly digitizes both positive and negative volt-

ages (down to 50mV below ground) from a single ground

referencedsupply,includessensorburn-outdetectionand

allowsexternalprotection/anti-aliasingcircuitswithoutthe

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 LTM2985 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 LTM2985 automatically generates the

excitation current, simultaneously measures the sense

resistorandthermistor/RTDvoltage,calculatesthesensor

resistance and reports the result in °C. The LTM2985 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 LTM2985 provides the means of inserting custom

coefficients for both RTDs and thermistors.

The LTM2985 includes the capability to measure active

analog output temperature sensors. These sensors output

voltage as a function of temperature. The relationship

between voltage and temperature can be stored in the

LTM2985. These sensors can be used as a stand alone

temperature sensor or as the cold junction compensation

for thermocouple measurements.

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