LTC2424CG datasheet(34/36 Pages) LINER

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LTC2424/LTC2428

could be used to measure the case temperature of the

RTD-based thermal power measurement scheme described

previously. In general, thermistors yield very good resolu-

tion over a limited temperature range. For the half-bridge

arrangement shown, the LTC2428 can measure tempera-

ture changes over nearly 5 orders of magnitude.

Connected to the LTC2428’s CH6 input, an infrared ther-

mocouple (Omega Engineering OS36-1) can be used in

limited range, noncontact temperature measurement ap-

plications or applications where high levels of infrared

light must be measured. Given the LTC2428’s 1.2ppmRMS

noise performance, measurement resolution using infra-

red thermocouples is approximately 0.25

°C—equivalent

to the resolution of a conventional Type J thermocouple.

These infrared thermocouples are self-contained: 1) they

do not require external cold junction compensation; 2)

they cannot use conventional open thermocouple detec-

tion schemes; and 3) their output impedances are high,

approximately 3k

Ω. Alternatively, conventional thermo-

couples can be connected directly to the LTC2428 (not

shown) and cold junction compensation can be provided

by an external temperature sensor connected to a different

channel (see the thermistor circuit on CH5) or by using the

LT1025, a monolithic cold-junction compensator IC.

The components connected to CH7 are used to sense

daylight or photodiode current with a resolution of 300pA.

In the figure, the photodiode is biased in photoconduc-

tive mode; however, the LTC2428 can accommodate

either photovoltaic or photoconductive configurations.

The photodiode chosen (Hammatsu S1336-5BK) pro-

duces an output of 500mA per watt of optical illumination.

The output of the photodiode is dependent on two factors:

active detector area (2.4mm • 2.4mm) and illumination

intensity. With the 5k resistor, optical intensities up to

368W/m2 at 960nM (direct sunlight is approximately

1000W/m2) can be measured by the LTC2428. With a

resolution of 1nA, the optical dynamic range covers 5

orders of magnitude.

The application circuits shown connected to the LTC2428

demonstrate the mix-and-match capabilities of this multi-

plexed-input, high resolution

∆Σ ADC. Very low level

signals and high level signals can be accommodated with

a minimum of additional circuitry.

APPLICATIONS INFORMATION

LTC2424CG Datasheet(PDF) 34 Page - Linear Technology

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