LTC2400CS8 datasheet(34/40 Pages) LINER

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LTC2400

TYPICAL APPLICATIONS

thermocouple with the highest output is type E, at about

70mV. This circuit does not provide curvature correction

for the Seebeck effect at the cold junction. If the applica-

tion requires very high accuracy, the temperature of the

cold junction should be determined via a separate input

to the A/D, using an RTD for example. The cold junction

compensation can be performed by implementing the

thermocouple’s NBS polynominal curvature correction

in software. (The input to the LTC2400 can be multi-

plexed using the LTC1391 with little degradation.) If a

separate temperature sensor is used to monitor the cold

junction, the connection from the thermocouple to the

LTC2400 can be direct. The junctions formed at the point

where the thermocouple leads meet different metal (e.g.,

copper traces) must be equal in temperature, and the

cold junction sensor must be mounted at that point. Any

temperature differential between the leads, or any differ-

ential between the leads and the temperature sensor will

introduce an error into the reading.

Figure 36 shows an inexpensive circuit with removal of the

DC offset. The output of the LT

®1077 is attenuated in order

to produce the required coefficient, as well as reduce the

noise and offset error contribution. If used with a ther-

mistor, this circuit can be modified to produce curvature

correction. The removal of the offset associated with diode

forward voltage, or the 273

°K overhead on some mono-

lithic temperature sensors, simplifies the use of substan-

tial gain after the thermocouple. Chopper amplifiers such

as the LTC1050 can extend the noise floor of the LTC2400

by as much as a factor of 10 to 20. The use of a gain of 20

in front of the LTC2400 can extend the resolution of a

thermocouple application to 0.02

°C or better.

If absolute accuracy is not important, the use of a low

noise bipolar amplifier, such as the LT1028, can extend

the resolution an additional order of magnitude.

Note that achieving high accuracy in the circuit in Figure 36

requires a calibration sequence for circuit offset and gain

correction.

VIN

SDO

SCK

6.1

µV/°C

1mV/

°C

–

174k*

V +

V –

*RECOMMENDED 0.1%,

±5ppm IRC AFD SERIES CHIP RESISTORS

10k

60Hz

SELECT R3 FOR

THERMOCOUPLE TYPE

S: 6.19

Ω

K: 39.2

Ω

J: 49.9

Ω

E: 61.9

Ω

50Hz

VREF

VCC

0.1

µF

GND

LTC2400

2400 F35

1k*

6.19

Ω

226

Ω*

10k*

–

LT1077

LM334

SO-8

Figure 36. Inexpensive Amplifier Improves Cold Junction Compensation

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