LTC2400CS8 datasheet(33/40 Pages) LINER

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LTC2400

Thermocouple Applications

Figure 35 shows a thermocouple interface circuit that

demonstrates the practicality of direct connection to the

LTC2400 using even the lowest output thermocouples (in

this case, a type S thermocouple, with a full-scale output

of 18mV).

This topology is the least costly solution for thermocouple

sensing. As shown, it is capable of resolving approxi-

mately 0.25

°Cwithoutaveraging.SincetheLTC2400does

not exhibit any easily discernible quantization effects,

averaging can significantly extend the resolution for slow

changing processes.

In this circuit, a 1N4148 diode provides cold junction

compensation by producing, at the positive terminal of the

thermocouple, an approximation of the average Seebeck

coefficient for a type S thermocouple over the temperature

range expected at the cold junction (0

°C to 40°C). If the

operating range is less, the coefficient can be adjusted to

produce a better match for the range anticipated. This

basic circuit can be used with other thermocouples by

changing the divide ratio to suit the Seebeck coefficient of

the type chosen (see table).

TYPICAL APPLICATIONS

This circuit produces a DC offset at the cold junction

reference point, of 1mV to 15mV, which must be nulled out

in software. This DC offset, resulting from the forward

voltage of the diode, is variable from device to device and

must be calibrated for each unit.

Since the temperature coefficient of the 1N4148 diode is

not guaranteed, a trim should be provided to accommo-

date a coefficient from 1.7mV/

°C to 2.3mV/°C. Alterna-

tively, a transistor can be used as a sensor with Omega

Engineering thermocouple circuit board connectors that

are available with TO-92 transistor retainer clips, placing

the transistor in physical contact with the cold junction.

The 1M resistor RTC shown is intended as an open-circuit

detection scheme, producing full scale at the input of the

LTC2400. Note that this resistor contributes to the offset

and must have low TC, as should the resistors R2 and R3.

Since R1 provides forward bias for the diode, its tempera-

ture coefficient is not as critical.

The circuit in Figure 35 uses only 12% of the LTC2400’s

input range and is able to accommodate the full-scale

output of all thermocouple types. The commonly used

VIN

SDO

SCK

THERMOCOUPLE

43.2k

10k

VREF

VCC

0.1

µF

GND

LTC2400

2400 F35

RTC

–

1N4148

COLD JUNCTION

ISOTHERMAL

R2*

– 2mV/

°C

60Hz

*25ppm, 1% TOLERANCE

SINGLE POINT GROUND

50Hz

–SB

R3*

100

Ω

*20

°C ≤ TA ≤ 50°C

THERMOCOUPLE

TYPE

SEEBECK

COEFFICIENT*

50.2

µV/°C

39.2

µV/°C

6.15

µV/°C

3.83k

4.99k

32.4k

Figure 35. Diode Cold Junction Compensation

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