3

LT1025

1025fb

TYPICAL PERFOR A CE CHARACTERISTICS

10mV/

°C Output Temperature

Error LT1025

10mV/

°C Output Temperature

Error LT1025A

Supply Current

TEMPERATURE

TO BE MEASURED

T1

T2

Fe

Cu

Cu

CONSTANTAN

LT1025 MUST BE LOCATED

NEXT TO COLD JUNCTION

FOR TEMPERATURE TRACKING

}

LT1025 • AG01

JUNCTION TEMPERATURE (

°C)

*ERROR CURVE FACTORS IN THE NONLINEARITY

TERM BUILT IN TO THE LT1025. SEE THEORY OF

OPERATION IN APPLICATION GUIDE SECTION

–50

–10

–8

–4

–2

0

10

4

0

50

75

LT1025 • G01

–6

6

8

2

–25

25

100

125

GUARANTEED LIMITS*

LT1025

JUNCTION TEMPERATURE (

°C)

*ERROR CURVE FACTORS IN THE NONLINEARITY

TERM BUILT IN TO THE LT1025. SEE THEORY OF

OPERATION IN APPLICATION GUIDE SECTION

–50

–5

–4

–2

–1

0

5

2

0

50

75

LT1025 • G02

–3

3

4

1

–25

25

100

125

GUARANTEED LIMITS*

LT1025A

SUPPLY VOLTAGE (V)

0

0

20

60

80

100

200

140

10

20

25

LT1025 • G03

40

160

180

120

515

30

35

40

DOES NOT INCLUDE 30

µA

PULL-DOWN CURRENT

REQUIRED FOR TEMPERATURES

BELOW 0

°C

PIN 4 TIED TO PIN 5

The LT1025 was designed to be extremely easy to use, but

the following ideas and suggestions should be helpful in

obtaining the best possible performance and versatility

from this new cold junction compensator.

Theory of Operation

A thermocouple consists of two dissimilar metals joined

together. A voltage (Seebeck EMF) will be generated if the

two ends of the thermocouple are at different

temperatures. In Figure 1, iron and constantan are joined

at the temperature measuring point T1. Two additional

thermocouple junctions are formed where the iron and

constantan connect to ordinary copper wire. For the

purposes of this discussion it is assumed that these two

junctions are at the same temperature, T2. The Seebeck

The junctions at T2 are commonly called the cold junction

because a common practice is to immerse the T2 junction

in 0

°C ice/water slurry to make T2 independent of room

temperature variations. Thermocouple tables are based

on a cold-junction temperature of 0

°C.

To date, IC manufacturers efforts to make microminiature

thermos bottles have not been totally successful. There-

fore, an electronically simulated cold-junction is required

for most applications. The idea is basically to add a

sum is the same as if the T2 junction were at a constant 0

°C

instead of at room temperature. This voltage source is

called a cold junction compensator. Its output is designed

to be 0V at 0

°C and have a slope equal to the Seebeck

coefficient over the expected range of T2 temperatures.

Figure 1

To operate properly, a cold junction compensator must be

at exactly the same temperature as the cold junction of the

thermocouple (T2).

Therefore, it is important to locate the

LT1025 physically close to the cold junction with local

temperature gradients minimized. If this is not possible,

APPLICATIO S I FOR ATIO