LT1025ACJ8 Datasheet, PDF (3/12 Pages) Linear Technology – Micropower Thermocouple Cold Junction Compensator

TYPICAL PERFOR A CE CHARACTERISTICS
10mV/°C Output Temperature
Error LT1025
10
8
6
GUARANTEED LIMITS*
LT1025
4
2
0
–2
–4
–6
–8
–10
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
LT1025 • G01
*ERROR CURVE FACTORS IN THE NONLINEARITY
TERM BUILT IN TO THE LT1025. SEE THEORY OF
OPERATION IN APPLICATION GUIDE SECTION
10mV/°C Output Temperature
Error LT1025A
5
4
3
GUARANTEED LIMITS*
LT1025A
2
1
0
–1
–2
–3
–4
–5
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
LT1025 • G02
*ERROR CURVE FACTORS IN THE NONLINEARITY
TERM BUILT IN TO THE LT1025. SEE THEORY OF
OPERATION IN APPLICATION GUIDE SECTION
LT1025
Supply Current
200
DOES NOT INCLUDE 30µA
180 _{PULL-DOWN CURRENT}
160
REQUIRED FOR TEMPERATURES
BELOW 0°C
140
T_J = 125°C
120
100
80
T_J = 25°C
60
40
20
T_J = –55°C
PIN 4 TIED TO PIN 5
0
0 5 10 15 20 25 30 35 40
SUPPLY VOLTAGE (V)
LT1025 • G03
APPLICATIO S I FOR ATIO
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
voltage, V_S, is the product of the Seebeck coefficient α,
and the temperature difference, T1 – T2; V_S = α (T1 – T2).
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
temperature dependent voltage to V_S such that the voltage
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.
TEMPERATURE ^T1
TO BE MEASURED
Fe
T2
CONSTANTAN
Figure 1
Cu
}^VS
Cu
LT1025 MUST BE LOCATED
NEXT TO COLD JUNCTION
FOR TEMPERATURE TRACKING
LT1025 • AG01
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,
1025fb
3