LT1025
Micropower Thermocouple
Cold Junction Compensator
FEATURES
s
s
s
s
DESCRIPTIO
s
s
80µA Supply Current
4V to 36V Operation
0.5°C Initial Accuracy (A Version)
Compatible with Standard Thermocouples
(E, J, K, R, S, T)
Auxiliary 10mV/°C Output
Available in 8-Lead PDIP and SO Packages
The LT
®
1025 is a micropower thermocouple cold junction
compensator for use with type E, J, K, R, S, and T
thermocouples. It utilizes wafer level and post-package
trimming to achieve 0.5°C initial accuracy. Special curvature
correction circuitry is used to match the “bow” found in all
thermocouples so that accurate cold junction compensation
is maintained over a wider temperature range.
The LT1025 will operate with a supply voltage from 4V to 36V.
Typical supply current is 80µA, resulting in less than 0.1°C
internal temperature rise for supply voltages under 10V.
A 10mV/°C output is available at low impedance, in addition
to the direct thermocouple voltages of 60.9µV/°C (E),
51.7µV/°C (J), 40.3µV/°C (K, T) and 5.95µV/°C (R, S). All
outputs are essentially independent of power supply voltage.
A special kit is available (LTK001) which contains an LT1025
and a custom tailored thermocouple amplifier. The amplifier
and compensator are matched to allow a much tighter specifi-
cation of temperature error than would be obtained by adding
the compensator and amplifier errors on a worst-case basis.
The amplifier from this kit is available separately as LTKA0x.
The LT1025 is available in either an 8-pin PDIP or 8-pin SO
package for temperatures between 0°C and 70°C.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
Thermocouple Cold Junction Compensator
Centigrade Thermometer
Temperature Compensation Network
BLOCK DIAGRA
TYPICAL APPLICATIO
R2
100Ω
FULL-SCALE TRIM
R1
1k
1%
Type K 10mV/°C Thermometer
E 60.9µV/°C
V
IN
J 51.7µV/°C
R3**
255k
1%
+
BOW*
CORRECTION
VOLTAGE
10mV/°C
TEMPERATURE
SENSOR
BUFFER
K,T 40.6µV/°C
R, S 6µV/°C
V
+
V
IN
K
–
V
+
–
LTKA0x
††
R– COMMON
V
0
10mV/°C
–
+
+
V
–
LT1025
V
GND R
– O
TYPE K
R4*
V
–
GND
*CORRECTS FOR BOW
IN
COLD JUNCTION,
NOT
IN PROBE (HOT JUNCTION)
C1
0.1µF
V
–
*R4
≤
30µA , R4 IS NOT REQUIRED
(OPEN) FOR LT1025 TEMPERATURES
≥
0°C
**SELECTED FOR 0°C TO 100°C RANGE
EQUIVALENT. SEE
“AMPLIFIER CONSIDERATIONS”
LT1025 • TA01
††
OR
LT1025 • BD01
U
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C2
0.1µF
V
OUT
10mV/°C
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LT1025
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
E
60.9µV/°C 1
V
IN
2
V
O
3
10mV/°C
GND 4
N8 PACKAGE
8-LEAD PDIP
J
8 51.7µV/°C
K, T
7 40.6µV/°C
R, S
6 6µV/°C
–
5 R
COMMON
S8 PACKAGE
8-LEAD PLASTIC SO
Input Supply Voltage .......................................... 36V
Output Voltage (Forced)........................................ 5V
Output Short-Circuit Duration ..................... Indefinite
Operating Temperature Range
LT1025AC, LT1025C ............................0°C to 70°C
LT1025AM, LT1025M .................. – 55°C to 125°C
Storage Temperature Range ............ – 55°C to 150°C
ORDER PART
NUMBER
LT1025ACN8
LT1025CN8
LT1025CS8
S8 PART MARKING
1025
LT1025AMJ8
LT1025MJ8
T
JMAX
= 150°C,
θ
JA
= 130°C/W (N8)
T
JMAX
= 150°C,
θ
JA
= 190°C/W (S8)
J8 PACKAGE 8-LEAD CERDIP
T
JMAX
= 150°C,
θ
JA
= 100°C/W
OBSOLETE PACKAGE
Consider the N8 Package for Alternate Source
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Temperature Error at
10mV/°C Output (Notes 4, 5)
CONDITIONS
T
J
= 25°C
LT1025A
LT1025
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specificatons are at T
A
= 25°C. V
S
= 5V, Pin 5 tied to Pin 4, unless otherwise noted.
MIN
TYP
0.3
0.5
q
MAX
0.5
2.0
UNITS
°C
°C
Full Temperature Span
Resistor Divider Accuracy
(Notes 2, 4)
V
OUT
= 10mV/°C
LT1025A
E
J
K, T
R, S
E
J
K, T
R, S
See Curve
60.6
51.4
40.3
5.8
60.4
51.2
40.2
5.75
60.9
51.7
40.6
5.95
60.9
51.7
40.6
5.95
80
61.3
52.1
41.0
6.2
61.6
52.3
41.2
6.3
100
150
200
0.02
0.2
µV/°C
µV/°C
µV/°C
µV/°C
µV/°C
µV/°C
µV/°C
µV/°C
µA
µA
µA
°C/V
°C
kΩ
kΩ
kΩ
kΩ
0.05
µA/V
LT1025
Supply Current
4V
≤
V
IN
≤
36V
LT1025AC, LT1025C
LT1025AM, LT1025M
4V
≤
V
IN
≤
36V
0
≤
I
O
≤
1mA
E
J
K, T
R, S
4V
≤
V
IN
≤
36V
q
q
q
q
50
50
Line Regulation (Note 3)
Load Regulation (Note 3)
Divider Impedance
0.003
0.04
2.5
2.1
4.4
3.8
0.01
Change in Supply Current
Note 1:
Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2:
Divider accuracy is measured by applying a 10.000V signal to the
output divider and measuring the individual outputs.
Note 3:
Regulation does not include the effects of self-heating. See
“Internal Temperature Rise” in Application Guide. Load regulation is
30µA
≤
I
O
≤
1mA for T
A
≤
0°C.
Note 4:
To calculate total temperature error at individual thermocouple
outputs, add 10mV/°C output error to the resistor divider error. Total error
for type K output at 25°C with an LT1025A is 0.5°C plus (0.4µV/°C)(25°C)/
(40.6µV/°C) = 0.5°C + 0.25°C = 0.75°C.
Note 5:
Temperature error is defined as the deviation from the following
formula: V
OUT
= 10mV(T) + (10mV)(5.5 • 10
-4
)(T – 25°C)
2
. The second
term is a built-in nonlinearity designed to help compensate the nonlinearity
of the
cold junction.
This “bow” is
≈
0.34°C for a 25°C temperature change.
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LT1025
TYPICAL PERFOR A CE CHARACTERISTICS
10mV/°C Output Temperature
Error LT1025
10
8
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
6
4
2
0
–2
–4
–6
–8
GUARANTEED LIMITS*
LT1025
1
0
–1
–2
–3
–4
CURRENT (µA)
–10
50
25
0
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
*ERROR CURVE FACTORS IN THE NONLINEARITY
TERM BUILT IN TO THE LT1025. SEE THEORY OF
OPERATION IN APPLICATION GUIDE SECTION
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.
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LT1025 • G01
10mV/°C Output Temperature
Error LT1025A
5
4
3
2
GUARANTEED LIMITS*
LT1025A
Supply Current
200
180
160
140
120
100
80
60
40
20
125
DOES NOT INCLUDE 30µA
PULL-DOWN CURRENT
REQUIRED FOR TEMPERATURES
BELOW 0°C
T
J
= 125°C
T
J
= 25°C
T
J
= –55°C
PIN 4 TIED TO PIN 5
125
–5
50
25
0
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
0
0
5
10 15 20 25 30
SUPPLY VOLTAGE (V)
35
40
LT1025 • G02
LT1025 • G03
*ERROR CURVE FACTORS IN THE NONLINEARITY
TERM BUILT IN TO THE LT1025. SEE THEORY OF
OPERATION IN APPLICATION GUIDE SECTION
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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
Cu
Cu
}
V
S
LT1025 MUST BE LOCATED
NEXT TO COLD JUNCTION
FOR TEMPERATURE TRACKING
LT1025 • AG01
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,
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LT1025
APPLICATIO S I FOR ATIO
an extender made of matching thermocouple wire can be
used. This shifts the cold junction from the user termina-
tion to the end of the extender so that the LT1025 can be
located remotely from the user termination as shown in
Figure 2.
LT1025
Fe
“HOT”
JUNCTION
CN
FRONT PANEL
CONNECTOR
CN
EXTENDER
Cu
“NEW” COLD
JUNCTION
LT1025 • AG02
Fe
Cu
AMPLIFIER
Figure 2
The four thermocouple outputs on the LT1025 are
60.9µV/°C (E), 51.7µV/°C (J), 40.6µV/°C (K and T), and
6µV/°C (R and S). These particular coefficients are chosen
to match the room temperature (25°C) slope of the
thermocouples. Over wide temperature ranges, however,
the slope of thermocouples changes, yielding a quasi-
parabolic error compared to a constant slope. The LT1025
outputs have a deliberate parabolic “bow” to help
compensate for this effect. The outputs can be mathemati-
cally described as the sum of a linear term equal to room
temperature slope plus a quadratic term proportional to
temperature deviation from 25°C
squared.
The coefficient
(ß) of the quadratic term is a compromise value chosen to
offer improvement in all the outputs.
V
OUT
=
αT
+
αß(T
–25°)
2
ß
≈
5.5 • 10
– 4
The actual ß term which would be required to best
compensate each thermocouple type in the temperature
range of 0°C to 50°C is: E, 6.6 • 10
–4
; J, 4.8 • 10
–4
;
K, 4.3 • 10
–4
; R, 1.9 • 10
–3
, S, 1.9 • 10
–3
; T, 1 • 10
–3
.
The temperature error specification for the LT1025
10mV/
°
C output (shown as a graph) assumes a ß of
5.5 •10
–4
. For example, an LT1025 is considered “perfect”
if its 10mV/
°
C output fits the equation V
O
= 10mV(T) +
(10mV)(5.5 • 10
–4
)(T – 25
°
C)
2
.
Operating at Negative Temperatures
The LT1025 is designed to operate with a single positive
supply. It therefore cannot deliver proper outputs for
4
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temperatures below zero unless an external pull-down
resistor is added to the V
O
output. This resistor can be
connected to any convenient negative supply. It should be
selected to sink at least 30µA of current. Suggested value
for a – 5V supply is 150kΩ, and for a – 15V supply, 470kΩ.
Smaller resistors must be used if an external load is
connected to the 10mV/°C output. The LT1025 can source
up to 1mA of current, but there is a trade-off with internal
temperature rise.
Internal Temperature Rise
The LT1025 is specified for temperature accuracy assum-
ing no internal temperature rise. At low supply voltages
this rise is usually negligible (≈ 0.05°C at 5V), but at higher
supply voltages or with external loads or pull-down cur-
rent, internal rise could become significant. This effect can
be calculated from a simple thermal formula,
∆T
= (θ
JA
)
(V
+
)(I
Q
+ I
L
), where
θ
JA
is thermal resistance from junction
to ambient, (≈130°C/W), V
+
is the LT1025 supply voltage,
I
Q
is the LT1025 supply current (≈ 80µA) and I
L
is the total
load current including actual load to ground and any pull-
down current needed to generate negative outputs. A
sample calculation with a 15V supply and 50µA pull-down
current would yield, (130°C/W) (15V) (80µA + 50µA) =
0.32°C. This is a significant rise in some applications. It
can be reduced by lowering supply voltage (a simple fix is
to insert a 10V zener in the V
IN
lead) or the system can be
calibrated and specified after an initial warm-up period of
several minutes.
Driving External Capacitance
The direct thermocouple drive pins on the LT1025 (J, K,
etc.) can be loaded with as much capacitance as desired,
but the 10mV/°C output should not be loaded with more
than 50pF unless external pull-down current is added, or
a compensation network is used.
Thermocouple Effects in Leads
Thermocouple voltages are generated whenever dissimi-
lar materials are joined.
This includes the leads of IC
packages,
which may be kovar in TO-5 cans, alloy 42 or
copper in dual-in-line packages, and a variety of other
materials in plating finishes and solders. The net effect
of these thermocouples is “zero” if all are at exactly the
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LT1025
APPLICATIO S I FOR ATIO
same temperature, but temperature gradients exist within
IC packages and across PC boards whenever power is
dissipated. For this reason, extreme care must be used to
ensure that no temperature gradients exist in the vicinity
of the thermocouple terminations, the LT1025, or the
thermocouple amplifier. If a gradient cannot be eliminated,
leads should be positioned isothermally, especially the
LT1025 R
–
and appropriate output pins, the amplifier input
pins, and the gain setting resistor leads. An effect to watch
for is amplifier offset voltage warm-up drift caused by
mismatched thermocouple materials in the wire-bond/
lead system of the IC package. This effect can be as high
as tens of microvolts in TO-5 cans with kovar leads. It has
nothing to do with the actual offset drift specification of the
amplifier and can occur in amplifiers with measured “zero”
drift. Warm-up drift is directly proportional to amplifier
power dissipation. It can be minimized by avoiding TO-5
cans, using low supply current amplifiers, and by using the
lowest possible supply voltages. Finally, it can be accom-
modated by calibrating and specifying the system after a
five minute warm-up period.
Reversing the Polarity of the 10mV/°C Output
The LT1025 can be made to “stand on its head” to achieve
a minus 10mV/°C output point. This is done as shown in
Figure 3. The normal output (V
O
) is grounded and feed-
back is established between the ground pin and the
positive supply pin by feeding both of them with currents
while coupling them with a 6V zener. The ground pin will
V
+
(15V)
I
+
R2
15k
V
IN
V
O
D1
V
Z
≈
6V
LT1025
GND
V
OUT
–10mV/°C
I
–
R1
47k I
L
R
L
V
–
(–15V)
LT1025 • AG03
Figure 3
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now be forced by feedback to generate –10mV/°C
as long
as the grounded output is supplying a net “source” current
into ground.
This condition is satisfied by selecting R1
such that the current through R1 (I
–
) is more than the sum
of the LT1025 supply current, the maximum load current
(I
L
), and the minimum zener current (≈ 50µA). R2 is then
selected to supply more current than I
–
.
V
+
– V
Z
(
≈
6 V)
V
–
R1
=
, R2
=
–
300
µ
A
+
I
L
V /R1
+
280
µ
A
For
±15V
supplies, with I
L
= 20µA maximum, R1 = 47k and
R2 = 15k.
Amplifier Considerations
Thermocouple amplifiers need very low offset voltage and
drift, and fairly low bias current if an input filter is used. The
best precision bipolar amplifiers should be used for type
J, K, E, and T thermocouples which have Seebeck coeffi-
cients of 40µV/°C to 60µV/°C. In particularly critical appli-
cations or for R and S thermocouples (6µV/°C to 15µV/°C),
a chopper-stabilized amplifier is required. Linear Technol-
ogy offers three amplifiers specifically tailored for thermo-
couple applications. The LTKA0x is a bipolar design with
extremely low offset (< 35µV), low drift (<1.5µV/°C), very
low bias current (<1nA), and almost negligible warm-up
drift (supply current is
≈
400µA). It is very cost effective
even when compared with “jellybean” op amps with vastly
inferior specifications.
For the most demanding applications, the LTC1050 and
LTC1052 CMOS chopper-stabilized amplifiers offer 5µV
offset and 0.05µV/°C drift (even over the full military
temperature range). Input bias current is 30pA, and gain
is typically 30 million. These amplifiers should be used for
R and S thermocouples, especially if no offset adjust-
ments can be tolerated, or a large ambient temperature
swing is expected.
Regardless of amplifier type, it is suggested that for best
possible performance, dual-in-line (DIP) packages be
used to avoid thermocouple effects in the kovar leads of
TO-5 metal can packages if amplifier supply current ex-
ceeds 500µA. These leads can generate both DC and AC
offset terms in the presence of thermal gradients in the
package and/or external air motion.
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