LT1025ACJ8 Datasheet, PDF (5/12 Pages) Linear Technology – Micropower Thermocouple Cold Junction Compensator

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
D1
V_Z ≈ 6V
V_IN
V_O
LT1025
GND
I^–
R1
47k
I_L
V_OUT
–10mV/°C
R_L
V^– (–15V)
LT1025 • AG03
Figure 3
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^–.
R1= ^V– ,
300µA + I_L
R2
=
V⁺ – V_Z(≈6V)
V^– /R1+ 280µA
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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