Datasheet

6
LT1025
1025fb
Figure 4. Thermocouple Nonlinearity, 0°C to 400°C
Figure 5. Offset Curve Fitting
In many situations, thermocouples are used in high noise
environments, and some sort of input filter is required.
(See discussion of input filters). To reject 60Hz pick-up
with reasonable capacitor values, input resistors in the
10k-100k range are needed. Under these conditions, bias
current for the amplifier needs to be less than 1nA to avoid
offset and drift effects.
To avoid gain error, high open loop gain is necessary for
single-stage thermocouple amplifiers with 10mV/°C or higher
outputs. A type K amplifier, for instance, with 100mV/°C
output, needs a closed loop gain of ≈2,500. An ordinary op
amp with a minimum open loop of 50,000 would have an
initial gain error of (2,500)/(50,000) = 5%! Although closed
loop gain is commonly trimmed, temperature drift of open
loop gain will have a very deleterious effect on output
accuracy. Minimum suggested open loop gain for type E, J,
K, and T thermocouples is 250,000. This gain is adequate for
type R and S if output scaling is 10mV/°C or less.
Suggested Amplifier Types
SUPPLY VOLTAGE
THERMOCOUPLE ±15V ±5V SINGLE SUPPLY
E, J, K, T LTKA0x LTKA0x LTC1050
LT1012 LT1012 LTC1052
LT1001 LT1001 LT1006
LTC1050
LTC1052
LT1006
R, S LTKA0x LTC1050 LTC1050
LT1012 LTC1052 LTC1052
LTKA0x LT1006
Thermocouple Nonlinearities
Thermocouples are linear over relatively limited temperature
spans if accuracies of better than 2°C are needed. The graph
in Figure 4 shows thermocouple nonlinearity for the
temperature range of 0°C to 400°C. Nonlinearities can be
dealt with in hardware by using offsets, breakpoints, or power
series generators. Software solutions include look-up tables,
power series expansions, and piece-wise approximations.
For tables and power series coefficients, the reader is referred
to the ASTM Publication 470A.
Hardware correction for nonlinearity can be as simple as an
offset term. This is shown in Figure 5. The thermocouple
shown in the figure has an increasing slope (α) with
temperature. The temperature range of interest is between T
L
and T
H
, with a calibration point at T
M
. If a simple amplifier is
used and calibrated at T
M
, the output will be very high at T
L
and very low at T
H
. Adding the proper offset term and
calibrating at T1/6 or T5/6 can significantly reduce errors. The
technique is as follows:
1. Calculate amplifier gain:
G = (SF) (T
H
– T
L
)/(V
H
– V
L
)
SF = Output scale factor, e.g., 10mV/°C
V
H
= Thermocouple output at T
H
V
L
= Thermocouple output at T
L
2. Use precision resistors to set gain or calibrate gain by
introducing a precision “delta” input voltage and trimming
for proper “delta” output.
APPLICATIO S I FOR ATIO
WUUU
TEMPERATURE (°C)
0
ERROR TYPE E AND T (°C)
20
15
12.5
10
5
100
200
250
LT1025 • G04
17.5
2.5
0
7.5
8
6
5
4
2
7
1
0
3
ERROR TYPE J AND K (°C)
50 150
300
350
400
K→SCALE
J→SCALE
SCALE←E
SCALE←T
TEMPERATURE (°C)
0
OUTPUT (V)
V
L
V
H
LT1025 • G05
T1/6
T
H
T
L
T
M
T5/6
THERMOCOUPLE
SIMPLE AMPLIFIER
OFFSET AMPLIFIER
ERROR AFTER OFFSETTING
ERROR BEFORE OFFSETTING