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參數(shù)資料
| 型號(hào)： | LT1025ACN8 |
| 廠商： | Linear Technology |
| 文件頁(yè)數(shù)： | 9/12頁(yè) |
| 文件大?。?/td> | 0K |
| 描述： | IC THERMO COMPNSATR MICRPWR 8DIP |
| 標(biāo)準(zhǔn)包裝： | 50 |
| 類型： | 熱電偶調(diào)節(jié)器 |
| 輸入類型： | 電壓 |
| 輸出類型： | 電壓 |
| 電流 - 電源： | 150µA |
| 安裝類型： | 通孔 |
| 封裝/外殼： | 8-DIP（0.300"，7.62mm） |
| 供應(yīng)商設(shè)備封裝： | 8-PDIP |
| 包裝： | 管件 |
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-stagethermocoupleamplifierswith10mV/
°Corhigher
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
LTC1050
LT1012
LTC1052
LT1001
LT1006
LTC1050
LTC1052
LT1006
R, S
LTKA0x
LTC1050
LT1012
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
dealtwithinhardwarebyusingoffsets,breakpoints,orpower
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 TL
and TH, with a calibration point at TM. If a simple amplifier is
used and calibrated at TM, the output will be very high at TL
and very low at TH. 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) (TH – TL)/(VH – VL)
SF = Output scale factor, e.g., 10mV/
°C
VH = Thermocouple output at TH
VL = Thermocouple output at TL
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
WU
UU
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)
VL
VH
LT1025 G05
T1/6
TH
TL
TM
T5/6
THERMOCOUPLE
SIMPLE AMPLIFIER
OFFSET AMPLIFIER
ERROR AFTER OFFSETTING
ERROR BEFORE OFFSETTING
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