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```Application Note
AN2148
Measuring Temperature Using a Thermocouple
By: M. Ganesh Raaja
Associated Project: Yes
Associated Part Family:CY8C26xxx
PSoC Designer Version: 4.00
Associated Application Notes: AN2099, AN2038, AN2101
Summary
This Application Note explains how to measure temperature using a J Type Thermocouple with a
PSoC. Following the principle, any thermocouple input can be measured.
Introduction
Thermocouples are widely used in industrial
applications for the following reasons:
They are robust
They measure over a wide range (-270°
to 3000° C)
They are available in a wide variety of
packages and probes
Thermocouple Principle
A thermocouple consists of two pieces of
dissimilar metals in the form of wire fused at one
end. This is called the hot junction. The other end
is connected to a measuring circuit. This is called
the cold junction. The difference in temperature
between the hot and cold junctions causes an
EMF to develop. This EMF can be measured by
the measuring circuit.
Because the thermocouple is a reference device,
the absolute temperature (hot junction) can be
measured only if the reference (cold junction) is
known. The reference temperature is called the
Cold Junction Temperature. Adding the
thermocouple equivalent EMF of this temperature
to the one measured from the thermocouple is
called Cold Junction Compensation.
There are two methods that can be used to
acquire an accurate temperature. Using a multiorder polynomial equation (Equation 1), the
temperature can be calculated accurately as
close as +0.02°C.
T = (a0 + a1V + a2V2 + ... + anVn) Equation 1
The polynomial coefficients for different types of
thermocouples can be found in Table-3 in the
Appendix.
This method is quite complicated and involves
high precision mathematics, which heavily taxes
the resources of an 8-bit device.
The second method is the use of lookup tables.
Here, we divide the whole measurement range of
the thermocouple into many regions and identify
the coefficient for each region. The higher the
number of regions, the better the accuracy. We
will use this method in this Application Note.
Steps Involved
The whole process can be broken into the
following steps.
1.
There are different types of thermocouples such
as J, K, T, R, and S just to name a few. Each
type has its own temperature coefficient and
range of measurement.
2.
3.
The output of the thermocouple is not linear
throughout the measurement range.
4.
5.
3/16/2004
Revision A
Find Cold Junction Temperature and
thermocouple voltage corresponding to
that temperature. This is the Cold
Junction Compensation voltage.
Measure thermocouple voltage.
Add the Cold Junction Compensation
voltage to the thermocouple voltage.
Find coefficient.
Find temperature.
-1-
AN2148
1. Find Cold Junction Compensation:
Many circuits can be used to measure the
reference temperature; Thermistor, RTD, and
Diode come to mind.
In this circuit, the LM335 Precision Temperature
Sensor is used to sense the Cold Junction
Temperature. This IC has a linear output of 10
mV/°C and can give excellent accuracy without
any calibration. The output of this IC is 0V at
absolute zero (0°K). At 0°C, the output voltage is
2.7315V (273.15°K). The LM335 output is
connected to P2[1], so that it can be directly
configured as an ADC input without using any
Continuous Time analog PSoC blocks.
A diode can also be used to sense room
temperature. Any common diode like 1N4148 can
be used. A diode exhibits a temperature
coefficient of –2.2 mV/°C. A single resistance can
be used to set the diode current. But the absolute
voltage will have some tolerance and will have to
be calibrated.
Correlated Double Sampling (CDS) is performed
while measuring Cold Junction Temperature.
CDS is explained in step #2. First, the ADC input
is shorted to AGND and the output measured.
The LM335 output is connected to the ADC input
and the ADC output is measured. The zero value
is subtracted from this value to get an offsetcorrected reading.
Now, considering a 12-bit ADC and full-scale
voltage of 1.3V (REFHI), the following equation
gives the LM335 voltage.
V = (1.3V / 2048) * ADC Count Equation 2
The LM335 output is at 10 mV/°C which modifies
the equation to:
T = (130 / 2048) * ADC Count Equation 3
This yields temperature directly (in °K). As the
output of LM335 at 0°C is 2.731V with reference
to VSS, and as ADC measurement is with
reference to AGND which is 2.6V, subtracting 13
from the calculated value will give Cold Junction
Temperature directly in °C.
From this temperature, find the corresponding
thermocouple voltage from table reference
(coldJunction[]) in the Appendix.
When we have to measure such small voltages,
the problems of offset error and signal-to-noise
ratio come into consideration. To overcome these
problems we will use CDS and Infinite Impulse
Response (IIR) filter techniques.
Correlated Double Sampling (CDS):
This method reduces offset errors present in the
signal-conditioning amplifiers and the ADC. The
following steps are involved.
1.
2.
3.
Short the inputs. Measure ADC output.
Store as Zero.
Connect inputs to thermocouple.
Measure output. Store as Signal.
Subtract Zero from Signal.
Infinite Impulse Response (IIR) Filter:
This is a low-pass filter implemented in software.
This averages and effectively reduces the noise
from the input signal.
In this application, the IIR filter constant has been
set to 4. This results in poor response time but
very good noise rejection. Most industrial
applications that measure high temperatures do
not need fast response time. For faster response,
the filter constant can be reduced.
For details on modifying the filter constant and
other IIR techniques, see Application Note
AN2099 “Single-Pole IIR Filters. To Infinity
And Beyond!”
The ADC output after CDS is passed through the
low-pass IIR filter. From the output of this filter,
the thermocouple voltage can be calculated by
the following formula:
volts = ADC Counts * Range / fullScale
Equation 4
For calibration purposes, the range is set to 50
mV and fullScale is the ADC Counts when input is
50 mV. For better resolution, ADC Counts is
multiplied by 5,000 and divided by fullScale. The
resolution of the result is 10 uV/count.
3. Cold Junction Compensation:
2. Measure Thermocouple Voltage:
The thermocouple input is fed to an INSAMP
User Module with a gain of 16. The output of J
Type Thermocouple is 69.55 mV at 1200°C.
3/16/2004
The output of the INSAMP is roughly 1.11V at
this temperature. This is fed to an ADCINC12
User Module.
Revision A
Add the Cold Junction Compensation voltage
calculated in step #2 to the measured
thermocouple voltage to get a cold junction
compensated output.
-2-
AN2148
4. Find Coefficient:
Let us consider a J Type Thermocouple. The
EMF table of J Type Thermocouple can be found
in Tables-1 and 2 in the Appendix.
First, we have to build a lookup table from the
EMF table of the J Type Thermocouple. Let us
divide the whole table into 0.64 mV divisions and
identify the coefficient for each division.
Compiling them will give us the lookup table.
As resolution of the measured voltage is 10 uV,
the coefficient is also calculated for °C/10 uV.
Multiply this fractional number by 10,000 to
convert it to an integer.
As the measured voltage is in tens of microvolts,
the thermocouple voltage is divided by 64 to get
the lookup table offset. Sixty-four has been
selected because it is easier to perform the
division by rotating the voltage 6 times to the
right.
The procedure of building the lookup table can be
found in the Excel .xls file attached with the
project.
5. Find Temperature:
Once the thermocouple voltage and coefficient
are known, temperature can be calculated by
multiplying the thermocouple voltage by the
sensitivity
coefficient
for
that
type
of
thermocouple. Some examples of thermocouple
sensitivies are given in Table 1.
Table 1. Thermocouple Coefficients
Type
Sensitivity Coefficient
K
41 uV/°C
E
68 uV/°C
R
10 uV/°C
N
10 uV/°C
The software consists of three main components.
1.
2.
3.
CheckCalibration()
This routine reads from both the thermocouple
and LM335, performs CDS and IIR filtering and
updates vTc and vColdJunction.
This
routine
calculates
Cold
Junction
Temperature from vColdJunction, performs the
Cold Junction Compensation on vTc and
calculates the temperature after finding the
coefficient. It then updates the LCD display with
the
measured
temperature
and
room
temperature.
If the voltage is positive, the coefficient is taken
from the positive lookup table. If the voltage is
negative, the coefficient is taken from the
negative lookup table.
The total time for calculation of temperature and
updating the display is 2.7 mS at a CPU speed of
24 MHz.
CheckCalibration():
This routine performs calibration. A push-button
switch connected to P2[3] is used for Full Scale
Calibration. Another push button connected to
P2[5] is used for Zero Calibration.
When the Zero Calibration switch is pressed, the
value of vTc is stored as zero.
When the Full-Scale Calibration switch is
pressed, the value of vTc is stored as fullScale.
Calibration:
In PSoC Designer, some assembly routines have
been written for 16-bit multiplication and 24-bit
division. These routines are called from C. This is
to minimize the time taken in math operations.
For details on 16-bit multiplication, refer to
Application Note AN2038, and for details on 24bit division, refer to Application Note AN2101.
To support signed division, first the dividend is
tested to determine if it is negative. If it is
negative, the sign is saved, the value is made
positive and division is performed. Then the sign
is restored by 2's complementing the result.
3/16/2004
Software
Revision A
Though CDS takes care of the zero offset error,
for optimum accuracy, a software zero calibration
has been added. This takes care of any residual
offset error.
Apply 0 mV at the thermocouple input
Wait until the display stabilizes
Press the Zero Calibration push button
Apply 50 mV at the thermocouple input
Wait until the display stabilizes
Press the Full-Scale Calibration push
button
The instrument is now calibrated and accurate to
+2°C. For test results see Table-4 in the
Appendix.
-3-
AN2148
The zero calibration procedure can be omitted if
some inaccuracy can be tolerated. In this case,
the error can go up to +4°C.
Name:
Contact:
M. Ganesh Raaja
[email protected]
Conclusion
Using the described method, temperature can be
measured for any thermocouple input. As there
are minimal external components, a low-cost
temperature controller can be built using a PSoC,
and adding some keys, a 4-digit LED display, and
one or two relay outputs. The controller can be
designed to measure various thermocouples like
J, K, R, T, etc. A feature can be implemented to
select the thermocouple and program the set
point using the front panel keys.
TC Input
AGND
VCC
1
27
2
26
3
25
4
24
2.2K
2
9
19
+
LM335
-
P07
P06
P05
P04
P03
P02
P01
P00
P17
P16
P15
P14
P13
P12
P11
P10
SMP
RESET
P27
P26
P25
P24
P23
P22
P21
P20
10
18
11
17
12
16
13
15
LCD_RW
LCD_RS
LCD_RE
D7
D6
D5
D4
1.5K
VCC
LCD_RS
LCD_RW
LCD_E
5
23
6
22
7
21
8
20
D4
D5
D6
D7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
VCC
1
3
LCD CONNECTOR
CY8C26443
1
2
CAL ZERO
CAL-FS
VCC
Figure 1. Schematic Diagram
3/16/2004
Revision A
-4-
AN2148
APPENDIX
Table 1. J Type Thermocouple EMF Table for +ve Temperatures (in mV)
°C
0
1
2
3
4
5
6
7
8
9
0
0.000
0.050
0.101
0.151
0.202
0.253
0.303
0.354
0.405
0.456
10
0.507
0.558
0.609
0.660
0.711
0.762
0.814
0.865
0.916
0.968
20
1.019
1.071
1.122
1.174
1.226
1.277
1.329
1.381
1.433
1.485
30
1.537
1.589
1.641
1.693
1.745
1.797
1.849
1.902
1.954
2.006
40
2.059
2.111
2.164
2.216
2.269
2.322
2.374
2.427
2.480
2.532
50
2.585
2.638
2.691
2.744
2.797
2.850
2.903
2.956
3.009
3.062
60
3.116
3.169
3.222
3.275
3.329
3.382
3.436
3.489
3.543
3.596
70
3.650
3.703
3.757
3.810
3.864
3.918
3.971
4.025
4.079
4.133
80
4.187
4.240
4.294
4.348
4.402
4.456
4.510
4.564
4.618
4.672
90
4.726
4.781
4.835
4.889
4.943
4.997
5.052
5.106
5.160
5.215
100
5.269
5.323
5.378
5.432
5.487
5.541
5.595
5.650
5.705
5.759
110
5.814
5.868
5.923
5.977
6.032
6.087
6.141
6.196
6.251
6.306
120
6.360
6.415
6.470
6.525
6.579
6.634
6.689
6.744
6.799
6.854
130
6.909
6.964
7.019
7.074
7.129
7.184
7.239
7.294
7.349
7.404
140
7.459
7.514
7.569
7.624
7.679
7.734
7.789
7.844
7.900
7.955
150
8.010
8.065
8.120
8.175
8.231
8.286
8.341
8.396
8.452
8.507
160
8.562
8.618
8.673
8.728
8.783
8.839
8.894
8.949
9.005
9.060
170
9.115
9.171
9.226
9.282
9.337
9.392
9.448
9.503
9.559
9.614
180
9.669
9.725
9.780
9.836
9.891
9.947
10.002
10.057
10.113
10.168
190
10.224
10.279
10.335
10.390
10.446
10.501
10.557
10.612
10.668
10.723
200
10.779
10.834
10.890
10.945
11.001
11.056
11.112
11.167
11.223
11.278
210
11.334
11.389
11.445
11.501
11.556
11.612
11.667
11.723
11.778
11.834
220
11.889
11.945
12.000
12.056
12.111
12.167
12.222
12.278
12.334
12.389
230
12.445
12.500
12.556
12.611
12.667
12.722
12.778
12.833
12.889
12.944
240
13.000
13.056
13.111
13.167
13.222
13.278
13.333
13.389
13.444
13.500
250
13.555
13.611
13.666
13.722
13.777
13.833
13.888
13.944
13.999
14.055
260
14.110
14.166
14.221
14.277
14.332
14.388
14.443
14.499
14.554
14.609
270
14.665
14.720
14.776
14.831
14.887
14.942
14.998
15.053
15.109
15.164
280
15.219
15.275
15.330
15.386
15.441
15.496
15.552
15.607
15.663
15.718
290
15.773
15.829
15.884
15.940
15.995
16.050
16.106
16.161
16.216
16.272
300
16.327
16.383
16.438
16.493
16.549
16.604
16.659
16.715
16.770
16.825
310
16.881
16.936
16.991
17.046
17.102
17.157
17.212
17.268
17.323
17.378
320
17.434
17.489
17.544
17.599
17.655
17.710
17.765
17.820
17.876
17.931
330
17.986
18.041
18.097
18.152
18.207
18.262
18.318
18.373
18.428
18.483
340
18.538
18.594
18.649
18.704
18.759
18.814
18.870
18.925
18.980
19.035
350
19.09
19.146
19.201
19.256
19.311
19.366
19.422
19.477
19.532
19.587
360
19.642
19.697
19.753
19.808
19.863
19.918
19.973
20.028
20.083
20.139
370
20.194
20.249
20.304
20.359
20.414
20.469
20.525
20.58
20.635
20.69
380
20.745
20.8
20.855
20.911
20.966
21.021
21.076
21.131
21.186
21.241
390
21.297
21.352
21.407
21.462
21.517
21.572
21.627
21.683
21.738
21.793
400
21.848
21.903
21.958
22.014
22.069
22.124
22.179
22.234
22.289
22.345
410
22.4
22.455
22.51
22.565
22.62
22.676
22.731
22.786
22.841
22.896
420
22.952
23.007
23.062
23.117
23.172
23.228
23.283
23.338
23.393
23.449
430
23.504
23.559
23.614
23.67
23.725
23.78
23.835
23.891
23.946
24.001
440
24.057
24.112
24.167
24.223
24.278
24.333
24.389
24.444
24.499
24.555
3/16/2004
Revision A
-5-
AN2148
450
24.61
24.665
24.721
24.776
24.832
24.887
24.943
24.998
25.053
25.109
460
25.164
25.22
25.275
25.331
25.386
25.442
25.497
25.553
25.608
25.664
470
25.72
25.775
25.831
25.886
25.942
25.998
26.053
26.109
26.165
26.22
480
26.276
26.332
26.387
26.443
26.499
26.555
26.61
26.666
26.722
26.778
490
26.834
26.889
26.945
27.001
27.057
27.113
27.169
27.225
27.281
27.337
°C
0
1
2
3
4
5
6
7
8
9
500
27.393
27.449
27.505
27.561
27.617
27.673
27.729
27.785
27.841
27.897
510
27.953
28.010
28.066
28.122
28.178
28.234
28.291
28.347
28.403
28.460
520
28.516
28.572
28.629
28.685
28.741
28.798
28.854
28.911
28.967
29.024
530
29.080
29.137
29.194
29.250
29.307
29.363
29.420
29.477
29.534
29.590
540
29.647
29.704
29.761
29.818
29.874
29.931
29.988
30.045
30.102
30.159
550
30.216
30.273
30.330
30.387
30.444
30.502
30.559
30.616
30.673
30.730
560
30.788
30.845
30.902
30.960
31.017
31.074
31.132
31.189
31.247
31.304
570
31.362
31.419
31.477
31.535
31.592
31.650
31.708
31.766
31.823
31.881
580
31.939
31.997
32.055
32.113
32.171
32.229
32.287
32.345
32.403
32.461
590
32.519
32.577
32.636
32.694
32.752
32.810
32.869
32.927
32.985
33.044
600
33.102
33.161
33.219
33.278
33.337
33.395
33.454
33.513
33.571
33.630
610
33.689
33.748
33.807
33.866
33.925
33.984
34.043
34.102
34.161
34.220
620
34.279
34.338
34.397
34.457
34.516
34.575
34.635
34.694
34.754
34.813
630
34.873
34.932
34.992
35.051
35.111
35.171
35.230
35.290
35.350
35.410
640
35.470
35.530
35.590
35.650
35.710
35.770
35.830
35.890
35.950
36.010
650
36.071
36.131
36.191
36.252
36.312
36.373
36.433
36.494
36.554
36.615
660
36.675
36.736
36.797
36.858
36.918
36.979
37.040
37.101
37.162
37.223
670
37.284
37.345
37.406
37.467
37.528
37.590
37.651
37.712
37.773
37.835
680
37.896
37.958
38.019
38.081
38.142
38.204
38.265
38.327
38.389
38.450
690
38.512
38.574
38.636
38.698
38.760
38.822
38.884
38.946
39.008
39.070
700
39.132
39.194
39.256
39.318
39.381
39.443
39.505
39.568
39.630
39.693
710
39.755
39.818
39.880
39.943
40.005
40.068
40.131
40.193
40.256
40.319
720
40.382
40.445
40.508
40.570
40.633
40.696
40.759
40.822
40.886
40.949
730
41.012
41.075
41.138
41.201
41.265
41.328
41.391
41.455
41.518
41.581
740
41.645
41.708
41.772
41.835
41.899
41.962
42.026
42.090
42.153
42.217
750
42.281
42.344
42.408
42.472
42.536
42.599
42.663
42.727
42.791
42.855
760
42.919
42.983
43.047
43.111
43.175
43.239
43.303
43.367
43.431
43.495
770
43.559
43.624
43.688
43.752
43.817
43.881
43.945
44.010
44.074
44.139
780
44.203
44.267
44.332
44.396
44.461
44.525
44.590
44.655
44.719
44.784
790
44.848
44.913
44.977
45.042
45.107
45.171
45.236
45.301
45.365
45.430
800
45.494
45.559
45.624
45.688
45.753
45.818
45.882
45.947
46.011
46.076
810
46.141
46.205
46.270
46.334
46.399
46.464
46.528
46.593
46.657
46.722
820
46.786
46.851
46.915
46.980
47.044
47.109
47.173
47.238
47.302
47.367
830
47.431
47.495
47.560
47.624
47.688
47.753
47.817
47.881
47.946
48.010
840
48.074
48.138
48.202
48.267
48.331
48.395
48.459
48.523
48.587
48.651
850
48.715
48.779
48.843
48.907
48.971
49.034
49.098
49.162
49.226
49.290
860
49.353
49.417
49.481
49.544
49.608
49.672
49.735
49.799
49.862
49.926
870
49.989
50.052
50.116
50.179
50.243
50.306
50.369
50.432
50.495
50.559
880
50.622
50.685
50.748
50.811
50.874
50.937
51.000
51.063
51.126
51.188
890
51.251
51.314
51.377
51.439
51.502
51.565
51.627
51.690
51.752
51.815
900
51.877
51.940
52.002
52.064
52.127
52.189
52.251
52.314
52.376
52.438
910
52.500
52.562
52.624
52.686
52.748
52.810
52.872
52.934
52.996
53.057
920
53.119
53.181
53.243
53.304
53.366
53.427
53.489
53.550
53.612
53.673
3/16/2004
Revision A
-6-
AN2148
930
53.735
53.796
53.857
53.919
53.980
54.041
54.102
54.164
54.225
54.286
940
54.347
54.408
54.469
54.530
54.591
54.652
54.713
54.773
54.834
54.895
950
54.956
55.016
55.077
55.138
55.198
55.259
55.319
55.380
55.440
55.501
960
55.561
55.622
55.682
55.742
55.803
55.863
55.923
55.983
56.043
56.104
970
56.164
56.224
56.284
56.344
56.404
56.464
56.524
56.584
56.643
56.703
980
56.763
56.823
56.883
56.942
57.002
57.062
57.121
57.181
57.240
57.300
990
57.360
57.419
57.479
57.538
57.597
57.657
57.716
57.776
57.835
57.894
°C
0
1
2
3
4
5
6
7
8
9
1000
57.953
58.013
58.072
58.131
58.190
58.249
58.309
58.368
58.427
58.486
1010
58.545
58.604
58.663
58.722
58.781
58.840
58.899
58.957
59.016
59.075
1020
59.134
59.193
59.252
59.310
59.369
59.428
59.487
59.545
59.604
59.663
1030
59.721
59.780
59.838
59.897
59.956
60.014
60.073
60.131
60.190
60.248
1040
60.307
60.365
60.423
60.482
60.540
60.599
60.657
60.715
60.774
60.832
1050
60.890
60.949
61.007
61.065
61.123
61.182
61.240
61.298
61.356
61.415
1060
61.473
61.531
61.589
61.647
61.705
61.763
61.822
61.880
61.938
61.996
1070
62.054
62.112
62.170
62.228
62.286
62.344
62.402
62.460
62.518
62.576
1080
62.634
62.692
62.750
62.808
62.866
62.924
62.982
63.040
63.098
63.156
1090
63.214
63.271
63.329
63.387
63.445
63.503
63.561
63.619
63.677
63.734
1100
63.792
63.850
63.908
63.966
64.024
64.081
64.139
64.197
64.255
64.313
1110
64.370
64.428
64.486
64.544
64.602
64.659
64.717
64.775
64.833
64.890
1120
64.948
65.006
65.064
65.121
65.179
65.237
65.295
65.352
65.410
65.468
1130
65.525
65.583
65.641
65.699
65.756
65.814
65.872
65.929
65.987
66.045
1140
66.102
66.160
66.218
66.275
66.333
66.391
66.448
66.506
66.564
66.621
1150
66.679
66.737
66.794
66.852
66.910
66.967
67.025
67.082
67.140
67.198
1160
67.255
67.313
67.370
67.428
67.486
67.543
67.601
67.658
67.716
67.773
1170
67.831
67.888
67.946
68.003
68.061
68.119
68.176
68.234
68.291
68.348
1180
68.406
68.463
68.521
68.578
68.636
68.693
68.751
68.808
68.865
68.923
1190
68.980
69.037
69.095
69.152
69.209
69.267
69.324
69.381
69.439
69.496
1200
69.553
Table 2. J Type Thermocouple EMF Table for -ve Temperatures (in mV)
°C
0
-210
-8.095
-1
-2
-3
-4
-5
-6
-7
-8
-9
-200
-190
-7.890
-7.912
-7.934
-7.955
-7.976
-7.996
-8.017
-8.037
-8.057
-8.076
-7.659
-7.683
-7.707
-7.731
-7.755
-7.778
-7.801
-7.824
-7.846
-7.868
-180
-7.403
-7.429
-7.456
-7.482
-7.508
-7.534
-7.559
-7.585
-7.610
-7.634
-170
-7.123
-7.152
-7.181
-7.209
-7.237
-7.265
-7.293
-7.321
-7.348
-7.376
-160
-6.821
-6.853
-6.883
-6.914
-6.944
-6.975
-7.005
-7.035
-7.064
-7.094
-150
-6.500
-6.533
-6.566
-6.598
-6.631
-6.663
-6.695
-6.727
-6.759
-6.790
-140
-6.159
-6.194
-6.229
-6.263
-6.298
-6.332
-6.366
-6.400
-6.433
-6.467
-130
-5.801
-5.838
-5.874
-5.910
-5.946
-5.982
-6.018
-6.054
-6.089
-6.124
-120
-5.426
-5.465
-5.503
-5.541
-5.578
-5.616
-5.653
-5.690
-5.727
-5.764
-110
-5.037
-5.076
-5.116
-5.155
-5.194
-5.233
-5.272
-5.311
-5.350
-5.388
-100
-4.633
-4.674
-4.714
-4.755
-4.796
-4.836
-4.877
-4.917
-4.957
-4.997
-90
-4.215
-4.257
-4.300
-4.342
-4.384
-4.425
-4.467
-4.509
-4.550
-4.591
-80
-3.786
-3.829
-3.872
-3.916
-3.959
-4.002
-4.045
-4.088
-4.130
-4.173
-70
-3.344
-3.389
-3.434
-3.478
-3.522
-3.566
-3.610
-3.654
-3.698
-3.742
-60
-2.893
-2.938
-2.984
-3.029
-3.075
-3.120
-3.165
-3.210
-3.255
-3.300
-50
-2.431
-2.478
-2.524
-2.571
-2.617
-2.663
-2.709
-2.755
-2.801
-2.847
-40
-1.961
-2.008
-2.055
-2.103
-2.150
-2.197
-2.244
-2.291
-2.338
-2.385
3/16/2004
Revision A
-7-
AN2148
-30
-1.482
-1.530
-1.578
-1.626
-1.674
-1.722
-1.770
-1.818
-1.865
-1.913
-20
-0.995
-1.044
-1.093
-1.142
-1.190
-1.239
-1.288
-1.336
-1.385
-1.433
-10
-0.501
-0.550
-0.600
-0.650
-0.699
-0.749
-0.798
-0.847
-0.896
-0.946
0
0.000
-0.050
-0.101
-0.151
-0.201
-0.251
-0.301
-0.351
-0.401
-0.451
Table 3. Polynomial Constants for Different Thermocouples
Type
E
J
K
R
S
T
a0
0.0
0.0
0.0
0.0
0.0
0.0
a1
1.7057035E-2
1.978425E-2
2.508355E-2
1.8891380E-1
1.84949460E-1
2.592800E-2
a2
-2.3301759E-7
-2.00120204E-7
7.860106E-8
-9.3835290E-5
-8.00504062E-5
-7.602961E-7
a3
6.543558E-12
1.036969E-11
-2.503131E-10
1.3068619E-7
1.02237430E-7
4.637791E-11
a4
-7.3562749E-17
-2.549687E-16
8.315270E-14
-2.2703580E-10
-1.52248592E-10
-2.165394E-15
a5
-1.7896001E-21
3.585153E-21
-1.228034E-17
3.5145659E-13
1.88821343E-13
6.048144E-20
a6
8.4036165E-26
-5.344285E-26
9.804036E-22
-3.8953900E-16
-1.59085941E-16
-7.293422E-25
a7
-1.3735879E-30
5.099890E-31
-4.413030E-26
2.8239471E-19
8.23027880E-20
a8
1.0629823E-35
1.057734E-30
-1.2607281E-22
-2.34181944E-23
a9
-3.2447087E-41
-1.052755E-35
3.1353611E-26
2.79786260E-27
a10
-3.3187769E-30
Table 4. Test Results
Input
Compensated
Expected
Actual
Error
Error on FS
mV
mV
°C
%
-7.90
-6.42
-148
-147
-1
0.08
-4.63
-3.15
-66
-67
1
-0.08
0.00
1.49
29
29
0
0.00
5.27
6.75
127
127
0
0.00
10.78
12.26
227
228
-1
0.08
16.33
17.81
328
328
0
0.00
21.85
23.33
427
428
-1
0.08
27.39
28.88
526
525
1
-0.08
33.10
34.59
625
625
0
0.00
39.13
40.62
724
724
0
0.00
45.49
46.98
824
824
0
0.00
51.88
53.36
924
924
0
0.00
57.95
59.44
1027
1025
2
-0.17
63.79
65.28
1126
1124
2
-0.17
66.68
68.16
1176
1175
1
-0.08
Max Error = 2°C
Test Condition:
Room Temperature = 29°C
Cold Junction Compensation = 1.49 mV
3/16/2004
Revision A
-8-
AN2148
Cypress MicroSystems, Inc.
nd
2700 162 Street SW, Building D
Lynnwood, WA 98037
Phone: 800.669.0557
Fax: 425.787.4641