AD537
REV. C
8
THERMOCOUPLE INPUT
The output of a Chromel-Constantan (Type E) thermocouple,
using a reference junction at 0癈, varies from 0mV to 53.14 mV
over the temperature range 0癈 to +700癈 with a slope of
80.678礦/degree over most of its range and some nonlinearity
over the range 0癈 to +200癈. For this example, we assume
that it is desired to indicate temperature in Degrees Celsius
using a counter/display with a 100ms gate width. Thus, the V-F
converter must deliver an output of 7kHz for an input of
53.14mV. If very precise operation down to 0癈 is imperative,
some sort of linearizing is necessary (see, for example, Analog
Devices Nonlinear Circuits Handbook, pp. 9297) but in many
cases operation is only needed over part of the range.
The circuit shown in Figure 14 provides good accuracy from
+300癈 to +700癈. The extrapolation of the temperature volt-
age curve back to 0癈 shows that an offset of 3.34mV is
required to fit the curve most exactly. This small amount of
voltage can be introduced without an additional calibration step
using the +1.00V output of the AD537. To adjust the scale, the
thermocouple should be raised to a known reference tempera-
ture near 500癈 and the frequency adjusted to value using R1.
The error should be within ?.2% over the range 400癈 to
700癈.
1
2
14
13
5
6
7
10
9
8
3
4
12
11
AD537
+5V
V
LOGIC
10Hz/?/SPAN>C
0.005礔
47k
I
OFFSET
H 21?/SPAN>A
120
R1 SCALE
50
T
MEAS
T
REF
(0?/SPAN>C)
0 TO 53mV
BUF
DRIVER
CURR-
TO-FREQ
CONV
PRECISION
VOLTAGE
REFERENCE
V
T
V
R
360礎
F
S
Figure 14. Thermocouple Interface with First-Order
Linearization
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14-Lead Side-Brazed Ceramic DIP (TO116)
(D14)
PIN 1
0.310 (7.87)
0.220 (5.59)
0.098 (2.49) MAX
0.005 (0.13) MIN
8
7
14
1
0.200
(5.08)
MAX
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36)
0.070 (1.78)
0.030 (0.76)
0.100
(2.54)
BSC
0.060 (1.52)
0.015 (0.38)
0.150
(3.81)
MIN
SEATING
PLANE
0.785 (19.94) MAX
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
10-Lead Metal Can (TO-100)
(H-10A)
36?/SPAN>
BSC
0.034 (0.86)
0.028 (0.71)
0.045 (1.14)
0.029 (0.74)
0.115
(2.92)
BSC
0.230
(5.84)
BSC
6
8
5
7
1
4
2
3
9
10
REFERENCE PLANE
BASE & SEATING PLANE
0.355 (9.02)
0.305 (7.75)
0.370 (9.40)
0.335 (8.51)
0.562 (14.30)
0.500 (12.70)
0.040 (1.01)
0.010 (0.25)
0.050
(1.27)
MAX
0.019 (0.48)
0.016 (0.41)
0.021 (0.53)
0.016 (0.41)
0.185 (4.70)
0.165 (4.19)
0.044 (1.12)
0.032 (0.81)
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