AD537
THERMOCOUPLE INPUT
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14
8
0.310 (7.87)
0.220 (5.59)
PIN 1
1
0.785 (19.94) MAX
0.200
(5.08)
MAX
0.200 (5.08)
0.125 (3.18)
7
0.060 (1.52)
0.015 (0.38)
0.320 (8.13)
0.290 (7.37)
The circuit shown in Figure 14 provides good accuracy from
+300°C to +700°C. The extrapolation of the temperature volt-
age curve back to 0°C shows that an offset of –3.34 mV is
required to fit the curve most exactly. This small amount of
voltage can be introduced without an additional calibration step
using the +1.00 V output of the AD537. To adjust the scale, the
thermocouple should be raised to a known reference tempera-
ture near 500°C and the frequency adjusted to value using R1.
The error should be within
±
0.2% over the range 400°C to
700°C.
AD537
T
MEAS
T
REF
(0
°
C)
0 TO 53mV
R1 SCALE
50
360µA 120
F
S
47k
I
OFFSET
≈
21µA
1
2
3
4
5
6
7
V
T
V
R
PRECISION
VOLTAGE
REFERENCE
CURR-
BUF TO-FREQ
CONV
DRIVER
14
13
12
0.005µF
11
10
9
8
V
LOGIC
10Hz/
°
C
+5V
0.150
(3.81)
MIN
0.015 (0.38)
0.008 (0.20)
0.023 (0.58)
0.014 (0.36)
0.100
(2.54)
BSC
0.070 (1.78)
0.030 (0.76)
SEATING
PLANE
10-Lead Metal Can (TO-100)
(H-10A)
REFERENCE PLANE
0.185 (4.70)
0.165 (4.19)
0.050
(1.27)
MAX
6
5
0.355 (9.02)
0.305 (7.75)
0.370 (9.40)
0.335 (8.51)
2
0.019 (0.48)
0.016 (0.41)
0.021 (0.53)
0.016 (0.41)
BASE & SEATING PLANE
0.115
(2.92)
BSC
1
10
0.230
(5.84)
BSC
4
3
7
8
9
0.045 (1.14)
0.029 (0.74)
0.562 (14.30)
0.500 (12.70)
0.044 (1.12)
0.032 (0.81)
0.040 (1.01)
0.010 (0.25)
0.034 (0.86)
0.028 (0.71)
36
°
BSC
Figure 14. Thermocouple Interface with First-Order
Linearization
–8–
REV. C
PRINTED IN U.S.A.
C397f–0–4/00 (rev. C)
The output of a Chromel-Constantan (Type E) thermocouple,
using a reference junction at 0°C, varies from 0 mV to 53.14 mV
over the temperature range 0°C to +700°C with a slope of
80.678
µV/degree
over most of its range and some nonlinearity
over the range 0°C to +200°C. For this example, we assume
that it is desired to indicate temperature in Degrees Celsius
using a counter/display with a 100 ms gate width. Thus, the V-F
converter must deliver an output of 7 kHz for an input of
53.14 mV. If very precise operation down to 0°C is imperative,
some sort of linearizing is necessary (see, for example,
Analog
Devices’ Nonlinear Circuits Handbook,
pp. 92–97) but in many
cases operation is only needed over part of the range.
14-Lead Side-Brazed Ceramic DIP (TO–116)
(D–14)
0.005 (0.13) MIN
0.098 (2.49) MAX
AD [ ANALOG DEVICES ]
