AD537 Analog Devices, Inc., AD537 Datasheet - Page 8

• Manufacturer Part Number: AD537
• Description: Integrated Circuit Voltage-to-frequency Converter
• Manufacturer: Analog Devices, Inc.

AD537
THERMOCOUPLE INPUT
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.
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.
T
MEAS
T
(0
REF
°
C)
Linearization
Figure 14. Thermocouple Interface with First-Order
0 TO 53mV
R1 SCALE
360µA
50
F
S
I
OFFSET
≈ 21µA
120
47k
2
3
4
5
6
7
1
V
V
T
R
BUF
REFERENCE
PRECISION
VOLTAGE
TO-FREQ
DRIVER
CURR-
CONV
AD537
14
13
12
11
10
9
8
+5V
V
LOGIC
10Hz/
0.005µF
°
C
–8–
0.370 (9.40)
0.335 (8.51)
0.355 (9.02)
0.305 (7.75)
0.200 (5.08)
0.125 (3.18)
(5.08)
0.200
MAX
PIN 1
0.005 (0.13) MIN
14-Lead Side-Brazed Ceramic DIP (TO–116)
0.044 (1.12)
0.032 (0.81)
0.023 (0.58)
0.014 (0.36)
0.040 (1.01)
0.010 (0.25)
0.185 (4.70)
0.165 (4.19)
14
1
(1.27)
0.050
MAX
Dimensions shown in inches and (mm).
10-Lead Metal Can (TO-100)
OUTLINE DIMENSIONS
0.785 (19.94) MAX
0.100
(2.54)
BSC
0.562 (14.30)
0.500 (12.70)
REFERENCE PLANE
0.019 (0.48)
0.016 (0.41)
BASE & SEATING PLANE
0.021 (0.53)
0.016 (0.41)
(H-10A)
(D–14)
0.070 (1.78)
0.030 (0.76)
0.098 (2.49) MAX
0.230
(5.84)
BSC
8
7
(2.92)
0.115
BSC
SEATING
PLANE
0.310 (7.87)
0.220 (5.59)
0.060 (1.52)
0.015 (0.38)
4
3
0.150
(3.81)
MIN
5
2
1
6
0.034 (0.86)
0.028 (0.71)
0.015 (0.38)
0.008 (0.20)
0.320 (8.13)
0.290 (7.37)
10
7
BSC
36
8
9
°
0.045 (1.14)
0.029 (0.74)
REV. C