Datasheet AD537 (Analog Devices) - 8

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