AD537

REV. C

–8–

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.

1

2

14

13

5

6

7

10

9

8

3

4

12

11

AD537

+5V

10Hz/

0.005µF

47k

≈ 21µA

120

R1 SCALE

50

(0

0 TO 53mV

BUF

DRIVER

CURR-

TO-FREQ

CONV

PRECISION

VOLTAGE

REFERENCE

360µA

Figure 14. Thermocouple Interface with First-Order

Linearization

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

14-Lead Side-Brazed Ceramic DIP (TO–116)

(D–14)

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

°

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

57

1

4

2

39

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)