AD594/AD595

REV. C

–8–

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-116 (D) Package

0.100

(2.54)

BSC

0.700

±0.010

(17.78

±0.25)

0.180

±0.030

(4.57

±0.76)

0.085 (2.16)

0.035

±0.010

(0.89

±0.25)

0.047

±0.007

(1.19

±0.18) 0.017

+0.003

–0.002

0.43 +0.08

–0.05

(

(

0.125

(3.18)

MIN

0.31

±0.01

(7.87

±0.25)

0.01

±0.002

(0.25

±0.05)

0.095 (2.41)

0.30 (7.62) REF

0.430 (10.92)

14

1

7

8

0.040

(1.02) R

PIN 1

0.265

(6.73)

0.290

±0.010

(7.37

±0.25)

Cerdip (Q) Package

14

1

7

8

PIN 1

0.77

±0.015

(19.55

±0.39)

0.310

(7.87)

0.260

±0.020

(6.6

±0.51)

SEATING

PLANE

0.100

(2.54)

BSC

0.035

±0.010

(0.889

±0.254)

0.180

±0.030

(4.57

±0.76)

0.125

3.175)

MIN

0.032

(0.812)

15

°

0

°

0.300 (7.62)

REF

0.148

±0.015

(3.76

±0.38)

0.010

±0.001

(0.254

±0.025)

0.018

(0.457)

0.600 (15.24)

BSC

THERMOCOUPLE BASICS

Thermocouples are economical and rugged; they have reason-

ably good long-term stability. Because of their small size, they

respond quickly and are good choices where fast response is im-

portant. They function over temperature ranges from cryogenics

to jet-engine exhaust and have reasonable linearity and accuracy.

Because the number of free electrons in a piece of metal de-

pends on both temperature and composition of the metal, two

pieces of dissimilar metal in isothermal and contact will exhibit

a potential difference that is a repeatable function of tempera-

ture, as shown in Figure 14. The resulting voltage depends on

the temperatures, T1 and T2, in a repeatable way.

CONSTANTAN

IRON

UNKNOWN

TEMPERATURE

ICE POINT

REFERENCE

T1

T2

CONSTANTAN

Cu

Cu

V1

Figure 14. Thermocouple Voltage with 0

°C Reference

Since the thermocouple is basically a differential rather than

absolute measuring device, a know reference temperature is

required for one of the junctions if the temperature of the other

is to be inferred from the output voltage. Thermocouples made

of specially selected materials have been exhaustively character-

ized in terms of voltage versus temperature compared to primary

temperature standards. Most notably the water-ice point of 0

°C

is used for tables of standard thermocouple performance.

An alternative measurement technique, illustrated in Figure 15,

is used in most practical applications where accuracy requirements

do not warrant maintenance of primary standards. The reference

junction temperature is allowed to change with the environment

of the measurement system, but it is carefully measured by some

type of absolute thermometer. A measurement of the thermo-

couple voltage combined with a knowledge of the reference

temperature can be used to calculate the measurement junction

temperature. Usual practice, however, is to use a convenient

thermoelectric method to measure the reference temperature

and to arrange its output voltage so that it corresponds to a ther-

mocouple referred to 0

°C. This voltage is simply added to the

thermocouple voltage and the sum then corresponds to the stan-

dard voltage tabulated for an ice-point referenced thermocouple.

CONSTANTAN

IRON

T1

V1

CuNi–

Cu

Cu

T3

FOR PROPERLY

V3

'

V1

'

V2

Figure 15. Substitution of Measured Reference

Temperature for Ice Point Reference

The temperature sensitivity of silicon integrated circuit transis-

tors is quite predictable and repeatable. This sensitivity is

exploited in the AD594/AD595 to produce a temperature re-

lated voltage to compensate the reference of “cold” junction of a

thermocouple as shown in Figure 16.

Cu

CONSTANTAN

IRON

T1

Cu

T3

Figure 16. Connecting Isothermal Junctions

Since the compensation is at the reference junction temperature,

it is often convenient to form the reference “junction” by connect-

ing directly to the circuit wiring. So long as these connections

and the compensation are at the same temperature no error will

result.