ADT70

REV. 0

–9–

would contribute to the difference in voltage drop between the

RTD and the reference resistor. Thus, an error in reading the ac-

tual temperature could occur.

Table I. Copper Wire Gauge Size to Resistance Table.

Lead-wire AWG

Ohms/foot at +25ºC

12

0.0016

14

0.0026

16

0.0041

18

0.0065

20

0.0103

22

0.0162

24

0.0257

26

0.0413

28

0.0651

30

0.1027

From Table I the amount of lead-wire resistance effect in the

circuit can be estimated. For example, connect 100 feet of

AWG 22 wire to a 100

Ω Platinum RTD (PF element). The

lead-wire resistance will be: R = 100 ft 3 0.0162

Ω/ft = 1.62 Ω.

Thus the total resistance you have with the PRTD will be:

R

TOTAL

.

.

=+

=

100

1 62

101 62

ΩΩ

Ω

Since the 100

Ω reference resistor is assumed to be relatively close

to the ADT70, the lead-wire resistance is negligible. This shows

1.62

Ω of inaccuracy.

From the PRTD’s data sheet, the PRTD’s sensitivity rating

(

Ω/°C) can be used with the lead-wire resistance to approximate

the accuracy error in temperature degree (

°C). Following the ex-

ample above, the sensitivity of the 100

Ω PRTD is 0.385 Ω/°C

(taken from PRTD data sheet). Hence the approximate error is:

Error

C

C

=° =

°

162

0 385

421

.

/ .

/

.

ΩΩ

assuming the reference resistor is constant at 100

Ω throughout

the temperature range.

As shown above, this is a significant inaccuracy, especially for ap-

plications where the PRTD would be hundreds of feet away from

the ADT70. To reduce lead-wire error it is recommended to use

a larger sensitivity RTD; 1 k

Ω instead of 100 Ω. Furthermore, in

the application circuit section, Figure 28 illustrates how to elimi-

nate such error by using the part’s general purpose op amp.

Self-Heating Effect

Another contributor to measurement error is the self-heating ef-

fect on the RTD. As with any resistive element, power is dissi-

pated in an amount equal to the square of the excitation current

times the resistance of the element. The error contribution of the

heat generated by this power dissipation can easily be calculated.

For example, if the package thermal resistance is 50

°C/W, the

RTD nominal resistance is 1 k

Ω and the element is excited with a

1 mA current source, then the artificial increase in temperature

(

ƼC) as a result of self-heating is:

∆° =

×

CI R

PACKAGE

2

0

θ

∆° =

CmA

C W

1

1000

50

2

/

∆° =

°

CC

005

.

where:

APPLICATION INFORMATION

As shown in Figure 27, using a 1 k

Ω PRTD, 1 kΩ reference

resistor, 49.9 k

Ω resistor between RG

VmV

R

OUT

PRTD RESISTANCE

REFERENCE RESISTANCE

=Ω ×

()

1 299

.

/

∆

−

RGA

RGB

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

ADT70

49.9k

INDEPENDENT

OP AMP

50k

+5V

1k

PRTD

1k

REF

RESISTOR

POTENTIOMETER

IS USED TO

ACHIEVE HIGHER

PRECISION OF

MATCHING

CURRENT.

1V <

Figure 27. Basic Operational Diagram