ADT70

REV. 0

–8–

FUNCTIONAL DESCRIPTION

The ADT70 provides excitation and signal conditioning for

resistance-temperature devices (RTDs). It is ideally suited for

1 k

Ω Platinum RTDs (PRTDs), which allow a much wider

range of temperature measurement than silicon-based sensors.

Using a low cost PRTD, the ADT70 can measure temperatures

in the range of –50

°C to +500°C.

The two main components in the ADT70 are the adjustable

current sources and the instrumentation amplifier. The current

sources provide matching excitation currents to the PRTD and

to the Reference Resistor. The instrumentation amplifier com-

pares the voltage drop across both the PRTD and Reference

Resistor, and provides an amplified output signal voltage that is

proportional to temperature.

Besides the matching current sources and the instrumentation

amplifier, there is a general purpose op amp for any application

desired. The ADT70 comes with a +2.5 V reference on board.

RGA RGB

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

ADT70

SHUTDOWN

Figure 26. Block Diagram

What is an RTD?

The measurable temperature range of the ADT70 heavily de-

pends on the characteristics of the resistance-temperature detec-

tor (RTD). Thus, it is important to choose the right RTD to

suit the actual application.

A basic physical property of any metal is that its electrical resis-

tivity changes with temperature. Some metals are known to have

a very predictable and repeatable change of resistance for a

given change in temperature. An RTD is fabricated from one of

these metals to a nominal ohmic value at a specified tempera-

ture. By measuring its resistance at some unknown temperature

and comparing this value to the resistor’s nominal value, the

change in resistance is determined. Because the temperature vs.

resistance characteristics are also known, the change in tempera-

ture from the point initially specified can be calculated. This

makes the RTD a practical temperature sensor, which in its bare

form is a resistive element.

Several types of metal can be chosen for fabricating RTDs.

These include: Copper, balco (an iron-nickel alloy), nickel,

tungsten, iridium and platinum. Platinum is by far the most

popular material used, due to its nearly linear response to tem-

perature, wide temperature operating range and superior long-

term stability. The price of Platinum Resistance Temperature

Detectors (PRTDs) are becoming more competitive through the

wide use of thin-film-type resistive elements.

Temperature Coefficient of Resistance

The temperature coefficient (TC, also referred to as

α) of an

RTD, describes the average resistance change per unit tempera-

ture from the ice point to the boiling point of water.

TCR

C

RR

CR

ΩΩ °

−

°×

100

0

0

100

°C

°C

TCR = Thermal Coefficient of Resistance.

For example, a platinum thermometer measuring 100

Ω at 0°C

and 138.5

Ω at 100°C, has TCR 0.00385 Ω/Ω/°C .

TCR

C

=

Ω−

Ω

Ω×

°

=

138 5

100

100

100

0 00385

.

.

The larger the TCR, the greater the change in resistance for a

given change in temperature. The most common use of TCR is

to distinguish between curves for platinum, which is available

with TCRs ranging from 0.00375 to 0.003927. The highest

TCR indicates the highest purity platinum and is mandated by

ITS-90 for standard platinum thermometers.

Basically, TCRs must be properly matched when replacing RTDs

or connecting them to instruments. There are no technical advan-

tages of one TCR over another in practical industrial applica-

tions. 0.00385 platinum is the most popular worldwide standard

and is available in both wire-wound and thin-film elements.

Understanding Error Source

The ADT70 uses an instrumentation amplifier that amplifies the

difference in voltage drop across the RTD and the reference resis-

tor, to output a voltage proportional to the measured temperature.

Thus, it is important to use a reference resistor that has stable resis-

tance over temperature. The accuracy of the reference resistor

should be determined by the end application.

The lead resistance of wires connecting to the RTD and the refer-

ence resistor can add inaccuracy to the ADT70. If the reference

resistor is located close to the part, while the RTD is located at a

remote location connected by wires, the lead-wires’ resistance

TEMPERATURE – C

0

50

0

10

25

25

50

75

100

125

20

30

40

50

= LOW TO HIGH

TURNING ON

= HIGH TO LOW

TURNING OFF

Figure 25. System Response Time from Shutdown vs.

Temperature