ADT70GN Analog Devices Inc, ADT70GN Datasheet - Page 8

• Manufacturer Part Number: ADT70GN
• Manufacturer: Analog Devices Inc

Specifications of ADT70GN

• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Operating Temperature Classification: Automotive
• Mounting: Through Hole
• Pin Count: 20
• Package Type: PDIP
• Lead Free Status / RoHS Status: Not Compliant

ADT70
Figure 25. System Response Time from Shutdown vs.
Temperature
FUNCTIONAL DESCRIPTION
The ADT70 provides excitation and signal conditioning for
resistance-temperature devices (RTDs). It is ideally suited for
1 k 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.
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.
I
I
+IN
OUTA
OUTB
IN
IA
IA
NULLA
50
40
30
20
10
RGA RGB
0
50
INST
AMP
SOURCES
MATCHED
CURRENT
NULLB
25
GND
SENSE
Figure 26. Block Diagram
TURNING OFF
TURNING ON
V
V
SHUTDOWN
SHUTDOWN
BIAS 2.5V
0
TEMPERATURE – C
OUT
25
IA
= LOW TO HIGH
= HIGH TO LOW
REFOUT
AGND
2.5V
REF
V
V
50
OUT
CC
= 5V SINGLE SUPPLY
V
OF IN AMP = 300mV
S
75
ADT70
SHUT-
DOWN
DGND
100
+V
OUT
+IN
125
SHUTDOWN
IN
S
OA
OA
OA
–8–
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 ) of an
RTD, describes the average resistance change per unit tempera-
ture from the ice point to the boiling point of water.
R
R
TCR = Thermal Coefficient of Resistance.
For example, a platinum thermometer measuring 100
and 138.5
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
0
100
= Resistance of the sensor at 0 C
= Resistance of the sensor at +100 C
at 100 C, has TCR 0.00385 / / C .
TCR
TCR
138 5
100
C
100
100
100
C
R
100
C R
0 00385
R
0
0
at 0 C
REV. 0