RGA RGB
ⴚIN
IA
+IN
IA
INST
AMP
SHUTDOWN
GND
SENSE
OUTIAAGND DGNDⴚV
S
SHUTDOWN
+IN
OA
ⴚIN
OA
OUT
OA
+V
S
2.5V
REF
I
OUTA
I
OUTB
MATCHED
CURRENT
SOURCES
NULLA NULLB BIAS 2.5V
REFOUT
ADT70
PRTD Conditioning Circuit
a
GENERAL DESCRIPTION
The ADT70 provides excitation and signal conditioning for
resistance-temperature devices (RTDs). It is ideally suited for
1 kΩ Platinum RTDs (PRTDs), allowing a very wide range of
temperature measurement. It can also easily interface to 100 Ω
PRTDs. Using a remote, low cost thin-film PRTD, the ADT70
can measure temperature in the range of –50°C to +500°C.
With high performance platinum elements, the temperature
change can be extended to 1000°C. Accuracy of the ADT70
and PRTD system over a –200°C to +1000°C temperature
range heavily depends on the quality of the PRTD. Typically
the ADT70 will introduce an error of only ±1°C over the
transducer's temperature range, and the error may be trimmed
to zero at a single calibration point.
The ADT70 consists of two matched 1 mA (nominal) current
sources for transducer and reference resistor excitation, a precision rail-to-rail output instrumentation amplifier, a 2.5 V reference and an uncommitted rail-to-rail output op amp. The
ADT70 includes a shutdown function for battery powered
equipment, which reduces the quiescent current from 4 mA to
less than 10␣ µA. The ADT70 operates from either single +5 V
or ±5 V supplies. Gain or full-scale range for the PRTD and
ADT70 system is set by a precision external resistor connected
to the instrumentation amplifier. The uncommitted op amp may
be used for scaling the internal voltage reference, providing a
“PRTD open” signal or “over-temperature” warning, a heater
switching signal, or other external conditioning determined by
the user.
The ADT70 is specified for operation from ⴚ40°C to ⴙ125°C
and is available in 20-lead DIP and SO packages.
Patent pending.
*
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
FEATURES
PRTD Temperature Measurement Range
Typical IC Measurement Error ⴞ1ⴗC
Includes Two Matched Current Sources
Rail-to-Rail Output Instrumentation Amp
Uncommitted, Rail-to-Rail Output Op Amp
On-Board ⴙ2.5 V Reference
Temperature Coefficient ⴞ25 ppm/ⴗC
ⴙ5 V or ⴞ5 V Operation
Supply Current 4 mA Max
10 A Max in Shutdown
APPLICATIONS
Temperature Controllers
Portable Instrumentation
Temperature Acquisition Cards
and Temperature Controller
ADT70*
FUNCTIONAL BLOCK DIAGRAM
PIN CONFIGURATIONS
20-Lead P-DIP
(N Suffix)
–V
1
S
AGND
2
V
REFOUT
BIAS
NULLA
NULLB
I
OUTA
I
OUTB
–IN
+IN
3
4
5
6
7
8
9
IA
10
IA
ADT70
TOP VIEW
(Not to Scale)
a
20-Lead SOIC
(R Suffix)
–V
1
S
AGND
2
V
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 1998
REFOUT
BIAS
NULLA
NULLB
I
OUTA
I
OUTB
2 IN
1 IN
3
4
5
6
7
8
9
IA
10
IA
ADT70
TOP VIEW
(Not to Scale)
a
+V
20
V
19
OUT OA
–IN
18
+IN
17
16
SHUTDOWN
DGND
15
V
14
OUT IA
GND SENSE
13
RGB
12
RGA
11
+V
20
V
19
OUT OA
–IN
18
+IN
17
16
SHUTDOWN
DGND
15
V
14
OUT IA
GND SENSE
13
RGB
12
RGA
11
S
OA
OA
S
OA
OA
ADT70–SPECIFICATIONS
(V
= ⴙ5 V, ⴚ40ⴗC ≤ TA ≤ ⴙ125ⴗC unless otherwise noted)
S
Parameter Symbol Conditions Min Typ Max Units
SYSTEM CONFIGURATION
Gain R
= 1 kΩ 1.234 1.295 1.364 mV/Ω
L
Line Regulation –2.25 ⫾0.35 2.25 %/V
CURRENT SOURCES
Output Current I
Output Current Mismatch I
Q1
Q1
, I
– I
Q2
Q2
R
= 1 kΩ 0.9 mA
L
R
= 1 kΩ –2 ⫾0.5 2 µA
L
Voltage Compliance –VS to +VS – 1.5 V
INSTRUMENTATION AMP
Input Offset Voltage V
Output Offset Voltage V
Input Bias Current I
Input Offset Current I
IOS
OOS
B
OS
Common-Mode Rejection CMR V
Output Voltage Swing V
OUT
T
= +25°C –500 ⫾100 500 µV
A
= +25°C–7⫾37mV
T
A
T
= +25°C –60 ⫾30 60 nA
A
= 0.5 V to 3 V 65 85 dB
CM
RL = ∞ , VS = ⫾5 V –VS + 25 +VS – 25 mV
–700 ⫾150 700 µV
–12 ⫾512mV
–75 ⫾40 75 nA
–3 ⫾13nA
Power Supply Rejection Ratio PSRR + 4.5 V ≤ VS ≤ ⫾5.5 V –2.5 ⫾0.5 2.5 mV/V
VOLTAGE REFERENCE
Output Voltage 2.485 2.5 2.515 V
T
= +25°C 2.49 2.5 2.51 V
Load Regulation I
A
= 0 mA to 1 mA 250 ppm/mA
L
Temperature Coefficient ⫾10 ppm/°C
Line Regulation + 4.5 V ≤ VS ≤ +5.5 V ⫾75 ppm/V
OPERATIONAL AMPLIFIER
Input Offset Voltage V
Input Offset Voltage Drift T
Input Bias Current I
Input Offset Current I
Open-Loop Voltage Gain A
Output Voltage Swing V
Common-Mode Rejection Ratio CMRR V
IOA
CVIOA
B
OS
VOL
OUTA
T
= +25°C –800 ⫾200 800 µV
A
T
= +25°C –60 ⫾30 60 nA
A
RL =
∞
RL =
∞
= 1 V to 4 V 85 105 dB
CM
T
= +25°C 88 110 dB
A
Power Supply Rejection Ratio PSRR ⫾3 V ≤ V
Slew Rate SR T
= +25°C, AV = 1, 0.17 V/µs
A
≤ ⫾6 V 100 150 dB
S
–1,000 ⫾400 1,000 µV
1 µV/°C
–75 ⫾40 75 nA
–3 ⫾13nA
2V/µV
–VS + 10 +VS – 10 mV
VIN = 0 V to 4 V
SHUTDOWN INPUT
Input Low Voltage V
Input High Voltage V
IL
IH
2.4 V
0.8 V
POWER SUPPLY
Supply Current I
Shutdown Supply Current I
Supply Voltage V
SY
SD
S
R
= 1 kΩ 3.5 5 mA
L
10 30 µA
+4.5 +5.5 V
Dual Supply Voltage ⫾4.5 ⫾5.5 V
Specifications subject to change without notice.
REV. 0–2–
ADT70
WARNING!
ESD SENSITIVE DEVICE
ABSOLUTE MAXIMUM RATINGS*
ORDERING GUIDE
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ⴙ16 V
Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Storage Temperature Range
N, R Package . . . . . . . . . . . . . . . . . . . . . . ⴚ65°C to ⴙ150°C
Operating Temperature Range . . . . . . . . . . ⴚ40°C to ⴙ125°C
Junction Temperature Range
Model Range Package
ADT70GR ⴚ40°C to ⴙ125°C 20-Lead SOIC
ADT70GN ⴚ40°C to ⴙ125°C 20-Lead PDIP
Temperature
N, R Package . . . . . . . . . . . . . . . . . . . . . . ⴚ65°C to ⴙ125°C
Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . ⴙ300°C
NOTE
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those listed in the operational sections
of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Package Type
*
JA
JC
Units
TRANSISTOR COUNT: 158
20-Lead SOIC (R) 74 24 °C/W
20-Lead PDIP (N) 102 31 °C/W
NOTE
*
θ
is specified for device in socket/soldered on circuit board (worst case conditions).
JA
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the ADT70 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
REV. 0 –3–
ADT70
5
4.5 VS = +5V, NO LOAD
4
3.5
3
2.5
2
1.5
SUPPLY CURRENT – mA
1
0.5
0
225
25 75 125
TEMPERATURE – 8C
Figure 1. Supply Current vs. Temperature
1.4
VS = +5V, NO LOAD
1.35
1.3
SYSTEM GAIN – mV/V
1.25
1.2
225
25 75 125
TEMPERATURE – 8C
Figure 2. System Gain vs. Temperature
100
80
VS = +5V, NO LOAD
60
40
20
0
220
240
OFFSET VOLTAGE – mV
260
INSTRUMENTATION AMPLIFIER INPUT
280
2100
225
25 75 125
TEMPERATURE – 8C
Figure 4. Instrumentation Amplifier Input Offset Voltage
vs. Temperature
10
8
VS = +5V, NO LOAD
6
4
2
0
22
24
OFFSET VOLTAGE – mV
26
28
INSTRUMENTATION AMPLIFIER OUTPUT
210
225
25 75 125
TEMPERATURE – 8C
Figure 5. Instrumentation Amplifier Output Offset Voltage
vs. Temperature
0.1
0.08
VS = +5V, NO LOAD
0.06
0.04
0.02
0
20.02
20.04
SYSTEM GAIN PSRR – %/V
20.06
20.08
20.1
225
25 75 125
TEMPERATURE – 8C
Figure 3. Total System Gain PSRR vs. Temperature
0
VS = +5V, NO LOAD
210
220
230
240
BIAS CURRENT – nA
250
260
INSTRUMENTATION AMPLIFIER INPUT
270
225
25 75 125
TEMPERATURE – 8C
Figure 6. Instrumentation Amplifier Input Bias Current vs.
Temperature
REV. 0–4–
ADT70
500
400
VS = +5V, NO LOAD
300
200
100
0
2100
2200
OFFSET CURRENT – pA
2300
INSTRUMENTATION AMPLIFIER INPUT
2400
2500
225
25 75 125
TEMPERATURE – 8C
Figure 7. Instrumentation Amplifier Input Offset Current
vs. Temperature
1.6
VS = +5V, NO LOAD
1.55
1.5
0
VS = +5V, NO LOAD
210
220
230
240
250
OP AMP INPUT BIAS CURRENT – nA
260
270
225
25 75 125
TEMPERATURE – 8C
Figure 10. Op Amp Input Bias Current vs. Temperature
500
VS = +5V, NO LOAD
400
300
200
1.45
INSTRUMENTATION AMPLIFIER GAIN – V/V
1.4
225
25 75 125
TEMPERATURE – 8C
Figure 8. Instrumentation Amplifier Gain vs. Temperature
100
80
VS = +5V, NO LOAD
60
40
20
0
220
240
260
OP AMP INPUT OFFSET VOLTAGE – mV
280
2100
225
25 75 125
TEMPERATURE – 8C
Figure 9. Op Amp Input Offset Voltage vs. Temperature
100
OP AMP INPUT OFFSET CURRENT – pA
0
225
25 75 125
TEMPERATURE – 8C
Figure 11. Op Amp Input Offset Current vs. Temperature
2.51
VS = +5V, NO LOAD
2.505
2.5
2.495
REFERENCE VOLTAGE – V
2.49
225
25 75 125
TEMPERATURE – 8C
Figure 12. Reference Voltage vs. Temperature
REV. 0 –5–
ADT70
1000
VCC = 5V
V
= 0
EE
= +258C
T
A
100
10
D RAIL OUTPUT VOLTAGE – mV
1
110
LOAD CURRENT – mA
DVCC,
SOURCING
CURRENT
DVEE,
SINKING
CURRENT
100 1k 10k
Figure 13. Op Amp Output Voltage from Rails vs.
Load Current
2.52
2.515
2.505
2.495
REFERENCE VOLTAGE – V
2.485
VS = +5V, DUT SOURCING
2.51
2.5
2.49
2.48
091
2345678
LOAD CURRENT – mA
Figure 14. Reference Voltage vs. Load Current
950
VCC = 5V
V
= 0V
EE
V
= 2.5V
REF
940
930
920
OUTPUT OF CURRENT SOURCE – mA
910
4.5 5.0 5.25 5.5
+1258C
+258C
2558C
4.75
SUPPLY VOLTAGE – Volts
Figure 16. Output of Current Source vs. Supply Voltage
140
120
100
80
60
CMRR – dB
40
20
0
10
AV = 1.4
100 1k 10k 100k 1M
FREQUENCY – Hz
AV = 14
Figure 17. In Amp CMRR vs. Frequency
4
TA = +258C
VCM INAMP = 1V
VEE = GND
3.8
3.6
3.4
, SUPPLY CURRENT – mA
SY
I
3.2
3
4.5
4.75 5.0 5.25 5.5
SUPPLY VOLTAGE – Volts
Figure 15. Supply Current vs. Supply Voltage
120
100
80
60
40
20
GAIN – dB
0
220
240
260
280
100
1k 10k 100k 1M 10M
FREQUENCY – Hz
270
225
180
135
90
45
0
245
290
2135
2180
Figure 18. Op Amp Open Loop Gain and Phase vs.
Frequency
REV. 0–6–
PHASE MARGIN – Degrees
FREQUENCY – Hz
10
CMRR – dB
100 1k 10k 100k 1M
0
20
40
60
80
100
120
FREQUENCY – Hz
10
PSRR – dB
100 1k 10k 100k 1M
220
20
40
60
80
100
120
0
+ PSRR
2 PSRR
FREQUENCY – Hz
100
CLOSED LOOP GAIN – dB
220
1k 10k 100k 1M 10M
210
0
10
20
30
40
50
A
VCL
= 0
A
VCL
= 100
A
VCL
= 10
TA = +258C
VCC = 4V
V
EE
= 21V
140
120
100
ADT70
80
60
PSRR – dB
40
20
0
220
10
2 PSRR
100 1k 10k 100k 1M
FREQUENCY – Hz
+ PSRR
Figure 19. In Amp PSRR vs. Frequency – AV = 1.4
140
120
100
80
60
PSRR – dB
40
20
0
220
10
2 PSRR
100 1k 10k 100k 1M
FREQUENCY – Hz
+ PSRR
Figure 20. In Amp PSRR vs. Frequency – AV = 14
Figure 22. Op Amp CMRR vs. Frequency
Figure 23. Op Amp PSRR vs. Frequency
Figure 21. In Amp Closed Loop Gain vs. Frequency
REV. 0 –7–
100
80
60
40
20
0
CLOSED LOOP GAIN – dB
220
240
260
100
1k 10k 100k 1M 10M
FREQUENCY – Hz
AV = 14
AV = 1.4
Figure 24. Op Amp Closed Loop Gain vs. Frequency
ADT70
50
40
30
20
SYSTEM RESPONSE TIME – ms
10
0
250
TURNING ON
TURNING OFF
225
V
SHUTDOWN
V
SHUTDOWN
0
TEMPERATURE – 8C
V
OF IN AMP = 300mV
OUT
VCC = 5V SINGLE SUPPLY
= LOW TO HIGH
= HIGH TO LOW
25 50 75 100 125
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 kΩ 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 compares 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.
NULLA NULLB BIAS 2.5V
I
OUTA
I
OUTB
+IN
ⴚIN
IA
IA
MATCHED
CURRENT
SOURCES
INST
AMP
RGA RGB
GND
SENSE
REFOUT
2.5V
REF
OUTIAAGND DGNDⴚV
S
ADT70
SHUTDOWN
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
Figure 26. Block Diagram
What is an RTD?
The measurable temperature range of the ADT70 heavily depends on the characteristics of the resistance-temperature detector (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 resistivity 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 temperature. 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 temperature 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 temperature, wide temperature operating range and superior longterm 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 temperature from the ice point to the boiling point of water.
RR
−
ΩΩ°
TCR C
()
R
= Resistance of the sensor at 0°C
0
= Resistance of the sensor at +100°C
R
100
=
100
100 0
CR
°×
0
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
=
100 100
Ω× °
Ω− Ω
C
.
0 00385
=
.
138 5 100
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 advantages of one TCR over another in practical industrial applications. 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 resistor, to output a voltage proportional to the measured temperature.
Thus, it is important to use a reference resistor that has stable resistance 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 reference 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
REV. 0–8–
ADT70
would contribute to the difference in voltage drop between the
RTD and the reference resistor. Thus, an error in reading the actual 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
. . =+ =100 1 62 101 62ΩΩ Ω
TOTAL
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 example above, the sensitivity of the 100 Ω PRTD is 0.385 Ω/°C
(taken from PRTD data sheet). Hence the approximate error is:
As shown above, this is a significant inaccuracy, especially for applications 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 kΩ instead of 100 Ω. Furthermore, in
the application circuit section, Figure 28 illustrates how to eliminate such error by using the part’s general purpose op amp.
Self-Heating Effect
Another contributor to measurement error is the self-heating effect on the RTD. As with any resistive element, power is dissipated 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 kΩ 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:
∆° = ×CIR
∆° =
∆° = °CC005.
2
θ
PACKAGE
0
2
×Ω×°CmA CW1 1000 50
/
()
where:
PACKAGE
R
= thermal resistance of package
= value of RTD resistance
0
APPLICATION INFORMATION
As shown in Figure 27, using a 1 kΩ PRTD, 1 kΩ reference
resistor, 49.9 kΩ resistor between RG
12), and shorting BIAS (Pin 4) with V
the output of OUT
VmVR
=Ω×
1 299. /
OUT
(Pin 14) will have a transfer function of
IA
∆
PRTD RESISTANCE REFERENCE RESISTANCE
()
(Pin 11) and RGB (Pin
A
(Pin 3) together,
REFOUT
−
Error C C=°=°162 0385 421. /. / .ΩΩ
assuming the reference resistor is constant at 100 Ω throughout
the temperature range.
POTENTIOMETER
IS USED TO
ACHIEVE HIGHER
PRECISION OF
MATCHING
CURRENT.
1k⍀
REF
RESISTOR
NULLA NULLB BIAS 2.5V
I
OUTA
I
OUTB
ⴚIN
IA
+IN
IA
1k⍀
PRTD
50k⍀
MATCHED
CURRENT
SOURCES
INST
AMP
RGA RGB
49.9k⍀
GND
OUTIAAGND DGNDⴚV
SENSE
V
OUT
Figure 27. Basic Operational Diagram
@ 5mV/ⴗC
+5V
REFOUT
2.5V
REF
ⴚ1V < ⴚVS < ⴚ5V
ADT70
SHUTDOWN
S
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
INDEPENDENT
OP AMP
REV. 0 –9–
ADT70
NULLA NULLB BIAS 2.5V
1k⍀
REF
NODE D
NODE A
PRTD
NODE B
RESISTOR
1k⍀
NODE C
I
OUTB
ⴚIN
+IN
I
OUTA
IA
IA
RGA RGB
50k⍀
MATCHED
CURRENT
SOURCES
INST
AMP
GND
SENSE
OUT
Figure 28. 4-Wire Lead-Wire Resistance Cancellation Circuit
If PRTD has a tempco resistance of 0.00385 Ω/Ω/°C or sensitivity of 3.85 Ω/°C, the system output voltage scaling factor will
be 5 mV/°C.
The gain of the instrumentation amplifier is normally at 1.30,
with a 49.9 kΩ gain resistor. It can be changed by changing the
gain resistor using the following equation.
Instrumentation Amp Gain
.
=
130
.
49 9
R
GAIN RESISTOR
k
Ω
In Figure 2 the ADT70 is powered by a dual power supply. In
order for the part to measure below 0°C, using a 1 kΩ PRTD,
has to be at least –1 V. –VS can be grounded when the mea-
–V
S
sured temperature is greater than 0°C using a 1 kΩ PRTD. GND
Sense (Pin 13), DGND (Pin 15), and AGND (Pin 2) are all connected to ground. If desired, GND Sense could be connected to
whatever potential desired for an output offset of the instrumentation amplifier. However, AGND and DGND must always be
connected to GND.
ADT70 will turn off if the
and will turn on when
SHUTDOWN is not used in the design, it should be con-
If
nected to +V
.
S
SHUTDOWN pin(GND) is low,
SHUTDOWN pin becomes high (+VS).
The undedicated op amp in the ADT70 can be used to transmit
measured signal to a remote location where noise might be introduced into the signal as it travels in a noisy environment. It can
also be used as a general purpose amplifier in any application desired. The op amp gain is set using standard feedback resistor
configurations.
Higher precision of matching the current sources can be
achieved by using a 50 kΩ potentiometer connected between
NULLA (Pin 5) and NULLB (Pin 6) with the center-tap of the
potentiometer connected to +V
(Pin 20). In Figure 27, the
S
ADT70’s Bias Pin (Pin 4) is generally connected to the
V
(Pin 3), but it can be connected to an external voltage
REFOUT
reference if different output current is preferred.
REFOUT
ADT70
2.5V
REF
SHUT-
DOWN
ⴚ5V
DGNDⴚV
S
AGND
IA
5V
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
Eliminating Lead-Wire Resistance by Using 4-Wire
Configuration
In applications where the lead-wire resistance can significantly
contribute error to the measured temperature, implementing a
4-wire lead-resistance canceling circuit can dramatically minimize the lead-wire resistance effect.
In Figure 28, I
OUTA
and I
provides matching excitation to
OUTB
the reference resistor and the PRTD respectively. The lead-resistance from the current source to the PRTD or reference resistor is not of concern because the instrumentation amplifier is
measuring the difference in potential directly on the PRTD
(Node A) and reference resistor (Node C). Since there is almost
no current going from Node A and Node C into the amplifier’s
input, there is no lead-wire resistance error.
A potential source of temperature measurement errors is the
possibility of voltage differences between the ground side of the
reference resistor and the PRTD. Differences in lead-wire resistance from ground to these two points, coupled with the 1 mA
excitation current, will lead directly to differential voltage errors
at the input of the instrumentation amplifier of the ADT70. By
connecting the ground side of the PRTD (Node B of Figure 28)
to the noninverting input of the op amp and connecting the
ground side of the reference resistor (Node D) to both the inverting input and the output of the op amp, the two points can
be forced to the same potential. It is not important that this potential is exactly at ground since the instrumentation amplifier
rejects common-mode signals at the input. Note that all three
connections should be made as close as possible to the body of
the reference resistor and the PRTD to minimize error.
Single Supply Operation
When using the ADT70 in single supply applications a few
simple but important points need to be considered. The most
important issue is ensuring that the ADT70 is properly biased.
To bias the ADT70, first consider the 1 kΩ PRTD sensor. The
PRTD typically changes from 230 Ω at –200°C to 4080 Ω at
800°C ± 1 Ω error. This impedance range results in an ADT70
output of –1 V to +4 V respectively, which is impossible to
REV. 0–10–
ADT70
achieve in a single supply application where the negative rail is
ground or 0 V. Therefore, to achieve full scale operation the
output of ADT70 should be shifted by 1 V to allow for operation in the 0 V to 5 V region.
The most straightforward method to shift the output voltage
incorporates the use of the GND SENSE as shown in Figure 29.
To shift output voltage range apply a potential equal to the necessary shift on the GND SENSE pin. For example, to shift the output voltage, OUT
, up to 1 V to GND SENSE, apply 1 V to
IA
GND SENSE. When applying a potential to GND SENSE, care
should be taken to ensure that the voltage source is capable of driv-
ing 2 kΩ and does not introduce excessive noise. Figure 29 uses the
on-board 2.5 V voltage reference for a low noise source. This reference is then divided to 1 V and buffered by the on-board op amp
to drive GND SENSE at a low impedance. A small 500 Ω potenti-
ometer can be used to calibrate the initial offset error to zero.
NULLA NULLB BIAS 2.5V
I
1k⍀
REF
RESISTOR
1k⍀
PRTD
SENSOR
49.9k⍀
RG
RG
OUTA
I
OUTB
ⴚIN
+IN
IA
IA
MATCHED
CURRENT
SOURCES
INST
AMP
GND
SENSE
OUT
IA
However, a voltage applied to GND SENSE is not the only
method to shift the voltage range. Placing a 768 Ω resistor in the
PRTD sensor path also shifts the output voltage by 1 V. This
second method, as shown in Figure 30, is usually not recommended for the following reasons; the input voltage range of the
op amps is limited to around 1 V from the negative and positive
rails and this could cause problems at high temperature, limiting
the upper range to 600°C; the physical location of this resistor
(if placed at a distance from the ADT70) may have an impact
on the noise performance. The method frees up the on-board op
amp for another function and achieves the lowest impedance
ground point for GND SENSE.
This brief section on ADT70 single supply operation has focused
on simple techniques to bias the ADT70 such that all output voltages are within operational range. However, these techniques may
not be useful in all single supply applications. For example, in Figure 3 the additional on-board op amp is operating at near ground
potential which will create problems in a single supply application
REFOUT
2.5V
REF
ADT70
SHUT-
DOWN
DGND
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
15k⍀
9.76k⍀
500⍀
POT
TO CONTROLLER
Figure 29. A Single Supply Application with Shifted Ground Sense Pin
+5V
ADT70
768⍀
RESISTOR
1K⍀
PRTD
RG
49.9k⍀
RG
1K⍀
REF
RESISTOR
NULLA NULLB BIAS 2.5V
I
OUTA
I
OUTB
ⴚIN
+IN
IA
IA
MATCHED
CURRENT
SOURCES
INST
AMP
GND
SENSE
REFOUT
2.5V
REF
SHUT-
DOWN
ⴚV
S
TO A/D CONVERTER
DGND
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
V
REF
TO CONTROLLER
Figure 30. A Basic Single Supply Operational Diagram with Bias Resistor in Sensor Path
REV. 0 –11–
ADT70
because the input voltage range of the on-board op amp only extends to about 1 V above the negative rail. If the application requires the inputs of either the on-board amp or instrumentation
amplifier to operate within 1 V of ground, it will be necessary to
generate a “pseudo-ground.” Figure 31 illustrates a typical
ADT70 “pseudo-ground” application. The Analog Devices’
ADR290, a 2.048 V reference, is being used to generate the
“pseudo-ground.” The ADR290 was selected for the following
reasons: low noise, ability to drive the required 5 mA in this
application, good temperature stability, which is usually important in a PRTD application. However, one undesired effect of
introducing the pseudo-ground is the loss in voltage range at
high temperature. In our example, the PRTD will only operate
from –200°C to +400°C corresponding to an input voltage
range of 1 V to 4 V.
100 ⍀ PRTD Application Circuit
A 1000 Ω PRTD sensor scales by 3.85 Ωs/°C, which is exactly
ten times the scale of the 100 Ω PRTD sensor. The ADT70
has been designed to allow for 1000 Ω or 100 Ω PRTD sen-
sors. Only the gain setting resistor RG needs to be altered. For
NULLA NULLB BIAS 2.5V
a 100 Ω PRTD 0.00385 sensor, change RG to 4.99 kΩ as illustrated in Figure 32. In single supply application, with a 100 Ω
PRTD sensor, a “pseudo-ground” will be necessary because the
inputs of the instrumentation amplifier will be within 1 V of the
negative rail. See the section on single supply applications for
more information.
ⴚIN
IA
RG
4.99k⍀
+IN
RG
IA
INST
AMP
GND
SENSE
OUT
Figure 32. 100␣Ω 0.00385 PRTD Application Showing
New Value for RG
REFOUT
ADT70
+5V
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
NODE D
1k⍀
PRTD
10F
1k⍀
REF
RESISTOR
+5V
NODE C
RG
49.9k⍀
RG
ADR290
I
OUTA
I
OUTB
ⴚIN
+IN
GND
MATCHED
CURRENT
SOURCES
2.5V
REF
IA
INST
AMP
IA
GND
SENSE
OUTIN
0.1F0.1F
OUT AGND
ⴚV
SHUTDOWN
S
DGND
Figure 31. Single Supply Application with an ADR290 “Pseudo-Ground”
REV. 0–12–
ADT70
American PRTD Application Circuit
The majority of PRTD sensors use a scale factor of 0.00385 Ω/Ω/°C.
This type of sensor is known as the European PRTD and is the most
common PRTD sensor. However, there is also an American PRTD
sensor that uses a scale factor of 0.00392 Ω/Ω/°C. Figure 33 illus-
trates the input section of the ADT70 configured for the Ameri-
can PRTD. The ideal value for RG is 50.98 kΩ when yielding a
5 mV/°C ADT70 output.
I
OUTA
I
OUTB
ⴚIN
IA
RG
INST
AMP
GND
SENSE
OUT
˜
1k⍀
PRTD
49.9k⍀
1k⍀
REF
RESISTOR
RG
2k⍀
+IN
IA
NOTE: IDEAL VALUE
= 51k⍀
FOR RG
Figure 33. Typical PRTD Application with American
0.003916
Ω/Ω/°
C Scale; 1 kΩ Scale
Strain Gauge Sensor Application Circuit
Figure 34 illustrates a typical strain gauge bridge circuit. The
versatility of the ADT70 allows the part to be used with most
bridge circuits that are within the 50 kΩ to 5 kΩ impedance
range. The sensor used in this circuit has two elements varying.
If a constant current is driven into the sensor, a linear V
OUT
is
obtained. In addition, the ADT70 will work with most bridge
circuits whether one-, two-, or all-element varying.
Securing Additional Current from the Current Sources
Some sensor applications need a higher excitation current to increase sensor sensitivity. There are two methods to increase the
current from the on-board current sources of the ADT70. The
most flexible method involves changing the voltage at the BIAS
node. The equation for determining the BIAS potential vs. Output current is 2.5 V for roughly 1 mA, or in other words, to
double the current output simply put 5 V into BIAS. The BIAS
node should be driven with a low-noise source, such as a reference, because output current is directly dependent on BIAS voltage. Directly tying BIAS to the positive supply rail may produce
too much current noise especially if the positive rail is not well
regulated. The second method involves tying the two ADT70
current outputs together which doubles the current. Of course,
this technique is most useful if, as illustrated in Figure 34, the application requires only one current source.
NULLA NULLB BIAS 2.5V
REFOUT
ADT70
I
OUTA
I
OUTB
RR
ⴚIN
RR
RG
RG
+IN
COLUMBIA RESEARCH LAB
MODEL DT3617
STRAIN SENSOR
R = 1k⍀
IA
IA
MATCHED
CURRENT
SOURCES
INST
AMP
2.5V
REF
ⴚ5V
SHUT-
DOWN
DGND
Figure 34. Typical Strain Sensor Application (Two Element Varying)
+5V
+V
S
OUT
OA
+IN
OA
ⴚIN
OA
SHUTDOWN
REV. 0 –13–
ADT70
0.210 (5.33)
MAX
0.160 (4.06)
0.115 (2.93)
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Lead Plastic DIP
(P-Suffix)
1.060 (26.90)
0.925 (23.50)
20
110
PIN 1
0.022 (0.558)
0.014 (0.356)
0.100
(2.54)
BSC
11
0.060 (1.52)
0.015 (0.38)
0.070 (1.77)
0.045 (1.15)
0.280 (7.11)
0.240 (6.10)
0.130
(3.30)
MIN
SEATING
PLANE
0.325 (8.25)
0.300 (7.62)
20-Lead SOIC
(S-Suffix)
0.5118 (13.00)
0.4961 (12.60)
C3395–8–7/98
0.195 (4.95)
0.115 (2.93)
0.015 (0.381)
0.008 (0.204)
20 11
PIN 1
0.0118 (0.30)
0.0040 (0.10)
0.0500
(1.27)
BSC
0.1043 (2.65)
0.0926 (2.35)
0.0192 (0.49)
0.0138 (0.35)
101
SEATING
PLANE
0.2992 (7.60)
0.2914 (7.40)
0.4193 (10.65)
0.3937 (10.00)
0.0125 (0.32)
0.0091 (0.23)
0.0291 (0.74)
0.0098 (0.25)
0.0500 (1.27)
8°
0°
0.0157 (0.40)
x 45°
PRINTED IN U.S.A.
REV. 0–14–
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