FUNCTIONAL BLOCK DIAGRAM

RGA RGB

2INIA

+INIAINSTAMP

SHUT-DOWN

GNDSENSE

OUTIA AGND DGND2VS

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

ADT70

PIN CONFIGURATIONS20-Lead P-DIP

(N Suffix)

SHUTDOWN

14

13

12

11

17

16

15

20

19

18

10

9

8

1

2

3

4

7

6

5

–VS

+INOA

–INOA

VOUT OA

+VS

AGND

VREFOUT

BIAS

VOUT IA

DGND

NULLA

NULLB

IOUTA

IOUTB

–INIA

+INIA RGA

RGB

GND SENSE

a

ADT70TOP VIEW

(Not to Scale)

20-Lead SOIC(R Suffix)

14

13

12

11

17

16

15

20

19

18

10

9

8

1

2

3

4

7

6

5

–VS

+INOA

–INOA

VOUT OA

+VS

AGND

VREFOUT

BIAS

VOUT IA

DGND

SHUTDOWNNULLA

NULLB

IOUTA

IOUTB

2 INIA

1 INIA RGA

RGB

GND SENSE

ADT70TOP VIEW

(Not to Scale)

a

REV. 0

Information furnished by Analog Devices is believed to be accurate andreliable. However, no responsibility is assumed by Analog Devices for itsuse, nor for any infringements of patents or other rights of third partieswhich may result from its use. No license is granted by implication orotherwise under any patent or patent rights of Analog Devices.

a PRTD Conditioning Circuitand Temperature Controller

ADT70*FEATURES

PRTD Temperature Measurement Range

Typical IC Measurement Error 618CIncludes Two Matched Current Sources

Rail-to-Rail Output Instrumentation Amp

Uncommitted, Rail-to-Rail Output Op Amp

On-Board 12.5 V Reference

Temperature Coefficient 625 ppm/8C15 V or 65 V Operation

Supply Current 4 mA Max

10 mA Max in Shutdown

APPLICATIONS

Temperature Controllers

Portable Instrumentation

Temperature Acquisition Cards

GENERAL DESCRIPTIONThe ADT70 provides excitation and signal conditioning forresistance-temperature devices (RTDs). It is ideally suited for1 kΩ Platinum RTDs (PRTDs), allowing a very wide range oftemperature measurement. It can also easily interface to 100 ΩPRTDs. Using a remote, low cost thin-film PRTD, the ADT70can measure temperature in the range of –50°C to +500°C.With high performance platinum elements, the temperaturechange can be extended to 1000°C. Accuracy of the ADT70and PRTD system over a –200°C to +1000°C temperaturerange heavily depends on the quality of the PRTD. Typicallythe ADT70 will introduce an error of only ±1°C over thetransducer's temperature range, and the error may be trimmedto zero at a single calibration point.

The ADT70 consists of two matched 1 mA (nominal) currentsources for transducer and reference resistor excitation, a preci-sion rail-to-rail output instrumentation amplifier, a 2.5 V refer-ence and an uncommitted rail-to-rail output op amp. TheADT70 includes a shutdown function for battery poweredequipment, which reduces the quiescent current from 4 mA toless than 10 µA. The ADT70 operates from either single +5 Vor ±5 V supplies. Gain or full-scale range for the PRTD andADT70 system is set by a precision external resistor connectedto the instrumentation amplifier. The uncommitted op amp maybe used for scaling the internal voltage reference, providing a“PRTD open” signal or “over-temperature” warning, a heaterswitching signal, or other external conditioning determined bythe user.

The ADT70 is specified for operation from 240°C to 1125°Cand is available in 20-lead DIP and SO packages.

*Patent pending.

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

OBSOLETE

Parameter Symbol Conditions Min Typ Max Units

SYSTEM CONFIGURATIONGain RL = 1 kΩ 1.234 1.295 1.364 mV/ΩLine Regulation –2.25 60.35 2.25 %/V

CURRENT SOURCESOutput Current IQ1, IQ2 RL = 1 kΩ 0.9 mAOutput Current Mismatch IQ1 – IQ2 RL = 1 kΩ –2 60.5 2 µAVoltage Compliance –VS to +VS – 1.5 V

INSTRUMENTATION AMPInput Offset Voltage VIOS –700 6150 700 µV

TA = +25°C –500 6100 500 µVOutput Offset Voltage VOOS –12 65 12 mV

TA = +25°C –7 63 7 mV Input Bias Current IB –75 640 75 nA

TA = +25°C –60 630 60 nAInput Offset Current IOS –3 61 3 nACommon-Mode Rejection CMR VCM = 0.5 V to 3 V 65 85 dBOutput Voltage Swing VOUT RL = ∞ , VS = 65 V –VS + 25 +VS – 25 mVPower Supply Rejection Ratio PSRR + 4.5 V ≤ VS ≤ 65.5 V –2.5 60.5 2.5 mV/V

VOLTAGE REFERENCEOutput Voltage 2.485 2.5 2.515 V

TA = +25°C 2.49 2.5 2.51 VLoad Regulation IL = 0 mA to 1 mA 250 ppm/mATemperature Coefficient 610 ppm/°CLine Regulation + 4.5 V ≤ VS ≤ +5.5 V 675 ppm/V

OPERATIONAL AMPLIFIERInput Offset Voltage VIOA –1,000 6400 1,000 µV

TA = +25°C –800 6200 800 µVInput Offset Voltage Drift TCVIOA 1 µV/°CInput Bias Current IB –75 640 75 nA

TA = +25°C –60 630 60 nAInput Offset Current IOS –3 61 3 nAOpen-Loop Voltage Gain AVOL RL = ∞ 2 V/µVOutput Voltage Swing VOUTA RL = ∞ –VS + 10 +VS – 10 mVCommon-Mode Rejection Ratio CMRR VCM = 1 V to 4 V 85 105 dB

TA = +25°C 88 110 dBPower Supply Rejection Ratio PSRR 63 V ≤ VS ≤ 66 V 100 150 dBSlew Rate SR TA = +25°C, AV = 1, 0.17 V/µs

VIN = 0 V to 4 V

SHUTDOWN INPUTInput Low Voltage VIL 0.8 VInput High Voltage VIH 2.4 V

POWER SUPPLYSupply Current ISY RL = 1 kΩ 3.5 5 mAShutdown Supply Current ISD 10 30 µASupply Voltage VS +4.5 +5.5 VDual Supply Voltage 64.5 65.5 V

Specifications subject to change without notice.

ADT70–SPECIFICATIONS

REV. 0–2–

(VS = 15 V, 2408C ≤ TA ≤ 11258C unless otherwise noted)

OBSOLETE

ADT70

REV. 0 –3–

ORDERING GUIDE

TemperatureModel Range Package

ADT70GR 240°C to 1125°C 20-Lead SOICADT70GN 240°C to 1125°C 20-Lead PDIP

ABSOLUTE MAXIMUM RATINGS*Supply Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 VOutput Short-Circuit Duration . . . . . . . . . . . . . . . . . IndefiniteStorage Temperature Range

N, R Package . . . . . . . . . . . . . . . . . . . . . . 265°C to 1150°COperating Temperature Range . . . . . . . . . . 240°C to 1125°CJunction Temperature Range

N, R Package . . . . . . . . . . . . . . . . . . . . . . 265°C to 1125°CLead Temperature (Soldering, 60 sec) . . . . . . . . . . . . 1300°CNOTE*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 thedevice at these or any other conditions above those listed in the operational sectionsof this specification is not implied. Exposure to absolute maximum rating condi-tions for extended periods may affect device reliability.

Package Type uJA* uJC Units

20-Lead SOIC (R) 74 24 °C/W20-Lead PDIP (N) 102 31 °C/W

NOTE*θJA is specified for device in socket/soldered on circuit board (worst case conditions).

TRANSISTOR COUNT: 158

CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readilyaccumulate on the human body and test equipment and can discharge without detection.Although the ADT70 features proprietary ESD protection circuitry, permanent damage mayoccur on devices subjected to high energy electrostatic discharges. Therefore, proper ESDprecautions are recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

OBSOLETE

ADT70

REV. 0–4–

TEMPERATURE – 8C

5

4

0

SU

PP

LY C

UR

RE

NT

– m

A

3

2

1

225 25 75 125

0.5

1.5

2.5

3.5

4.5 VS = +5V, NO LOAD

Figure 1. Supply Current vs. Temperature

TEMPERATURE – 8C

SY

ST

EM

GA

IN –

mV

/V

225 25 75 1251.2

1.25

1.3

VS = +5V, NO LOAD

1.4

1.35

Figure 2. System Gain vs. Temperature

TEMPERATURE – 8C

SY

ST

EM

GA

IN P

SR

R –

%/V

225 25 75 125

20.06

VS = +5V, NO LOAD

20.08

20.1

20.04

20.02

0

0.02

0.04

0.06

0.08

0.1

Figure 3. Total System Gain PSRR vs. Temperature

TEMPERATURE – 8C

INS

TR

UM

EN

TA

TIO

N A

MP

LIF

IER

INP

UT

OF

FS

ET

VO

LTA

GE

– m

V

225 25 75 125

260

VS = +5V, NO LOAD

280

2100

240

220

0

20

40

60

80

100

Figure 4. Instrumentation Amplifier Input Offset Voltagevs. Temperature

TEMPERATURE – 8C

INS

TR

UM

EN

TA

TIO

N A

MP

LIF

IER

OU

TP

UT

OF

FS

ET

VO

LTA

GE

– m

V

225 25 75 125

26

VS = +5V, NO LOAD

28

210

24

22

0

2

4

6

8

10

Figure 5. Instrumentation Amplifier Output Offset Voltagevs. Temperature

TEMPERATURE – 8C

INS

TR

UM

EN

TA

TIO

N A

MP

LIF

IER

INP

UT

BIA

S C

UR

RE

NT

– n

A

225 25 75 125

VS = +5V, NO LOAD

270

260

0

250

240

230

220

210

Figure 6. Instrumentation Amplifier Input Bias Current vs.Temperature

OBSOLETE

ADT70

REV. 0 –5–

TEMPERATURE – 8C

INS

TR

UM

EN

TA

TIO

N A

MP

LIF

IER

INP

UT

OF

FS

ET

CU

RR

EN

T –

pA

225 25 75 125

2300

VS = +5V, NO LOAD

2400

2500

2200

2100

500

0

100

200

300

400

Figure 7. Instrumentation Amplifier Input Offset Currentvs. Temperature

TEMPERATURE – 8C

INS

TR

UM

EN

TA

TIO

N A

MP

LIF

IER

GA

IN –

V/V

225 25 75 125

1.45

VS = +5V, NO LOAD

1.4

1.5

1.55

1.6

Figure 8. Instrumentation Amplifier Gain vs. Temperature

TEMPERATURE – 8C

OP

AM

P IN

PU

T O

FF

SE

T V

OLT

AG

E –

mV

225 25 75 125

260

VS = +5V, NO LOAD

280

2100

240

220

100

0

20

40

60

80

Figure 9. Op Amp Input Offset Voltage vs. Temperature

TEMPERATURE – 8C

OP

AM

P IN

PU

T B

IAS

CU

RR

EN

T –

nA

225 25 75 125

VS = +5V, NO LOAD

270

260

0

250

240

230

220

210

Figure 10. Op Amp Input Bias Current vs. Temperature

TEMPERATURE – 8C

OP

AM

P IN

PU

T O

FF

SE

T C

UR

RE

NT

– p

A

225 25 75 125

VS = +5V, NO LOAD

500

0

100

200

300

400

Figure 11. Op Amp Input Offset Current vs. Temperature

TEMPERATURE – 8C

RE

FE

RE

NC

E V

OLT

AG

E –

V

225 25 75 125

VS = +5V, NO LOAD

2.49

2.495

2.5

2.505

2.51

Figure 12. Reference Voltage vs. Temperature

OBSOLETE

ADT70

REV. 0–6–

LOAD CURRENT – mA

11 10

D R

AIL

OU

TP

UT

VO

LTA

GE

– m

V

10

100

1000

100 1k 10k

VCC = 5VVEE = 0TA = +258C

DVCC,SOURCINGCURRENT

DVEE,SINKINGCURRENT

Figure 13. Op Amp Output Voltage from Rails vs. Load Current

LOAD CURRENT – mA

2.480 91

RE

FE

RE

NC

E V

OLT

AG

E –

V

2 3 4 5 6 7 8

2.485

2.49

2.495

2.5

2.505

2.51

2.515

2.52

VS = +5V, DUT SOURCING

Figure 14. Reference Voltage vs. Load Current

SUPPLY VOLTAGE – Volts

4

34.5

I SY

, SU

PP

LY C

UR

RE

NT

– m

A

3.2

4.75 5.0 5.25 5.5

TA = +258CVCM INAMP = 1VVEE = GND

3.4

3.6

3.8

Figure 15. Supply Current vs. Supply Voltage

SUPPLY VOLTAGE – Volts

9104.75

OU

TP

UT

OF

CU

RR

EN

T S

OU

RC

E –

mA

920

930

940

950

4.5 5.0 5.25 5.5

VCC = 5VVEE = 0VVREF = 2.5V

+1258C

+258C

2558C

Figure 16. Output of Current Source vs. Supply Voltage

FREQUENCY – Hz

010

CM

RR

– d

B

100 1k 10k 100k 1M

20

40

60

80

100

120

140

AV = 14

AV = 1.4

Figure 17. In Amp CMRR vs. Frequency

FREQUENCY – Hz100

GA

IN –

dB

2801k 10k 100k 1M 10M

260

240

220

0

20

40

60

80

100

120

2135

290

245

0

45

90

135

PH

AS

E M

AR

GIN

– D

egre

es

2180

180

225

270

Figure 18. Op Amp Open Loop Gain and Phase vs. Frequency

OBSOLETE

ADT70

REV. 0 –7–

FREQUENCY – Hz10

CM

RR

– d

B

100 1k 10k 100k 1M0

20

40

60

80

100

120

Figure 22. Op Amp CMRR vs. Frequency

FREQUENCY – Hz10

PS

RR

– d

B

100 1k 10k 100k 1M220

20

40

60

80

100

120

0

+ PSRR

2 PSRR

Figure 23. Op Amp PSRR vs. Frequency

FREQUENCY – Hz100

CLO

SE

D L

OO

P G

AIN

– d

B

2201k 10k 100k 1M 10M

210

0

10

20

30

40

50

AVCL = 0

AVCL = 100

AVCL = 10

TA = +258CVCC = 4VVEE = 21V

Figure 24. Op Amp Closed Loop Gain vs. Frequency

FREQUENCY – Hz10

PS

RR

– d

B

100 1k 10k 100k 1M220

0

20

40

60

80

100

120

140

+ PSRR

2 PSRR

Figure 19. In Amp PSRR vs. Frequency – AV = 1.4

FREQUENCY – Hz10

PS

RR

– d

B

100 1k 10k 100k 1M220

0

20

40

60

80

100

120

140

+ PSRR

2 PSRR

Figure 20. In Amp PSRR vs. Frequency – AV = 14

FREQUENCY – Hz100

CLO

SE

D L

OO

P G

AIN

– d

B

2601k 10k 100k 1M 10M

240

220

0

20

40

60

80

100

AV = 14

AV = 1.4

Figure 21. In Amp Closed Loop Gain vs. Frequency

OBSOLETE

ADT70

REV. 0–8–

FUNCTIONAL DESCRIPTIONThe ADT70 provides excitation and signal conditioning forresistance-temperature devices (RTDs). It is ideally suited for1 kΩ Platinum RTDs (PRTDs), which allow a much widerrange of temperature measurement than silicon-based sensors.Using a low cost PRTD, the ADT70 can measure temperaturesin the range of –50°C to +500°C.

The two main components in the ADT70 are the adjustablecurrent sources and the instrumentation amplifier. The currentsources provide matching excitation currents to the PRTD andto the Reference Resistor. The instrumentation amplifier com-pares the voltage drop across both the PRTD and ReferenceResistor, and provides an amplified output signal voltage that isproportional to temperature.

Besides the matching current sources and the instrumentationamplifier, there is a general purpose op amp for any applicationdesired. The ADT70 comes with a +2.5 V reference on board.

RGA RGB

2INIA

+INIAINSTAMP

SHUT-DOWN

GNDSENSE

OUTIA AGND DGND2VS

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

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 tosuit the actual application.

A basic physical property of any metal is that its electrical resis-tivity changes with temperature. Some metals are known to havea very predictable and repeatable change of resistance for agiven change in temperature. An RTD is fabricated from one ofthese metals to a nominal ohmic value at a specified tempera-ture. By measuring its resistance at some unknown temperatureand comparing this value to the resistor’s nominal value, thechange 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. Thismakes the RTD a practical temperature sensor, which in its bareform 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 mostpopular 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 TemperatureDetectors (PRTDs) are becoming more competitive through thewide use of thin-film-type resistive elements.

Temperature Coefficient of ResistanceThe temperature coefficient (TC, also referred to as α) of anRTD, describes the average resistance change per unit tempera-ture from the ice point to the boiling point of water.

TCR C

R R

C R Ω Ω °( ) = −

° ×100 0

0100

R0 = Resistance of the sensor at 0°CR100 = Resistance of the sensor at +100°CTCR = Thermal Coefficient of Resistance.

For example, a platinum thermometer measuring 100 Ω at 0°Cand 138.5 Ω at 100°C, has TCR 0.00385 Ω/Ω/°C .

TCR

C= Ω − Ω

Ω × °=138 5 100

100 1000 00385

.

.

The larger the TCR, the greater the change in resistance for agiven change in temperature. The most common use of TCR isto distinguish between curves for platinum, which is availablewith TCRs ranging from 0.00375 to 0.003927. The highestTCR indicates the highest purity platinum and is mandated byITS-90 for standard platinum thermometers.

Basically, TCRs must be properly matched when replacing RTDsor 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 standardand is available in both wire-wound and thin-film elements.

Understanding Error SourceThe ADT70 uses an instrumentation amplifier that amplifies thedifference 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 resistorshould 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 referenceresistor is located close to the part, while the RTD is located at aremote location connected by wires, the lead-wires’ resistance

TEMPERATURE – 8C

0250

SY

ST

EM

RE

SP

ON

SE

TIM

E –

ms

0

10

225 25 50 75 100 125

20

30

40

50VOUT OF IN AMP = 300mVVCC = 5V SINGLE SUPPLY

= LOW TO HIGHTURNING ON

VSHUTDOWN

= HIGH TO LOWTURNING OFF

VSHUTDOWN

Figure 25. System Response Time from Shutdown vs.Temperature

OBSOLETE

ADT70

REV. 0 –9–

would contribute to the difference in voltage drop between theRTD 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 0.001614 0.002616 0.004118 0.006520 0.010322 0.016224 0.025726 0.041328 0.065130 0.1027

From Table I the amount of lead-wire resistance effect in thecircuit can be estimated. For example, connect 100 feet ofAWG 22 wire to a 100 Ω Platinum RTD (PF element). Thelead-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:

RTOTAL . . = + =100 1 62 101 62Ω Ω Ω

Since the 100 Ω reference resistor is assumed to be relatively closeto the ADT70, the lead-wire resistance is negligible. This shows1.62 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 approximatethe 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= ° = °1 62 0 385 4 21. / . / .Ω Ω

assuming the reference resistor is constant at 100 Ω throughoutthe temperature range.

As shown above, this is a significant inaccuracy, especially for ap-plications where the PRTD would be hundreds of feet away fromthe ADT70. To reduce lead-wire error it is recommended to usea larger sensitivity RTD; 1 kΩ instead of 100 Ω. Furthermore, inthe application circuit section, Figure 28 illustrates how to elimi-nate such error by using the part’s general purpose op amp.

Self-Heating EffectAnother 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 currenttimes the resistance of the element. The error contribution of theheat generated by this power dissipation can easily be calculated.For example, if the package thermal resistance is 50°C/W, theRTD nominal resistance is 1 kΩ and the element is excited with a1 mA current source, then the artificial increase in temperature(∆ºC) as a result of self-heating is:

∆° = ×C I R PACKAGE2

0 θ

∆° = ( ) × Ω × °C mA C W1 1000 50

2 /

∆° = °C C0 05.where:uPACKAGE = thermal resistance of packageR0 = value of RTD resistance

APPLICATION INFORMATIONAs shown in Figure 27, using a 1 kΩ PRTD, 1 kΩ referenceresistor, 49.9 kΩ resistor between RGA (Pin 11) and RGB (Pin12), and shorting BIAS (Pin 4) with VREFOUT (Pin 3) together,the output of OUTIA (Pin 14) will have a transfer function of

V mV ROUT PRTD RESISTANCE REFERENCE RESISTANCE= Ω × ( )1 299. / ∆ −

RGA RGB

+INIA

2INIA

INSTAMP

SHUT-DOWN

GNDSENSE

OUTIA AGND DGND2VS

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

ADT70

49.9kV

VOUT @ 5mV/8C

INDEPENDENTOP AMP

50kV

+5V

1kVPRTD

1kVREF

RESISTOR

POTENTIOMETERIS USED TOACHIEVE HIGHERPRECISION OFMATCHINGCURRENT.

21V < 2VS < 25V

Figure 27. Basic Operational Diagram

OBSOLETE

ADT70

REV. 0–10–

If PRTD has a tempco resistance of 0.00385 Ω/Ω/°C or sensi-tivity of 3.85 Ω/°C, the system output voltage scaling factor willbe 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 thegain resistor using the following equation.

Instrumentation Amp Gain

k

RGAIN RESISTOR

. .

= Ω

1 30

49 9

In Figure 2 the ADT70 is powered by a dual power supply. Inorder for the part to measure below 0°C, using a 1 kΩ PRTD, –VS has to be at least –1 V. –VS can be grounded when the mea-sured temperature is greater than 0°C using a 1 kΩ PRTD. GNDSense (Pin 13), DGND (Pin 15), and AGND (Pin 2) are all con-nected to ground. If desired, GND Sense could be connected towhatever potential desired for an output offset of the instrumen-tation amplifier. However, AGND and DGND must always beconnected to GND.

ADT70 will turn off if the SHUTDOWN pin(GND) is low,and will turn on when SHUTDOWN pin becomes high (+VS).If SHUTDOWN is not used in the design, it should be con-nected to +VS.

The undedicated op amp in the ADT70 can be used to transmitmeasured signal to a remote location where noise might be intro-duced into the signal as it travels in a noisy environment. It canalso be used as a general purpose amplifier in any application de-sired. The op amp gain is set using standard feedback resistorconfigurations.

Higher precision of matching the current sources can beachieved by using a 50 kΩ potentiometer connected betweenNULLA (Pin 5) and NULLB (Pin 6) with the center-tap of thepotentiometer connected to +VS (Pin 20). In Figure 27, theADT70’s Bias Pin (Pin 4) is generally connected to theVREFOUT (Pin 3), but it can be connected to an external voltagereference if different output current is preferred.

Eliminating Lead-Wire Resistance by Using 4-WireConfigurationIn applications where the lead-wire resistance can significantlycontribute error to the measured temperature, implementing a4-wire lead-resistance canceling circuit can dramatically mini-mize the lead-wire resistance effect.

In Figure 28, IOUTA and IOUTB provides matching excitation tothe reference resistor and the PRTD respectively. The lead-re-sistance from the current source to the PRTD or reference resis-tor is not of concern because the instrumentation amplifier ismeasuring the difference in potential directly on the PRTD(Node A) and reference resistor (Node C). Since there is almostno current going from Node A and Node C into the amplifier’sinput, there is no lead-wire resistance error.

A potential source of temperature measurement errors is thepossibility of voltage differences between the ground side of thereference resistor and the PRTD. Differences in lead-wire resis-tance from ground to these two points, coupled with the 1 mAexcitation current, will lead directly to differential voltage errorsat the input of the instrumentation amplifier of the ADT70. Byconnecting the ground side of the PRTD (Node B of Figure 28)to the noninverting input of the op amp and connecting theground side of the reference resistor (Node D) to both the in-verting input and the output of the op amp, the two points canbe forced to the same potential. It is not important that this po-tential is exactly at ground since the instrumentation amplifierrejects common-mode signals at the input. Note that all threeconnections should be made as close as possible to the body ofthe reference resistor and the PRTD to minimize error.

Single Supply OperationWhen using the ADT70 in single supply applications a fewsimple but important points need to be considered. The mostimportant issue is ensuring that the ADT70 is properly biased.To bias the ADT70, first consider the 1 kΩ PRTD sensor. ThePRTD typically changes from 230 Ω at –200°C to 4080 Ω at800°C ± 1 Ω error. This impedance range results in an ADT70output of –1 V to +4 V respectively, which is impossible to

RGA RGB

+INIA

2INIA

INSTAMP

SHUT-DOWN

GNDSENSE

OUTIA DGND2VS

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

ADT70

50kV

1kVPRTD

NODE D

1kVREF

RESISTOR NODE C

NODE B

NODE A

5V

AGND

25V

Figure 28. 4-Wire Lead-Wire Resistance Cancellation Circuit

OBSOLETE

ADT70

REV. 0 –11–

achieve in a single supply application where the negative rail isground or 0 V. Therefore, to achieve full scale operation theoutput of ADT70 should be shifted by 1 V to allow for opera-tion in the 0 V to 5 V region.

The most straightforward method to shift the output voltageincorporates the use of the GND SENSE as shown in Figure 29.To shift output voltage range apply a potential equal to the neces-sary shift on the GND SENSE pin. For example, to shift the out-put voltage, OUTIA, up to 1 V to GND SENSE, apply 1 V toGND SENSE. When applying a potential to GND SENSE, careshould be taken to ensure that the voltage source is capable of driv-ing 2 kΩ and does not introduce excessive noise. Figure 29 uses theon-board 2.5 V voltage reference for a low noise source. This refer-ence is then divided to 1 V and buffered by the on-board op ampto drive GND SENSE at a low impedance. A small 500 Ω potenti-ometer can be used to calibrate the initial offset error to zero.

However, a voltage applied to GND SENSE is not the onlymethod to shift the voltage range. Placing a 768 Ω resistor in thePRTD sensor path also shifts the output voltage by 1 V. Thissecond method, as shown in Figure 30, is usually not recom-mended for the following reasons; the input voltage range of theop amps is limited to around 1 V from the negative and positiverails and this could cause problems at high temperature, limitingthe upper range to 600°C; the physical location of this resistor(if placed at a distance from the ADT70) may have an impacton the noise performance. The method frees up the on-board opamp for another function and achieves the lowest impedanceground point for GND SENSE.

This brief section on ADT70 single supply operation has focusedon simple techniques to bias the ADT70 such that all output volt-ages are within operational range. However, these techniques maynot be useful in all single supply applications. For example, in Fig-ure 3 the additional on-board op amp is operating at near groundpotential which will create problems in a single supply application

+INIA

2INIA

INSTAMP

SHUT-DOWN

GNDSENSE

OUTIA DGND

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

1kVPRTDSENSOR

RG

RG

49.9kV1kVREF

RESISTORTO CONTROLLER

15kV

9.76kV

500VPOT

ADT70

Figure 29. A Single Supply Application with Shifted Ground Sense Pin

+INIA

2INIA

INSTAMP

SHUT-DOWN

GNDSENSE

DGND

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

1KVPRTD

RG

RG

49.9kV768VRESISTOR

TO CONTROLLER

1KVREFRESISTOR 2VS

TO A/D CONVERTER

VREF

+5V

ADT70

Figure 30. A Basic Single Supply Operational Diagram with Bias Resistor in Sensor Path

OBSOLETE

ADT70

REV. 0–12–

because the input voltage range of the on-board op amp only ex-tends to about 1 V above the negative rail. If the application re-quires the inputs of either the on-board amp or instrumentationamplifier to operate within 1 V of ground, it will be necessary togenerate a “pseudo-ground.” Figure 31 illustrates a typicalADT70 “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 followingreasons: low noise, ability to drive the required 5 mA in thisapplication, good temperature stability, which is usually impor-tant in a PRTD application. However, one undesired effect ofintroducing the pseudo-ground is the loss in voltage range athigh temperature. In our example, the PRTD will only operatefrom –200°C to +400°C corresponding to an input voltagerange of 1 V to 4 V.

100 V PRTD Application CircuitA 1000 Ω PRTD sensor scales by 3.85 Ωs/°C, which is exactlyten times the scale of the 100 Ω PRTD sensor. The ADT70has been designed to allow for 1000 Ω or 100 Ω PRTD sen-sors. Only the gain setting resistor RG needs to be altered. For

a 100 Ω PRTD 0.00385 sensor, change RG to 4.99 kΩ as illus-trated in Figure 32. In single supply application, with a 100 ΩPRTD sensor, a “pseudo-ground” will be necessary because theinputs of the instrumentation amplifier will be within 1 V of thenegative rail. See the section on single supply applications formore information.

+INIA

2INIA

INSTAMP

RG

RG

4.99kV

GNDSENSE

OUT

Figure 32. 100 Ω 0.00385 PRTD Application ShowingNew Value for RG

+INIA

2INIA

INSTAMP

SHUT-DOWN

GNDSENSE

DGND

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

1kVPRTD

RG

RG

49.9kV

2VS

OUTINGND

ADR290

0.1mF0.1mF10mF

OUT AGND

1kVREFRESISTOR

NODE D

NODE C

+5V

+5V

ADT70

Figure 31. Single Supply Application with an ADR290 “Pseudo-Ground”OBSOLETE

ADT70

REV. 0 –13–

American PRTD Application CircuitThe 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 mostcommon PRTD sensor. However, there is also an American PRTDsensor 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 a5 mV/°C ADT70 output.

+INIA

2INIA

INSTAMP

RG

RG

49.9kV

2kV

1kVREFRESISTOR

1kVPRTD

IOUTA

IOUTB

NOTE: IDEAL VALUEFOR RG = 51kV

GNDSENSE

OUT

Figure 33. Typical PRTD Application with American0.003916 Ω/ Ω /°C Scale; 1 kΩ Scale

+INIA

2INIA

INSTAMP

SHUT-DOWN

DGND

SHUTDOWN

+INOA

2INOA

OUTOA

+VS

2.5VREF

IOUTA

IOUTB

MATCHEDCURRENTSOURCES

NULLA NULLB BIAS 2.5VREFOUT

RG

RG

+5V

RR

RR

COLUMBIA RESEARCH LABMODEL DT3617STRAIN SENSORR = 1kV

25V

ADT70

Figure 34. Typical Strain Sensor Application (Two Element Varying)

Strain Gauge Sensor Application CircuitFigure 34 illustrates a typical strain gauge bridge circuit. Theversatility of the ADT70 allows the part to be used with mostbridge circuits that are within the 50 kΩ to 5 kΩ impedancerange. The sensor used in this circuit has two elements varying.If a constant current is driven into the sensor, a linear VOUT isobtained. In addition, the ADT70 will work with most bridgecircuits whether one-, two-, or all-element varying.

Securing Additional Current from the Current SourcesSome sensor applications need a higher excitation current to in-crease sensor sensitivity. There are two methods to increase thecurrent from the on-board current sources of the ADT70. Themost flexible method involves changing the voltage at the BIASnode. The equation for determining the BIAS potential vs. Out-put current is 2.5 V for roughly 1 mA, or in other words, todouble the current output simply put 5 V into BIAS. The BIASnode should be driven with a low-noise source, such as a refer-ence, because output current is directly dependent on BIAS volt-age. Directly tying BIAS to the positive supply rail may producetoo much current noise especially if the positive rail is not wellregulated. The second method involves tying the two ADT70current outputs together which doubles the current. Of course,this technique is most useful if, as illustrated in Figure 34, the ap-plication requires only one current source.

OBSOLETE

ADT70

REV. 0–14–

OUTLINE DIMENSIONSDimensions shown in inches and (mm).

20-Lead Plastic DIP(P-Suffix)

20

1 10

11

1.060 (26.90)0.925 (23.50)

0.280 (7.11)0.240 (6.10)

PIN 1

SEATINGPLANE

0.022 (0.558)0.014 (0.356)

0.060 (1.52)0.015 (0.38)

0.210 (5.33)MAX 0.130

(3.30)MIN

0.070 (1.77)0.045 (1.15)

0.100(2.54)BSC

0.160 (4.06)0.115 (2.93)

0.325 (8.25)0.300 (7.62)

0.015 (0.381)0.008 (0.204)

0.195 (4.95)0.115 (2.93)

20-Lead SOIC(S-Suffix)

SEATINGPLANE

0.0118 (0.30)0.0040 (0.10)

0.0192 (0.49)0.0138 (0.35)

0.1043 (2.65)0.0926 (2.35)

0.0500(1.27)BSC

0.0125 (0.32)0.0091 (0.23)

0.0500 (1.27)0.0157 (0.40)

8°0°

0.0291 (0.74)0.0098 (0.25)

x 45°

20 11

101

0.5118 (13.00)0.4961 (12.60)

0.41

93 (

10.6

5)0.

3937

(10

.00)

0.29

92 (

7.60

)0.

2914

(7.

40)

PIN 1

C33

95–8

–7/9

8P

RIN

TE

D IN

U.S

.A.OBSOLETE