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a
FEATURES PRTD Temperature Measurement Range Typical IC Measurement Error 1 C Includes Two Matched Current Sources Rail-to-Rail Output Instrumentation Amp Uncommitted, Rail-to-Rail Output Op Amp On-Board 2.5 V Reference Temperature Coefficient 25 ppm/ C 5 V or 5 V Operation Supply Current 4 mA Max 10 A Max in Shutdown APPLICATIONS Temperature Controllers Portable Instrumentation Temperature Acquisition Cards GENERAL DESCRIPTION
PRTD Conditioning Circuit and Temperature Controller ADT70*
FUNCTIONAL BLOCK DIAGRAM
NULLA NULLB BIAS 2.5VREFOUT
ADT70
+VS OUTOA
IOUTA IOUTB
MATCHED CURRENT SOURCES 2.5V REF
+INOA INOA
+INIA INIA
INST AMP
SHUTDOWN
SHUTDOWN
RGA RGB GND SENSE
OUTIA AGND
VS
DGND
The ADT70 provides excitation and signal conditioning for resistance-temperature devices (RTDs). It is ideally suited for 1 k Platinum RTDs (PRTDs), allowing a very wide range of temperature measurement. It can also easily interface to 100 PRTDs. Using a remote, low cost thin-film PRTD, the ADT70 can measure temperature in the range of -50C to +500C. With high performance platinum elements, the temperature change can be extended to 1000C. Accuracy of the ADT70 and PRTD system over a -200C to +1000C temperature range heavily depends on the quality of the PRTD. Typically the ADT70 will introduce an error of only 1C 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 A. 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 40C to 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.
PIN CONFIGURATIONS 20-Lead P-DIP (N Suffix)
-VS 1 AGND 2 VREFOUT 3 BIAS 4 NULLA 5 20 +VS 19 VOUT OA 18 -INOA 17 +INOA
ADT70
TOP VIEW NULLB 6 (Not to Scale) 15 DGND IOUTA 7 IOUTB 8 -INIA 9 +INIA 10
a
16 SHUTDOWN
14 VOUT IA 13 GND SENSE 12 RGB 11 RGA
20-Lead SOIC (R Suffix)
-VS 1 AGND 2 VREFOUT 3 BIAS 4 NULLA 5 20 +VS 19 VOUT OA 18 -INOA 17 +INOA
ADT70
16 SHUTDOWN
TOP VIEW NULLB 6 (Not to Scale) 15 DGND IOUTA 7 IOUTB 8 INIA 9 INIA 10
a
125C
14 VOUT IA 13 GND SENSE 12 RGB 11 RGA
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 (c) Analog Devices, Inc., 1998
ADT70-SPECIFICATIONS (V =
S
5 V,
40 C T A
125 C unless otherwise noted)
Min
1.234 -2.25
Parameter
SYSTEM CONFIGURATION Gain Line Regulation CURRENT SOURCES Output Current Output Current Mismatch Voltage Compliance INSTRUMENTATION AMP Input Offset Voltage Output Offset Voltage Input Bias Current Input Offset Current Common-Mode Rejection Output Voltage Swing Power Supply Rejection Ratio VOLTAGE REFERENCE Output Voltage Load Regulation Temperature Coefficient Line Regulation OPERATIONAL AMPLIFIER Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Offset Current Open-Loop Voltage Gain Output Voltage Swing Common-Mode Rejection Ratio Power Supply Rejection Ratio Slew Rate SHUTDOWN INPUT Input Low Voltage Input High Voltage POWER SUPPLY Supply Current Shutdown Supply Current Supply Voltage Dual Supply Voltage
Specifications subject to change without notice.
Symbol
Conditions
RL = 1 k
Typ
1.295 0.35 0.9 0.5 -V S to +VS - 1.5 150 100 5 3 40 30 1 85 0.5 2.5 2.5 250 10 75 400 200 1 40 30 1 2
Max
1.364 2.25
Units
mV/ %/V mA A V V V mV mV nA nA nA dB mV mV/V V V ppm/mA ppm/C ppm/V V V V/C nA nA nA V/V mV dB dB dB V/s
I Q1, IQ2 I Q1 - IQ2
RL = 1 k RL = 1 k
-2
2
VIOS TA = +25C VOOS TA = +25C IB TA = +25C I OS CMR VOUT PSRR VCM = 0.5 V to 3 V RL = , VS = 5 V + 4.5 V VS 5.5 V
-700 -500 -12 -7 -75 -60 -3 65 -V S + 25 -2.5 2.485 2.49
700 500 12 7 75 60 3 +V S - 25 2.5 2.515 2.51
TA = +25C I L = 0 mA to 1 mA + 4.5 V VS +5.5 V VIOA TA = +25C TCVIOA IB TA = +25C I OS AVOL VOUTA CMRR PSRR SR RL = RL = VCM = 1 V to 4 V TA = +25C 3 V VS 6 V TA = +25C, AV = 1, VIN = 0 V to 4 V
-1,000 -800 -75 -60 -3 -V S + 10 85 88 100
1,000 800 75 60 3 +V S - 10
105 110 150 0.17
VIL VIH I SY I SD VS RL = 1 k
0.8 2.4 3.5 10 +4.5 4.5 5 30 +5.5 5.5
V V mA A V V
-2-
REV. 0
ADT70
ABSOLUTE MAXIMUM RATINGS* ORDERING GUIDE
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 V Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite Storage Temperature Range N, R Package . . . . . . . . . . . . . . . . . . . . . . 65C to 150C Operating Temperature Range . . . . . . . . . . 40C to 125C Junction Temperature Range N, R Package . . . . . . . . . . . . . . . . . . . . . . 65C to 125C Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . 300C
NOTE *Stresses above those listed under Absolute Maximum Ratings may cause permanent 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.
Model ADT70GR ADT70GN
Temperature Range 40C to 40C to 125C 125C
Package 20-Lead SOIC 20-Lead PDIP
TRANSISTOR COUNT: 158
Package Type 20-Lead SOIC (R) 20-Lead PDIP (N)
* JA
JC
Units C/W C/W
74 102
24 31
NOTE * JA is specified for device in socket/soldered on circuit board (worst case conditions).
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.
WARNING!
ESD SENSITIVE DEVICE
REV. 0
-3-
ADT70
5
INSTRUMENTATION AMPLIFIER INPUT OFFSET VOLTAGE - V
100 VS = +5V, NO LOAD 80 60 40 20 0 20 40 60 80 100 25 25 75 TEMPERATURE - C 125 VS = +5V, NO LOAD
4.5 4 SUPPLY CURRENT - mA 3.5 3 2.5 2 1.5 1 0.5 0
25
25 75 TEMPERATURE - C
125
Figure 1. Supply Current vs. Temperature
Figure 4. Instrumentation Amplifier Input Offset Voltage vs. Temperature
1.4 INSTRUMENTATION AMPLIFIER OUTPUT OFFSET VOLTAGE - mV VS = +5V, NO LOAD 1.35 SYSTEM GAIN - mV/
10 8 6 4 2 0 2 4 6 8 10 25 25 75 TEMPERATURE - C 125 VS = +5V, NO LOAD
1.3
1.25
1.2
25
25 75 TEMPERATURE - C
125
Figure 2. System Gain vs. Temperature
Figure 5. Instrumentation Amplifier Output Offset Voltage vs. Temperature
0.1
INSTRUMENTATION AMPLIFIER INPUT BIAS CURRENT - nA
0 VS = +5V, NO LOAD
0.08 0.06 SYSTEM GAIN PSRR - %/V 0.04 0.02 0 0.02 0.04 0.06 0.08 0.1
10
VS = +5V, NO LOAD
20 30
40 50 60 70
25
25 75 TEMPERATURE - C
125
25
25 75 TEMPERATURE - C
125
Figure 3. Total System Gain PSRR vs. Temperature
Figure 6. Instrumentation Amplifier Input Bias Current vs. Temperature
-4-
REV. 0
ADT70
500
INSTRUMENTATION AMPLIFIER INPUT OFFSET CURRENT - pA
0
400 300 200 100 0 100 200 300 400 500 25 25 75 TEMPERATURE - C 125 70 25 25 75 TEMPERATURE - C 125 OP AMP INPUT BIAS CURRENT - nA VS = +5V, NO LOAD 10 VS = +5V, NO LOAD
20 30
40 50 60
Figure 7. Instrumentation Amplifier Input Offset Current vs. Temperature
Figure 10. Op Amp Input Bias Current vs. Temperature
1.6 INSTRUMENTATION AMPLIFIER GAIN - V/V OP AMP INPUT OFFSET CURRENT - pA
500
VS = +5V, NO LOAD 1.55
VS = +5V, NO LOAD 400
300
1.5
200
1.45
100
1.4 25 25 75 TEMPERATURE - C 125
0
25
25 75 TEMPERATURE - C
125
Figure 8. Instrumentation Amplifier Gain vs. Temperature
Figure 11. Op Amp Input Offset Current vs. Temperature
100 OP AMP INPUT OFFSET VOLTAGE - V 80 VS = +5V, NO LOAD 60 REFERENCE VOLTAGE - V
2.51
VS = +5V, NO LOAD 2.505
40 20 0 20 40 60 80 100 25 25 75 TEMPERATURE - C 125
2.5
2.495
2.49
25
25 75 TEMPERATURE - C
125
Figure 9. Op Amp Input Offset Voltage vs. Temperature
Figure 12. Reference Voltage vs. Temperature
REV. 0
-5-
ADT70
1000 OUTPUT OF CURRENT SOURCE - A VCC = 5V VEE = 0 TA = +25 C VCC, SOURCING CURRENT 950 VCC = 5V VEE = 0V VREF = 2.5V 940 RAIL OUTPUT VOLTAGE - mV
100
+125 C 930 +25 C
10
VEE, SINKING CURRENT
920
55 C
1 1 10 100 LOAD CURRENT - A 1k 10k
910 4.5
4.75
5.0 5.25 SUPPLY VOLTAGE - Volts
5.5
Figure 13. Op Amp Output Voltage from Rails vs. Load Current
Figure 16. Output of Current Source vs. Supply Voltage
2.52 2.515 VS = +5V, DUT SOURCING REFERENCE VOLTAGE - V 2.51 CMRR - dB 2.505 2.5 2.495
140 120
100
80 AV = 14 60 AV = 1.4 40
2.49 2.485 2.48 20 0 10
0
1
2
3 4 5 6 LOAD CURRENT - mA
7
8
9
100
1k 10k FREQUENCY - Hz
100k
1M
Figure 14. Reference Voltage vs. Load Current
Figure 17. In Amp CMRR vs. Frequency
4 TA = +25 C VCM INAMP = 1V VEE = GND ISY, SUPPLY CURRENT - mA 3.8
120 100 80 60
270 225 180 135 90 45 0 45 90 135 180 1k 10k 100k FREQUENCY - Hz 1M 10M PHASE MARGIN - Degrees
3.6
GAIN - dB 4.75 5.0 5.25 SUPPLY VOLTAGE - Volts 5.5
40 20 0 20
3.4
3.2
40 60
3 4.5
80 100
Figure 15. Supply Current vs. Supply Voltage
Figure 18. Op Amp Open Loop Gain and Phase vs. Frequency
-6-
REV. 0
ADT70
140 120 100 100 80 PSRR - dB CMRR - dB 80 60 40 20 PSRR 0 20 0 20 120
60
+ PSRR
40
10
100
1k 10k FREQUENCY - Hz
100k
1M
10
100
1k 10k FREQUENCY - Hz
100k
1M
Figure 19. In Amp PSRR vs. Frequency - AV = 1.4
Figure 22. Op Amp CMRR vs. Frequency
140 120 100
120
100 80
PSRR - dB
+ PSRR 60 40 PSRR 20 0 20
PSRR - dB
80
60 + PSRR 40 PSRR 20 0
10
100
1k 10k FREQUENCY - Hz
100k
1M
20
10
100
1k 10k FREQUENCY - Hz
100k
1M
Figure 20. In Amp PSRR vs. Frequency - AV = 14
Figure 23. Op Amp PSRR vs. Frequency
100 80
CLOSED LOOP GAIN - dB
50 40 AVCL = 100 CLOSED LOOP GAIN - dB 30 20 AVCL = 10 10 0 AVCL = 0 10 20 100 TA = +25 C VCC = 4V VEE = 1V
60 40 20 AV = 14 0 20 AV = 1.4 40 60 100
1k
10k 100k FREQUENCY - Hz
1M
10M
1k
10k 100k FREQUENCY - Hz
1M
10M
Figure 21. In Amp Closed Loop Gain vs. Frequency
Figure 24. Op Amp Closed Loop Gain vs. Frequency
REV. 0
-7-
ADT70
50 VOUT OF IN AMP = 300mV VCC = 5V SINGLE SUPPLY SYSTEM RESPONSE TIME - s 40
30
TURNING ON VSHUTDOWN
= LOW TO HIGH
20
10
TURNING OFF VSHUTDOWN
= HIGH TO LOW
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.
125
0
50
25
0
25
50
75
100
TEMPERATURE - C
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 -50C to +500C. 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 2.5VREFOUT
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 ) of an RTD, describes the average resistance change per unit temperature from the ice point to the boiling point of water. TCR C =
(
)
R100 - R0 100C x R0
R0 = Resistance of the sensor at 0C R100 = Resistance of the sensor at +100C TCR = Thermal Coefficient of Resistance. For example, a platinum thermometer measuring 100 at 0C and 138.5 at 100C, has TCR 0.00385 //C . TCR = 138.5 - 100 = 0.00385 100 x 100C
ADT70
+VS OUTOA
IOUTA IOUTB
MATCHED CURRENT SOURCES 2.5V REF
+INOA INOA
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
+INIA INIA
INST AMP
SHUTDOWN
SHUTDOWN
RGA RGB GND SENSE
OUTIA AGND
VS
DGND
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.
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
-8-
REV. 0
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.
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 k instead of 100 . 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
Lead-wire AWG 12 14 16 18 20 22 24 26 28 30
Ohms/foot at +25C 0.0016 0.0026 0.0041 0.0065 0.0103 0.0162 0.0257 0.0413 0.0651 0.1027
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 50C/W, the RTD nominal resistance is 1 k and the element is excited with a 1 mA current source, then the artificial increase in temperature (C) as a result of self-heating is: C = I 2 R0 x PACKAGE C = 1 mA x 1000 x 50C /W C = 0.05C where:
PACKAGE = thermal resistance of package R0 = value of RTD resistance
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 Platinum RTD (PF element). The lead-wire resistance will be: R = 100 ft 3 0.0162 /ft = 1.62 . Thus the total resistance you have with the PRTD will be: RTOTAL = 100 + 1.62 = 101.62 Since the 100 reference resistor is assumed to be relatively close to the ADT70, the lead-wire resistance is negligible. This shows 1.62 of inaccuracy. From the PRTD's data sheet, the PRTD's sensitivity rating (/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 PRTD is 0.385 /C (taken from PRTD data sheet). Hence the approximate error is: Error = 1.62 / 0.385 / C = 4.21C assuming the reference resistor is constant at 100 throughout the temperature range.
()
2
APPLICATION INFORMATION
As shown in Figure 27, using a 1 k PRTD, 1 k reference resistor, 49.9 k resistor between RGA (Pin 11) and RGB (Pin 12), and shorting BIAS (Pin 4) with VREFOUT (Pin 3) together, the output of OUTIA (Pin 14) will have a transfer function of VOUT = 1.299 mV / x R ( PRTD RESISTANCE - REFERENCE RESISTANCE )
+5V
50k POTENTIOMETER IS USED TO NULLA ACHIEVE HIGHER PRECISION OF MATCHING CURRENT. IOUTA IOUTB NULLB BIAS 2.5VREFOUT
ADT70
+VS OUTOA
MATCHED CURRENT SOURCES 2.5V REF
+INOA INOA INDEPENDENT OP AMP
INIA +INIA 1k REF RESISTOR INST AMP SHUTDOWN SHUTDOWN
1k PRTD
RGA
RGB GND OUTIA AGND SENSE 1V < VOUT @ 5mV/ C
VS
DGND
49.9k
VS <
5V
Figure 27. Basic Operational Diagram
REV. 0
-9-
ADT70
NULLA NULLB BIAS 2.5VREFOUT 5V
ADT70
1k REF RESISTOR NODE C NODE D IOUTB IOUTA
+VS OUTOA
MATCHED CURRENT SOURCES 2.5V REF
+INOA INOA
INIA +INIA INST AMP SHUTDOWN SHUTDOWN
NODE A 1k PRTD NODE B
RGA
RGB GND SENSE
OUTIA
AGND
VS
DGND
50k 5V
Figure 28. 4-Wire Lead-Wire Resistance Cancellation Circuit
If PRTD has a tempco resistance of 0.00385 //C or sensitivity of 3.85 /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 k gain resistor. It can be changed by changing the gain resistor using the following equation. 49.9 k Instrumentation Amp Gain = 1.30 RGAIN RESISTOR In Figure 2 the ADT70 is powered by a dual power supply. In order for the part to measure below 0C, using a 1 k PRTD, -VS has to be at least -1 V. -VS can be grounded when the measured temperature is greater than 0C using a 1 k 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 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 connected to +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 k potentiometer connected between NULLA (Pin 5) and NULLB (Pin 6) with the center-tap of the potentiometer connected to +VS (Pin 20). In Figure 27, the ADT70's Bias Pin (Pin 4) is generally connected to the VREFOUT (Pin 3), but it can be connected to an external voltage reference if different output current is preferred.
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, IOUTA and IOUTB provides matching excitation to 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 k PRTD sensor. The PRTD typically changes from 230 at -200C to 4080 at 800C 1 error. This impedance range results in an ADT70 output of -1 V to +4 V respectively, which is impossible to -10- REV. 0
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, OUTIA, up to 1 V to GND SENSE, apply 1 V to GND SENSE. When applying a potential to GND SENSE, care should be taken to ensure that the voltage source is capable of driving 2 k 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 potentiometer can be used to calibrate the initial offset error to zero. However, a voltage applied to GND SENSE is not the only method to shift the voltage range. Placing a 768 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 600C; 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
2.5VREFOUT
NULLA
NULLB
BIAS
ADT70
+VS OUTOA
IOUTA IOUTB
MATCHED CURRENT SOURCES 2.5V REF
15k +INOA 9.76k INOA 500 POT
INIA 1k REF RESISTOR RG 49.9k RG +INIA GND SENSE OUTIA DGND INST AMP SHUTDOWN TO CONTROLLER SHUTDOWN
1k PRTD SENSOR
Figure 29. A Single Supply Application with Shifted Ground Sense Pin
+5V
ADT70
NULLA NULLB BIAS 2.5VREFOUT
+VS IOUTA IOUTB MATCHED CURRENT SOURCES 2.5V REF INIA RG 768 RESISTOR 49.9k RG +INIA 1K REF RESISTOR INST AMP SHUTDOWN OUTOA +INOA INOA
VREF
TO CONTROLLER SHUTDOWN
1K PRTD
GND SENSE
VS
DGND
TO A/D CONVERTER
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 -200C to +400C corresponding to an input voltage range of 1 V to 4 V.
100
a 100 PRTD 0.00385 sensor, change RG to 4.99 k as illustrated in Figure 32. In single supply application, with a 100 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.
INIA RG 4.99k RG +INIA GND SENSE OUT INST AMP
A 1000 PRTD sensor scales by 3.85 s/C, which is exactly ten times the scale of the 100 PRTD sensor. The ADT70 has been designed to allow for 1000 or 100 PRTD sensors. Only the gain setting resistor RG needs to be altered. For
PRTD Application Circuit
Figure 32. 100 0.00385 PRTD Application Showing New Value for RG
NULLA
NULLB
BIAS
2.5VREFOUT
ADT70
+5V
1k REF RESISTOR NODE C NODE D IOUTA IOUTB
+VS MATCHED CURRENT SOURCES 2.5V REF OUTOA +INOA INOA
INIA RG 49.9k RG +INIA 1k PRTD +5V ADR290 IN OUT GND GND SENSE OUT AGND VS DGND INST AMP SHUTDOWN SHUTDOWN
10 F
0.1 F
0.1 F
Figure 31. Single Supply Application with an ADR290 "Pseudo-Ground"
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ADT70
American PRTD Application Circuit
The majority of PRTD sensors use a scale factor of 0.00385 //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 //C. Figure 33 illustrates the input section of the ADT70 configured for the American PRTD. The ideal value for RG is 50.98 k when yielding a 5 mV/C ADT70 output.
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 k to 5 k impedance range. The sensor used in this circuit has two elements varying. If a constant current is driven into the sensor, a linear VOUT 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
IOUTA IOUTB INIA RG 49.9k 2k 1k PRTD 1k REF RESISTOR RG +INIA NOTE: IDEAL VALUE FOR RG = 51k GND SENSE OUT INST AMP
Figure 33. Typical PRTD Application with American 0.003916 / /C Scale; 1 k Scale
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
2.5VREFOUT
+5V
ADT70
+VS IOUTA IOUTB R R MATCHED CURRENT SOURCES 2.5V REF INIA R R RG INST AMP RG +INIA COLUMBIA RESEARCH LAB MODEL DT3617 STRAIN SENSOR R = 1k DGND 5V SHUTDOWN SHUTDOWN OUTOA +INOA INOA
Figure 34. Typical Strain Sensor Application (Two Element Varying)
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ADT70
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Lead Plastic DIP (P-Suffix)
1.060 (26.90) 0.925 (23.50)
20 1 11 10
0.280 (7.11) 0.240 (6.10) 0.325 (8.25) 0.300 (7.62) 0.195 (4.95) 0.115 (2.93)
PIN 1 0.210 (5.33) MAX 0.160 (4.06) 0.115 (2.93) 0.022 (0.558) 0.014 (0.356) 0.100 (2.54) BSC
0.060 (1.52) 0.015 (0.38) 0.130 (3.30) MIN 0.070 (1.77) SEATING 0.045 (1.15) PLANE
0.015 (0.381) 0.008 (0.204)
20-Lead SOIC (S-Suffix)
0.5118 (13.00) 0.4961 (12.60)
20 11
1
10
PIN 1
0.1043 (2.65) 0.0926 (2.35)
0.4193 (10.65) 0.3937 (10.00)
0.2992 (7.60) 0.2914 (7.40)
0.0291 (0.74) x 45 0.0098 (0.25)
0.0118 (0.30) 0.0040 (0.10)
8 0.0500 0.0192 (0.49) 0 (1.27) 0.0138 (0.35) SEATING 0.0125 (0.32) PLANE BSC 0.0091 (0.23)
0.0500 (1.27) 0.0157 (0.40)
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PRINTED IN U.S.A.
C3395-8-7/98
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