a 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 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. PRTD Conditioning Circuit and Temperature Controller ADT70* FUNCTIONAL BLOCK DIAGRAM NULLA NULLB BIAS 2.5VREFOUT ADT70 OUTOA IOUTA MATCHED CURRENT SOURCES +INOA IOUTB 2.5V REF +INIA ⴚINOA INST AMP SHUTDOWN SHUTDOWN ⴚINIA OUTIA AGND ⴚVS RGA RGB GND SENSE DGND PIN CONFIGURATIONS 20-Lead P-DIP (N Suffix) –VS 1 20 +VS AGND 2 19 VOUT OA VREFOUT 3 18 –INOA BIAS 4 17 +INOA ADT70 NULLA 5 16 SHUTDOWN TOP VIEW NULLB 6 (Not to Scale) 15 DGND IOUTA 7 14 VOUT IA IOUTB 8 13 GND SENSE –INIA 9 12 RGB 11 RGA +INIA 10 a 20-Lead SOIC (R Suffix) –VS 1 20 +VS AGND 2 19 VOUT OA VREFOUT 3 18 –INOA BIAS 4 NULLA 5 17 +INOA ADT70 16 SHUTDOWN TOP VIEW NULLB 6 (Not to Scale) 15 DGND IOUTA 7 14 VOUT IA IOUTB 8 13 GND SENSE 2 INIA 9 *Patent pending. +VS 12 RGB 11 RGA 1 INIA 10 a 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. 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 ADT70–SPECIFICATIONS (V = ⴙ5 V, ⴚ40ⴗC ≤ T ≤ ⴙ125ⴗC unless otherwise noted) A S Parameter Symbol SYSTEM CONFIGURATION Gain Line Regulation CURRENT SOURCES Output Current Output Current Mismatch Voltage Compliance INSTRUMENTATION AMP Input Offset Voltage I Q1, IQ2 I Q1 – IQ2 Conditions Min Typ Max Units RL = 1 kΩ 1.234 –2.25 1.295 ⫾0.35 1.364 2.25 mV/Ω %/V –2 0.9 ⫾0.5 –V S to +VS – 1.5 2 mA µA V ⫾150 ⫾100 ⫾5 ⫾3 ⫾40 ⫾30 ⫾1 85 700 500 12 7 75 60 3 RL = 1 kΩ RL = 1 kΩ VIOS TA = +25°C Output Offset Voltage VOOS Input Bias Current IB Input Offset Current Common-Mode Rejection Output Voltage Swing Power Supply Rejection Ratio I OS CMR VOUT PSRR TA = +25°C TA = +25°C VCM = 0.5 V to 3 V RL = ∞ , VS = ⫾5 V + 4.5 V ≤ VS ≤ ⫾5.5 V VOLTAGE REFERENCE Output Voltage TA = +25°C I L = 0 mA to 1 mA Load Regulation Temperature Coefficient Line Regulation OPERATIONAL AMPLIFIER Input Offset Voltage VIOA TCVIOA IB TA = +25°C Input Offset Current Open-Loop Voltage Gain Output Voltage Swing Common-Mode Rejection Ratio I OS AVOL VOUTA CMRR Power Supply Rejection Ratio Slew Rate PSRR SR SHUTDOWN INPUT Input Low Voltage Input High Voltage POWER SUPPLY Supply Current Shutdown Supply Current Supply Voltage Dual Supply Voltage RL = ∞ RL = ∞ VCM = 1 V to 4 V TA = +25°C ⫾3 V ≤ VS ≤ ⫾6 V TA = +25°C, AV = 1, VIN = 0 V to 4 V VIL VIH I SY I SD VS ⫾0.5 +V S – 25 2.5 µV µV mV mV nA nA nA dB mV mV/V 2.485 2.49 2.5 2.5 250 ⫾10 ⫾75 2.515 2.51 V V ppm/mA ppm/°C ppm/V –1,000 –800 ⫾400 ⫾200 1 ⫾40 ⫾30 ⫾1 2 1,000 800 µV µV µV/°C nA nA nA V/µV mV dB dB dB V/µs + 4.5 V ≤ VS ≤ +5.5 V TA = +25°C Input Offset Voltage Drift Input Bias Current –700 –500 –12 –7 –75 –60 –3 65 –V S + 25 –2.5 –75 –60 –3 –V S + 10 85 88 100 75 60 3 +V S – 10 105 110 150 0.17 0.8 V V 5 30 +5.5 ⫾5.5 mA µA V V 2.4 RL = 1 kΩ 3.5 10 +4.5 ⫾4.5 Specifications subject to change without notice. –2– REV. 0 ADT70 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 N, R Package . . . . . . . . . . . . . . . . . . . . . . ⴚ65°C to ⴙ125°C Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . ⴙ300°C Model Temperature Range Package ADT70GR ADT70GN ⴚ40°C to ⴙ125°C ⴚ40°C to ⴙ125°C 20-Lead SOIC 20-Lead PDIP TRANSISTOR COUNT: 158 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. Package Type JA* JC Units 20-Lead SOIC (R) 20-Lead PDIP (N) 74 102 24 31 °C/W °C/W NOTE * θJA 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. REV. 0 –3– WARNING! ESD SENSITIVE DEVICE ADT70 5 100 VS = +5V, NO LOAD INSTRUMENTATION AMPLIFIER INPUT OFFSET VOLTAGE – mV 4.5 SUPPLY CURRENT – mA 4 3.5 3 2.5 2 1.5 1 0.5 0 225 25 75 TEMPERATURE – 8C 20 0 220 240 260 280 225 25 75 TEMPERATURE – 8C 125 10 INSTRUMENTATION AMPLIFIER OUTPUT OFFSET VOLTAGE – mV VS = +5V, NO LOAD 1.35 SYSTEM GAIN – mV/V 40 Figure 4. Instrumentation Amplifier Input Offset Voltage vs. Temperature 1.4 1.3 1.25 1.2 225 25 75 TEMPERATURE – 8C 4 2 0 22 24 26 28 225 25 75 TEMPERATURE – 8C 125 0 INSTRUMENTATION AMPLIFIER INPUT BIAS CURRENT – nA VS = +5V, NO LOAD 0.06 0.04 0.02 0 20.02 20.04 20.06 20.08 20.1 VS = +5V, NO LOAD 6 Figure 5. Instrumentation Amplifier Output Offset Voltage vs. Temperature 0.1 0.08 8 210 125 Figure 2. System Gain vs. Temperature SYSTEM GAIN PSRR – %/V VS = +5V, NO LOAD 60 2100 125 Figure 1. Supply Current vs. Temperature 80 225 25 75 TEMPERATURE – 8C VS = +5V, NO LOAD 220 230 240 250 260 270 125 Figure 3. Total System Gain PSRR vs. Temperature 210 225 25 75 TEMPERATURE – 8C 125 Figure 6. Instrumentation Amplifier Input Bias Current vs. Temperature –4– REV. 0 ADT70 0 400 VS = +5V, NO LOAD 210 OP AMP INPUT BIAS CURRENT – nA INSTRUMENTATION AMPLIFIER INPUT OFFSET CURRENT – pA 500 300 200 100 0 2100 2200 2300 220 230 240 250 260 2400 2500 225 25 75 TEMPERATURE – 8C 270 125 Figure 7. Instrumentation Amplifier Input Offset Current vs. Temperature OP AMP INPUT OFFSET CURRENT – pA INSTRUMENTATION AMPLIFIER GAIN – V/V VS = +5V, NO LOAD 1.55 1.5 1.45 225 125 VS = +5V, NO LOAD 400 300 200 100 0 25 75 TEMPERATURE – 8C 125 225 25 75 TEMPERATURE – 8C 125 Figure 11. Op Amp Input Offset Current vs. Temperature 100 2.51 80 VS = +5V, NO LOAD VS = +5V, NO LOAD 60 REFERENCE VOLTAGE – V OP AMP INPUT OFFSET VOLTAGE – mV 25 75 TEMPERATURE – 8C 500 Figure 8. Instrumentation Amplifier Gain vs. Temperature 40 20 0 220 240 260 2.505 2.5 2.495 280 2100 225 25 75 TEMPERATURE – 8C 2.49 125 Figure 9. Op Amp Input Offset Voltage vs. Temperature REV. 0 225 Figure 10. Op Amp Input Bias Current vs. Temperature 1.6 1.4 VS = +5V, NO LOAD 225 25 75 TEMPERATURE – 8C 125 Figure 12. Reference Voltage vs. Temperature –5– ADT70 950 VCC = 5V VEE = 0 TA = +258C DVCC, SOURCING CURRENT 100 DVEE, SINKING CURRENT 10 VCC = 5V VEE = 0V VREF = 2.5V OUTPUT OF CURRENT SOURCE – mA D RAIL OUTPUT VOLTAGE – mV 1000 940 +1258C 930 +258C 920 910 4.5 1 1 10 100 1k 2558C 10k 4.75 LOAD CURRENT – mA 5.0 5.25 SUPPLY VOLTAGE – Volts 5.5 Figure 16. Output of Current Source vs. Supply Voltage Figure 13. Op Amp Output Voltage from Rails vs. Load Current 140 2.52 2.515 120 2.51 100 CMRR – dB 2.505 2.5 80 AV = 14 60 2.495 AV = 1.4 40 2.49 20 2.485 0 1 2 3 4 5 6 LOAD CURRENT – mA 7 8 0 10 9 100 1k 10k FREQUENCY – Hz 100k Figure 17. In Amp CMRR vs. Frequency Figure 14. Reference Voltage vs. Load Current 4 TA = +258C VCM INAMP = 1V VEE = GND GAIN – dB 3.8 3.6 3.4 3.2 120 270 100 225 80 180 60 135 40 90 20 45 0 0 220 245 240 290 260 3 4.5 4.75 5.0 5.25 SUPPLY VOLTAGE – Volts 1M 280 100 5.5 PHASE MARGIN – Degrees 2.48 ISY, SUPPLY CURRENT – mA REFERENCE VOLTAGE – V VS = +5V, DUT SOURCING 2135 1k 10k 100k FREQUENCY – Hz 1M 2180 10M Figure 18. Op Amp Open Loop Gain and Phase vs. Frequency Figure 15. Supply Current vs. Supply Voltage –6– REV. 0 ADT70 120 140 120 100 100 80 60 CMRR – dB PSRR – dB 80 + PSRR 40 60 40 20 2 PSRR 20 0 220 10 100 1k 10k FREQUENCY – Hz 100k 0 1M Figure 19. In Amp PSRR vs. Frequency – AV = 1.4 10 100 1k 10k FREQUENCY – Hz 100k 1M Figure 22. Op Amp CMRR vs. Frequency 140 120 120 100 100 80 PSRR – dB PSRR – dB 80 + PSRR 60 40 + PSRR 40 2 PSRR 2 PSRR 20 20 0 0 220 60 10 100 1k 10k FREQUENCY – Hz 100k 220 1M 100 100 50 80 40 40 20 AV = 14 0 220 1M 30 20 AVCL = 10 10 0 AVCL = 0 AV = 1.4 210 240 1k 10k 100k FREQUENCY – Hz 1M 220 100 10M Figure 21. In Amp Closed Loop Gain vs. Frequency REV. 0 100k TA = +258C VCC = 4V VEE = 21V AVCL = 100 60 260 100 1k 10k FREQUENCY – Hz Figure 23. Op Amp PSRR vs. Frequency CLOSED LOOP GAIN – dB CLOSED LOOP GAIN – dB Figure 20. In Amp PSRR vs. Frequency – AV = 14 10 1k 10k 100k FREQUENCY – Hz 1M 10M Figure 24. Op Amp Closed Loop Gain vs. Frequency –7– ADT70 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. 50 SYSTEM RESPONSE TIME – ms VOUT OF IN AMP = 300mV VCC = 5V SINGLE SUPPLY 40 TURNING ON 30 = LOW TO HIGH VSHUTDOWN 20 TURNING OFF 10 = HIGH TO LOW VSHUTDOWN 0 250 225 0 25 50 75 100 125 TEMPERATURE – 8C 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 2.5VREFOUT ADT70 +VS OUTOA IOUTA MATCHED CURRENT SOURCES +INOA IOUTB 2.5V REF +INIA INST AMP ⴚINOA SHUTDOWN SHUTDOWN ⴚINIA 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. ( ) TCR Ω Ω °C = R100 − R0 100°C × R0 R0 = Resistance of the sensor at 0°C R100 = Resistance of the sensor at +100°C TCR = Thermal Coefficient of Resistance. For example, a platinum thermometer measuring 100 Ω at 0°C and 138.5 Ω at 100°C, has TCR 0.00385 Ω/Ω/°C . TCR = 138.5 Ω − 100 Ω = 0.00385 100 Ω × 100°C 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 RGA RGB GND SENSE OUTIA AGND ⴚVS 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. 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. Table I. Copper Wire Gauge Size to Resistance Table. Lead-wire AWG Ohms/foot at +25ºC 12 14 16 18 20 22 24 26 28 30 0.0016 0.0026 0.0041 0.0065 0.0103 0.0162 0.0257 0.0413 0.0651 0.1027 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 (∆ºC) as a result of self-heating is: ∆°C = I 2 R0 × θ PACKAGE 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: ( ) 2 ∆°C = 1 mA × 1000 Ω × 50°C /W ∆°C = 0.05°C where: PACKAGE = thermal resistance of package R0 = value of RTD resistance RTOTAL = 100 Ω + 1.62 Ω = 101.62 Ω 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. APPLICATION INFORMATION As shown in Figure 27, using a 1 kΩ PRTD, 1 kΩ reference resistor, 49.9 kΩ 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 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: VOUT = 1.299 mV / Ω × ∆R ( PRTD RESISTANCE − REFERENCE RESISTANCE ) Error = 1.62 Ω / 0.385 Ω / °C = 4.21°C assuming the reference resistor is constant at 100 Ω throughout the temperature range. +5V 50k⍀ POTENTIOMETER IS USED TO NULLA ACHIEVE HIGHER PRECISION OF MATCHING CURRENT. NULLB BIAS 2.5VREFOUT ADT70 +VS OUTOA IOUTA MATCHED CURRENT SOURCES IOUTB +INOA ⴚINOA 2.5V REF ⴚINIA INST AMP +INIA 1k⍀ REF RESISTOR 1k⍀ PRTD RGA SHUTDOWN RGB GND OUTIA AGND SENSE 49.9k⍀ ⴚVS DGND ⴚ1V < ⴚVS < ⴚ5V VOUT @ 5mV/ⴗC Figure 27. Basic Operational Diagram REV. 0 –9– SHUTDOWN INDEPENDENT OP AMP ADT70 NULLA NULLB BIAS 2.5VREFOUT 5V ADT70 1k⍀ REF RESISTOR NODE C NODE D +VS OUTOA IOUTA MATCHED CURRENT SOURCES IOUTB +INOA ⴚINOA 2.5V REF ⴚINIA NODE A 1k⍀ PRTD INST AMP +INIA RGA NODE B RGB GND SENSE SHUTDOWN OUTIA AGND ⴚVS SHUTDOWN DGND 50k⍀ ⴚ5V 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. 49.9 kΩ 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 0°C, using a 1 kΩ PRTD, –VS has to be at least –1 V. –VS can be grounded when the measured 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 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 kΩ 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 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 –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. 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. 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 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 potentiometer can be used to calibrate the initial offset error to zero. NULLA NULLB BIAS 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 ADT70 2.5VREFOUT +VS OUTOA IOUTA IOUTB 15k⍀ MATCHED CURRENT SOURCES +INOA ⴚINOA 2.5V REF 9.76k⍀ 500⍀ POT ⴚINIA 1k⍀ REF RESISTOR RG INST AMP 49.9k⍀ SHUTDOWN RG TO CONTROLLER SHUTDOWN +INIA 1k⍀ PRTD SENSOR GND SENSE DGND OUTIA Figure 29. A Single Supply Application with Shifted Ground Sense Pin +5V ADT70 NULLA NULLB BIAS 2.5VREFOUT +VS IOUTA IOUTB OUTOA MATCHED CURRENT SOURCES VREF +INOA 2.5V REF ⴚINOA ⴚINIA RG 768⍀ RESISTOR 49.9k⍀ INST AMP SHUTDOWN RG 1K⍀ PRTD +INIA 1K⍀ REF RESISTOR ⴚVS GND SENSE TO CONTROLLER SHUTDOWN 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 –200°C to +400°C corresponding to an input voltage range of 1 V to 4 V. 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. ⴚINIA RG INST AMP 4.99k⍀ RG +INIA GND SENSE 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 sensors. Only the gain setting resistor RG needs to be altered. For NULLA 1k⍀ REF RESISTOR NODE C IOUTA NODE D NULLB OUT Figure 32. 100␣ Ω 0.00385 PRTD Application Showing New Value for RG BIAS 2.5VREFOUT ADT70 +5V +VS OUTOA MATCHED CURRENT SOURCES +INOA IOUTB 2.5V REF ⴚINOA ⴚINIA RG 49.9k⍀ INST AMP SHUTDOWN SHUTDOWN RG +INIA 1k⍀ PRTD 10F +5V GND SENSE ADR290 0.1F IN OUT GND OUT AGND ⴚVS DGND 0.1F Figure 31. Single Supply Application with an ADR290 “Pseudo-Ground” –12– REV. 0 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 illustrates the input section of the ADT70 configured for the American PRTD. The ideal value for RG is 50.98 kΩ 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 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 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 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. IOUTA IOUTB ⴚINIA RG INST AMP 49.9k⍀ 2k⍀ 1k⍀ PRTD 1k⍀ REF RESISTOR RG +INIA NOTE: IDEAL VALUE FOR RG =˜ 51k⍀ GND SENSE OUT Figure 33. Typical PRTD Application with American 0.003916 Ω / Ω / °C Scale; 1 kΩ Scale NULLA NULLB BIAS 2.5VREFOUT +5V ADT70 +VS IOUTA IOUTB R OUTOA MATCHED CURRENT SOURCES +INOA R 2.5V REF ⴚINOA ⴚINIA R R RG INST AMP SHUTDOWN RG SHUTDOWN +INIA COLUMBIA RESEARCH LAB MODEL DT3617 STRAIN SENSOR R = 1k⍀ DGND ⴚ5V Figure 34. Typical Strain Sensor Application (Two Element Varying) REV. 0 –13– ADT70 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). C3395–8–7/98 20-Lead Plastic DIP (P-Suffix) 1.060 (26.90) 0.925 (23.50) 20 11 1 10 0.280 (7.11) 0.240 (6.10) 0.060 (1.52) 0.015 (0.38) PIN 1 0.210 (5.33) MAX 0.325 (8.25) 0.300 (7.62) 0.195 (4.95) 0.115 (2.93) 0.130 (3.30) MIN 0.160 (4.06) 0.115 (2.93) 0.022 (0.558) 0.014 (0.356) 0.100 (2.54) BSC 0.015 (0.381) 0.008 (0.204) 0.070 (1.77) SEATING 0.045 (1.15) PLANE 20-Lead SOIC (S-Suffix) 11 1 10 0.0118 (0.30) 0.0040 (0.10) 0.1043 (2.65) 0.0926 (2.35) 0.0291 (0.74) x 45° 0.0098 (0.25) 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) PRINTED IN U.S.A. PIN 1 0.4193 (10.65) 0.3937 (10.00) 20 0.2992 (7.60) 0.2914 (7.40) 0.5118 (13.00) 0.4961 (12.60) –14– REV. 0