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| Precision Thermocouple Amplifiers with Cold Junction Compensation AD8494/AD8495/AD8496/AD8497 FEATURES Low cost and easy to use Pretrimmed for J or K type thermocouples Internal cold junction compensation High impedance differential input Standalone 5 mV/C thermometer Reference pin allows offset adjustment Thermocouple break detection Laser wafer trimmed to 1C initial accuracy and 0.025C/C ambient temperature rejection Low power: <1 mW at VS = 5 V Wide power supply range Single supply: 2.7 V to 36 V Dual supply: 2.7 V to 18 V Small, 8-lead MSOP FUNCTIONAL BLOCK DIAGRAM SENSE -IN 1M ESD AND OVP AD8494/AD8495/ AD8496/AD8497 A2 THERMOCOUPLE COLD JUNCTION COMPENSATION A3 OUT +IN ESD AND OVP A1 REF Figure 1. APPLICATIONS J or K type thermocouple temperature measurement Setpoint controller Celsius thermometer Universal cold junction compensator White goods (oven, stove top) temperature measurements Exhaust gas temperature sensing Catalytic converter temperature sensing Table 1. Device Temperature Ranges Thermocouple Type J K J K Optimized Temperature Range Ambient Temperature Measurement (Reference Junction) Junction 0C to 50C Full J type range 0C to 50C Full K type range 25C to 100C Full J type range 25C to 100C Full K type range Part No. AD8494 AD8495 AD8496 AD8497 GENERAL DESCRIPTION The AD8494/AD8495/AD8496/AD8497 are precision instrumentation amplifiers with thermocouple cold junction compensators on an integrated circuit. They produce a high level (5 mV/C) output directly from a thermocouple signal by combining an ice point reference with a precalibrated amplifier. They can be used as standalone thermometers or as switched output setpoint controllers using either a fixed or remote setpoint control. The AD8494/AD8495/AD8496/AD8497 can be powered from a single-ended supply (less than 3 V) and can measure temperatures below 0C by offsetting the reference input. To minimize selfheating, an unloaded AD849x typically operates with a total supply current of 180 A, but it is also capable of delivering in excess of 5 mA to a load. The AD8494 and AD8496 are precalibrated by laser wafer trimming to match the characteristics of J type (iron-constantan) thermocouples; the AD8495 and AD8497 are laser trimmed to match the characteristics of K type (chromel-alumel) thermocouples. See Table 1 for the optimized ambient temperature range of each part. The AD8494/AD8495/AD8496/AD8497 allow a wide variety of supply voltages. With a 5 V single supply, the 5 mV/C output allows the devices to cover nearly 1000 degrees of a thermocouple's temperature range. The AD8494/AD8495/AD8496/AD8497 work with 3 V supplies, allowing them to interface directly to lower supply ADCs. They can also work with supplies as large as 36 V in industrial systems that require a wide common-mode input range. PRODUCT HIGHLIGHTS 1. 2. 3. 4. 5. 6. Complete, precision laser wafer trimmed thermocouple signal conditioning system in a single IC package. Flexible pinout provides for operation as a setpoint controller or as a standalone Celsius thermometer. Rugged inputs withstand 4 kV ESD and provide overvoltage protection (OVP) up to VS 25 V. Differential inputs reject common-mode noise on the thermocouple leads. Reference pin voltage can be offset to measure 0C on single supplies. Available in a small, 8-lead MSOP that is fully RoHS compliant. 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 that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2010 Analog Devices, Inc. All rights reserved. 08529-020 AD8494/AD8495/AD8496/AD8497 TABLE OF CONTENTS Features .............................................................................................. 1 Applications ....................................................................................... 1 Functional Block Diagram .............................................................. 1 General Description ......................................................................... 1 Product Highlights ........................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 Thermal Resistance ...................................................................... 5 ESD Caution .................................................................................. 5 Pin Configuration and Function Descriptions ............................. 6 Typical Performance Characteristics ............................................. 7 Theory of Operation ...................................................................... 11 Thermocouples ........................................................................... 11 Thermocouple Signal Conditioner .......................................... 11 AD8494/AD8495/AD8496/AD8497 Architecture .................. 11 Maximum Error Calculation .................................................... 12 Recommendations for Best Circuit Performance .................. 13 Applications Information .............................................................. 14 Basic Connection ....................................................................... 14 Ambient Temperature Sensor ................................................... 14 Setpoint Controller .................................................................... 15 Measuring Negative Temperatures .......................................... 15 Reference Pin Allows Offset Adjustment ................................ 15 Outline Dimensions ....................................................................... 16 Ordering Guide .......................................................................... 16 REVISION HISTORY 7/10--Revision 0: Initial Version Rev. 0 | Page 2 of 16 AD8494/AD8495/AD8496/AD8497 SPECIFICATIONS +VS = 5 V, -VS = 0 V, V+IN = V-IN = 0 V, VREF = 0 V, TA = TRJ = 25C, RL = 100 k, unless otherwise noted. Specifications do not include gain and offset errors of the thermocouple itself. TA is the ambient temperature at the AD849x; TRJ is the thermocouple reference junction temperature; TMJ is the thermocouple measurement junction temperature. Table 2. Parameter TEMPERATURE ACCURACY Initial Accuracy AD8494/AD8495 AD8496/AD8497 Ambient Temperature Rejection 1 AD8494/AD8495 AD8496/AD8497 Gain Error 2, 3 AD8494/AD8495 AD8496/AD8497 Transfer Function INPUTS Input Voltage Range Overvoltage Range Input Bias Current 4 Input Offset Current Common-Mode Rejection Power Supply Rejection NOISE Voltage Noise Voltage Noise Density Current Noise Density REFERENCE INPUT Input Resistance Input Current Voltage Range Gain to Output OUTPUT Output Voltage Range Short-Circuit Current 5 DYNAMIC RESPONSE -3 dB Bandwidth AD8494 AD8495/AD8497 AD8496 Settling Time to 0.1% AD8494 AD8495/AD8497 AD8496 POWER SUPPLY Operating Voltage Range 6 Single Supply Dual Supply Quiescent Current Test Conditions/Comments Min A Grade Typ Max Min C Grade Typ Max Unit TA = TRJ = TMJ = 25C TA = TRJ = 60C, TMJ = 175C 3 3 1 1.5 C C TA = TRJ = 0C to 50C TA = TRJ = 25C to 100C VOUT = 0.125 V to 4.125 V 0.05 0.05 0.3 0.3 5 -VS - 0.2 +VS - 25 25 +VS - 1.6 -VS + 25 50 1.5 1 0.5 -VS - 0.2 +VS - 25 25 5 0.025 0.025 0.1 0.1 C/C C/C % % mV/C V V nA nA C/V C/V V p-p nV/Hz fA/Hz k A V V/V V mA VCM = 0 V to 3 V +VS = 2.7 V to 5 V f = 0.1 Hz to 10 Hz, TA = 25C f = 1 kHz, TA = 25C f = 1 kHz, TA = 25C 0.8 32 100 60 25 -VS 1 -VS + 0.025 7 +VS - 1.6 -VS + 25 50 0.5 0.3 0.5 0.8 32 100 60 25 +VS -VS 1 +VS - 0.1 -VS + 0.025 7 +VS - 0.1 +VS 30 25 31 4 V output step 36 40 32 30 25 31 36 40 32 kHz kHz kHz s s s 2.7 2.7 180 Rev. 0 | Page 3 of 16 36 18 250 2.7 2.7 180 36 18 250 V V A AD8494/AD8495/AD8496/AD8497 Parameter TEMPERATURE RANGE (TA) Specified Performance AD8494/AD8495 AD8496/AD8497 Operational 1 Test Conditions/Comments Min A Grade Typ Max Min C Grade Typ Max Unit 0 25 -40 50 100 +125 0 25 -40 50 100 +125 C C C Ambient temperature rejection specifies the change in the output measurement (in C) for a given change in temperature of the cold junction. For the AD8494 and AD8495, ambient temperature rejection is defined as the slope of the line connecting errors calculated at 0C and 50C ambient temperature. For the AD8496 and AD8497, ambient temperature rejection is defined as the slope of the line connecting errors calculated at 25C and 100C ambient temperature. 2 Error does not include thermocouple gain error or thermocouple nonlinearity. 3 With a 100 k load, measurement junction temperatures beyond approximately 880C for the AD8494 and AD8496 and beyond approximately 960C for the AD8495 and AD8497 require supply voltages larger than 5 V or a negative voltage applied to the reference pin. Measurement junction temperatures below 5C require either a positive offset voltage applied to the reference pin or a negative supply. 4 Input stage uses PNP transistors, so bias current always flows out of the part. 5 Large output currents can increase the internal temperature rise of the part and contribute to cold junction compensation (CJC) error. 6 Unbalanced supplies can also be used. Care should be taken that the common-mode voltage of the thermocouple stays within the input voltage range of the part. Rev. 0 | Page 4 of 16 AD8494/AD8495/AD8496/AD8497 ABSOLUTE MAXIMUM RATINGS Table 3. Parameter Supply Voltage Maximum Voltage at -IN or +IN Minimum Voltage at -IN or +IN REF Voltage Output Short-Circuit Current Duration Storage Temperature Range Operating Temperature Range Maximum IC Junction Temperature ESD Human Body Model Field-Induced Charged Device Model Rating 18 V +VS - 25 V -VS + 25 V VS Indefinite -65C to +150C -40C to +125C 140C 4.5 kV 1.5 kV THERMAL RESISTANCE JA is specified for a device on a 4-layer JEDEC PCB in free air. Table 4. Package 8-Lead MSOP (RM-8) JA 135 Unit C/W ESD CAUTION 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 indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Rev. 0 | Page 5 of 16 AD8494/AD8495/AD8496/AD8497 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS -IN 1 REF 2 -VS 3 NC 4 AD849x 8 - + 7 6 5 +IN +VS OUT SENSE 08529-002 TOP VIEW (Not to Scale) NC = NO CONNECT Figure 2. Pin Configuration Table 5. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7 8 Mnemonic -IN REF -VS NC SENSE OUT +VS +IN Description Negative Input. Reference. This pin must be driven by low impedance. Negative Supply. No Connect. Sense Pin. In measurement mode, connect to output; in setpoint mode, connect to setpoint voltage. Output. Positive Supply. Positive Input. Rev. 0 | Page 6 of 16 AD8494/AD8495/AD8496/AD8497 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, +VS = 5 V, RL = , unless otherwise noted. 100 AD8495/AD8497 AD8494 AD8496 10 TEMPERATURE READING (C) 1200 1000 CONNECTED THERMOCOUPLE 800 CMRR (C/V) 600 1 400 200 OPEN THERMOCOUPLE 0 THERMOCOUPLE CONNECTION AD849x OUTPUT TIME (50s/DIV) 0.1 1 10 100 1k FREQUENCY (Hz) 10k 100k 08529-035 0.01 0.1 -200 Figure 3. CMRR vs. Frequency Figure 6. Output Response to Open Thermocouple, -IN Connected to Ground Through a 1 M Resistor 1000 AD8495/AD8497 AD8494 AD8496 100 4.0 3.5 +0.05, +3.45 +4.91, +2.95 +0.05, +3.21 +4.91, +2.71 INPUT COMMON-MODE VOLTAGE (V) 3.0 2.5 2.0 1.5 1.0 0.5 0 PSRR (C/V) 10 1 +0.05, -0.36 -0.5 +0.05, -0.39 0.5 VREF = 0V VREF = 2.5V 1.5 2.5 3.5 OUTPUT VOLTAGE (V) +4.91, -0.37 +4.91, -0.39 4.5 5.5 08529-017 1 10 100 1k FREQUENCY (Hz) 10k 100k Figure 4. PSRR vs. Frequency 08529-036 0 -1.0 -0.5 Figure 7. Input Common-Mode Voltage Range vs. Output Voltage, +VS = 5 V, VREF = 0 V, and VREF = 2.5 V 50 40 35 2.00 1.75 1.50 IBIAS 1.25 1.00 0.75 0.50 0.25 IOS -20 0 20 40 60 TEMPERATURE (C) 80 100 120 08529-042 40 08529-019 30 30 25 20 15 10 5 20 10 AD8494 AD8496 AD8495/AD8497 0 -10 -20 100 1k 10k FREQUENCY (Hz) 100k 1M Figure 5. Frequency Response 08529-018 0 -40 0 Figure 8. Input Bias Current and Input Offset Current vs. Temperature Rev. 0 | Page 7 of 16 INPUT OFFSET CURRENT (nA) INPUT BIAS CURRENT (nA) GAIN (dB) AD8494/AD8495/AD8496/AD8497 3.00 2.75 12 2.50 2.25 VOUT 1.50 8 VOUT 2.5 2.0 1.5 IIN 1.0 0.5 0 -0.5 08529-024 2.00 16 3.0 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) INPUT CURRENT (mA) 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 IIN 1.00 4 0 -4 -8 0.50 0 -0.50 -12 08529-021 0 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.00 30 -16 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.0 30 Figure 9. AD8494 Input Overvoltage Performance, +VS = 2.7 V (Gain = 96.7) Figure 12. AD8494 Input Overvoltage Performance, VS = 15 V (Gain = 96.7) 3.00 2.75 2.00 16 12 3.0 2.5 VOUT 2.0 1.5 IIN 1.0 0.5 0 -0.5 08529-025 2.50 2.25 VOUT 1.50 8 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 IIN 1.00 4 0 -4 -8 0.50 0 -0.50 -12 08529-022 0 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.00 30 -16 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.0 30 Figure 10. AD8495/AD8497 Input Overvoltage Performance, +VS = 2.7 V (Gain = 122.4) Figure 13. AD8495/AD8497 Input Overvoltage Performance, VS = 15 V (Gain = 122.4) 3.00 2.75 2.00 16 12 3.0 2.5 VOUT 2.0 1.5 IIN 1.0 0.5 0 -0.5 08529-026 2.50 2.25 VOUT 1.50 8 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) INPUT CURRENT (mA) 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 IIN 1.00 4 0 -4 -8 0.50 0 -0.50 -12 08529-023 0 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.00 30 -16 -30 -25 -20 -15 -10 -5 0 5 10 INPUT VOLTAGE (V) 15 20 25 -1.0 30 Figure 11. AD8496 Input Overvoltage Performance, +VS = 2.7 V Gain = 90.35) Figure 14. AD8496 Input Overvoltage Performance, VS = 15 V (Gain = 90.35) Rev. 0 | Page 8 of 16 INPUT CURRENT (mA) INPUT CURRENT (mA) INPUT CURRENT (mA) INPUT CURRENT (mA) AD8494/AD8495/AD8496/AD8497 CL = 0pF CL = 1000pF CL = 0pF CL = 1000pF 08529-028 120s/DIV 120s/DIV Figure 15. AD8494/AD8496 Small-Signal Response with Various Capacitive Loads Figure 18. AD8495/AD8497 Small-Signal Response with Various Capacitive Loads AD8494/AD8496 AD8495/AD8497 2V/DIV 20mV/DIV 0.02%/DIV SETTLING TO 0.1% IN 36s 08529-027 120s/DIV 100s/DIV Figure 16. Small-Signal Response, RL = 100 k, CL = 1 nF Figure 19. AD8494 Large-Signal Step Response and Settling Time 2V/DIV 2V/DIV 0.02%/DIV SETTLING TO 0.1% IN 40s 0.02%/DIV SETTLING TO 0.1% IN 32s 08529-040 100s/DIV 100s/DIV Figure 17. AD8495/AD8497 Large-Signal Step Response and Settling Time Figure 20. AD8496 Large-Signal Step Response and Settling Time Rev. 0 | Page 9 of 16 08529-041 08529-039 08529-029 CL = 4700pF CL = 10000pF 20mV/DIV 20mV/DIV CL = 4700pF CL = 10000pF AD8494/AD8495/AD8496/AD8497 SUPPLY VOLTAGE (1.25V/DIV) OUTPUT VOLTAGE 5V POWER-UP 200nV/DIV 08529-030 OUTPUT VOLTAGE (50mV/DIV) 1s/DIV TIME (1.5ms/DIV) Figure 21. 0.1 Hz to 10 Hz RTI Voltage Noise Figure 24. Output Voltage Start-Up 5 +VS OUTPUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES (VS = 5V) 4 -0.4 -0.8 -1.2 (+) -40C (+) +25C (+) +85C (+) +125C OUTPUT VOLTAGE SWING (V) 3 2 1 0 -1 -2 -3 -4 (+) -40C (+) +25C (+) +85C (+) +125C (-) -40C (-) +25C (-) +85C (-) +125C +1.2 +0.8 +0.4 (-) -40C (-) +25C (-) +85C (-) +125C 08529-033 10k LOAD RESISTANCE () 100k 100 1m OUTPUT CURRENT (A) 5m Figure 22. Output Voltage Swing vs. Load Resistance, VS = 5 V Figure 25. Output Voltage Swing vs. Output Current, VS = 5 V 100 90 80 70 60 50 40 30 20 08529-031 NOISE (nV/ Hz) 10 1 10 100 1k FREQUENCY (Hz) 10k 100k Figure 23. Voltage Noise Spectral Density vs. Frequency Rev. 0 | Page 10 of 16 08529-034 -5 1k -VS 10 08529-032 AD8494/AD8495/AD8496/AD8497 THEORY OF OPERATION THERMOCOUPLES A thermocouple is a rugged, low cost temperature transducer whose output is proportional to the temperature difference between a measurement junction and a reference junction. It has a very wide temperature range. Its low level output (typically tens of microvolts per C) requires amplification. Variation in the reference junction temperature results in measurement error unless the thermocouple signal is properly compensated. A thermocouple consists of two dissimilar metals. These metals are connected at one end to form the measurement junction, also called the hot junction. The other end of the thermocouple is connected to the metal lines that lead to the measurement electronics. This connection forms a second junction: the reference junction, also called the cold junction. MEASUREMENT JUNCTION REFERENCE JUNCTION -IN 08529-004 Table 6. J Type Thermocouple Voltages and AD8494 Readings Measurement Junction Temperature (TMJ) 50C 50C 0C 0C Reference Junction Temperature (TRJ) 0C 50C 0C 50C Thermocouple Voltage +2.585 mV 0 mV 0 mV -2.585 mV AD8494 Reading 250 mV 250 mV 0 mV 0 mV AD8494/AD8495/AD8496/AD8497 ARCHITECTURE Figure 27 shows a block diagram of the AD849x circuitry. The AD849x consists of a low offset, fixed-gain instrumentation amplifier and a temperature sensor. SENSE PCB TRACES THERMOCOUPLE WIRES AD849x 1M ESD AND OVP AD8494/AD8495/ AD8496/AD8497 A2 Figure 26. Thermocouple Junctions THERMOCOUPLE COLD JUNCTION COMPENSATION A3 OUT To derive the temperature at the measurement junction (TMJ), the user must know the differential voltage created by the thermocouple. The user must also know the error voltage generated by the temperature at the reference junction (TRJ). Compensating for the reference junction error voltage is typically called cold junction compensation. The electronics must compensate for any changes in temperature at the reference (cold) junction so that the output voltage is an accurate representation of the hot junction measurement. +IN ESD AND OVP A1 REF Figure 27. Block Diagram THERMOCOUPLE SIGNAL CONDITIONER The AD8494/AD8495/AD8496/AD8497 thermocouple amplifiers provide a simple, low cost solution for measuring thermocouple temperatures. These amplifiers simplify many of the difficulties of measuring thermocouples. An integrated temperature sensor performs cold junction compensation. A fixed-gain instrumentation amplifier amplifies the small thermocouple voltage to provide a 5 mV/C output. The high common-mode rejection of the amplifier blocks common-mode noise that the long thermocouple leads can pick up. For additional protection, the high impedance inputs of the amplifier make it easy to add extra filtering. Table 6 shows an example of a J type thermocouple voltage for various combinations of 0C and 50C on the reference and measurement junctions. Table 6 also shows the performance of the AD8494 amplifying the thermocouple voltage and compensating for the reference junction temperature changes, thus eliminating the error. The AD849x output is a voltage that is proportional to the temperature at the measurement junction of the thermocouple (TMJ). To derive the measured temperature from the AD849x output voltage, use the following transfer function: TMJ = (VOUT - VREF)/(5 mV/C) An ideal AD849x achieves this output with an error of less than 2C, within the specified operating ranges listed in Table 7. Instrumentation Amplifier A thermocouple signal is so small that considerable gain is required before it can be sampled properly by most ADCs. The AD849x has an instrumentation amplifier with a fixed gain that generates an output voltage of 5 mV/C for J type and K type thermocouples. VOUT = (TMJ x 5 mV/C) + VREF To accommodate the nonlinear behavior of the thermocouple, each amplifier has a different gain so that the 5 mV/C is accurately maintained for a given temperature measurement range. * * The AD8494 and AD8496 (J type) have an instrumentation amplifier with a gain of 96.7 and 90.35, respectively. The AD8495 and AD8497 (K type) have an instrumentation amplifier with a gain of 122.4. Rev. 0 | Page 11 of 16 08529-020 AD8494/AD8495/AD8496/AD8497 The small thermocouple voltages mean that signals are quite vulnerable to interference, especially when measured with single-ended amplifiers. The AD849x addresses this issue in several ways. Low input bias currents and high input impedance allow for easy filtering at the inputs. The excellent common-mode rejection of the AD849x prevents variations in ground potential and other common-mode noise from affecting the measurement. MAXIMUM ERROR CALCULATION As is normally the case, the AD849x outputs are subject to calibration, gain, and temperature sensitivity errors. The user can calculate the maximum error from the AD849x using the following information. The five primary sources of AD849x error are described in this section. Temperature Sensor (Cold Junction Compensation) The AD849x also includes a temperature sensor for cold junction compensation. This temperature sensor is used to measure the reference junction temperature of the thermocouple and to cancel its effect. * The AD8494/AD8495 cold junction compensation is optimized for operation in a lab environment, where the ambient temperature is around 25C. The AD8494/AD8495 are specified for an ambient range of 0C to 50C. The AD8496/AD8497 cold junction compensation is optimized for operation in a less controlled environment, where the temperature is around 60C. The AD8496/AD8497 are specified for an ambient range of 25C to 100C. Application examples for the AD8496/AD8497 include automotive applications, autoclave, and ovens. AD849x Initial Calibration Accuracy Error at the initial calibration point can be easily calibrated out with a one-point temperature calibration. See Table 2 for the specifications. AD849x Ambient Temperature Rejection The specified ambient temperature rejection represents the ability of the AD849x to reject errors caused by changes in the ambient temperature/reference junction. For example, with 0.025C/C ambient temperature rejection, a 20C change in the reference junction temperature adds less than 0.5C error to the measurement. See Table 2 for the specifications. * AD849x Gain Error Gain error is the amount of additional error when measuring away from the measurement junction calibration point. For example, if the part is calibrated at 25C and the measurement junction is 100C with a gain error of 0.1%, the gain error contribution is (100C - 25C) x (0.1%) = 0.075C. This error can be calibrated out with a two-point calibration if needed, but it is usually small enough to ignore. See Table 2 for the specifications. Thermocouple Break Detection The AD849x offers open thermocouple detection. The inputs of the AD849x are PNP type transistors, which means that the bias current always flows out of the inputs. Therefore, the input bias current drives any unconnected input high, which rails the output. Connecting the negative input to ground through a 1 M resistor causes the AD849x output to rail high in an open thermocouple condition (see Figure 6, Figure 28, and the Ground Connection section). Manufacturing Tolerances of the Thermocouple Consult the data sheet for your thermocouple to find the specified tolerance of the thermocouple. Linearity Error of the Thermocouple Each part in the AD849x family is precision trimmed to optimize a linear operating range for a specific thermocouple type and for the widest possible measurement and ambient temperature ranges. The AD849x achieves a linearity error of less than 2C, within the specified operating ranges listed in Table 7. This error is due only to the nonlinearity of the thermocouple. Table 7. AD849x 2C Accuracy Temperature Ranges Thermocouple Type J K J K Max Error 2C 2C 2C 2C Ambient Temperature Range 0C to 50C 0C to 50C 25C to 100C 25C to 100C Measurement Temperature Range -35C to +95C -25C to +400C +55C to +565C -25C to +295C 1M Figure 28. Ground the Negative Input Through a 1 M Resistor for Open Thermocouple Detection Input Voltage Protection The AD849x has very robust inputs. Input voltages can be up to 25 V from the opposite supply rail. For example, with a +5 V positive supply and a -3 V negative supply, the part can safely withstand voltages at the inputs from -20 V to +22 V. Voltages at the reference and sense pins should not go beyond 0.3 V of the supply rails. 08529-008 Part AD8494 AD8495 AD8496 AD8497 For temperature ranges outside those listed in Table 7 or for instructions on how to correct for thermocouple nonlinearity error with software, see the product page for the AD8494, AD8495, AD8496, or AD8497, or contact an Analog Devices representative. Rev. 0 | Page 12 of 16 AD8494/AD8495/AD8496/AD8497 RECOMMENDATIONS FOR BEST CIRCUIT PERFORMANCE Input Filter A low-pass filter before the input of the AD849x is strongly recommended (see Figure 29), especially when operating in an electrically noisy environment. Long thermocouple leads can function as an excellent antenna and pick up many unwanted signals. The filter should be set to a low corner frequency that still allows the input signal to pass through undiminished. The primary purpose of the filter is to remove RF signals, which, if allowed to reach the AD849x, can be rectified and appear as temperature fluctuations. R R CONNECT WHEN THERMOCOUPLE TIP TYPE IS UNKNOWN CC CD CC 1M THERMOCOUPLE WIRES Keeping the AD849x at the Same Temperature as the Reference Junction The AD849x compensates for thermocouple reference junction temperature by using an internal temperature sensor. It is critical to keep the reference junction (thermocouple-to-PCB connection) as close to the AD849x as possible. Any difference in temperature between the AD849x and the reference junction appears directly as temperature error. Temperature difference between the device and the reference junction may occur if the AD849x is not physically close to the reference junction or if the AD849x is required to supply large amounts of output power. MEASUREMENT JUNCTION REFERENCE JUNCTION KEEP JUNCTION AND AD849x AT SAME TEMPERATURE PCB TRACES KEEP TRACES SHORT AD849x 08529-010 AD849x Figure 31. Compensating for Thermocouple Reference Junction Temperature 1 2R(2C D + CC) 08529-011 Driving the Reference Pin The AD849x comes with a reference pin, which can be used to offset the output voltage. This is particularly useful when reading a negative temperature in a single-supply system. INCORRECT CORRECT FILTER FREQUENCYDIFF = 1 FILTER FREQUENCYCM = 2RC C WHERE CD 10CC Figure 29. Filter for Any Thermocouple Style To prevent input offset currents from affecting the measurement accuracy, the filter resistor values should be less than 50 k. Ground Connection It is always recommended that the thermocouple be connected to ground through a 100 k to 1 M resistor placed at the negative (inverting) input of the amplifier on the PCB (see Figure 30). This solution works well regardless of the thermocouple tip style. V AD849x REF V AD849x REF + AD8613 - 08529-006 Figure 32. Driving the Reference Pin 1M Figure 30. Ground the Thermocouple with a 1 M Resistor For best performance, the reference pin should be driven with a low output impedance source, not a resistor divider. The AD8613 and the OP777 are good choices for the buffer amplifier. 08529-038 If there is no electrical connection at the measurement junction (insulated tip), the resistor value is small enough that no meaningful common-mode voltage is generated. If there is an electrical connection through a grounded or exposed tip, the resistor value is large enough that any current from the measurement tip to ground is very small, preventing measurement errors. The AD849x inputs require only one ground connection or source of common-mode voltage. Any additional ground connection is detrimental to performance because ground loops can form through the thermocouple, easily swamping the small thermocouple signal. Grounding the thermocouple through a resistor as recommended prevents such problems. Debugging Tip If the AD849x is not providing the expected performance, a useful debugging step is to implement the ambient temperature configuration in Figure 34. If the ambient temperature sensor does not work as expected, the problem is likely with the AD849x or with the downstream circuitry. If the ambient temperature sensor configuration is working correctly, the problem typically lies with how the thermocouple is connected to the AD849x. Common errors include an incorrect grounding configuration or lack of filtering. Rev. 0 | Page 13 of 16 AD8494/AD8495/AD8496/AD8497 APPLICATIONS INFORMATION BASIC CONNECTION Figure 33 shows an example of a basic connection for the AD849x, with a J type or K type thermocouple input. 5V 0.1F 7 AMBIENT TEMPERATURE SENSOR The AD849x can be configured as a standalone Celsius thermometer with a 5 mV/C output, as shown in Figure 34. The thermocouple sensing functionality is disabled by shorting both AD849x inputs to ground; the AD849x simply outputs the value from the on-board temperature sensor. As a temperature sensor, the AD8494 has a measurement temperature range of -40C to +125C with a precision output of VOUT = TA x 5 mV/C IN-AMP 6 5 +VS 10F COLD JUNCTION COMPENSATION +IN THERMOCOUPLE 1M 2 8 OUT SENSE COLD JUNCTION COMPENSATION 08529-012 -IN 1 5V +VS 7 AD849x 3 REF -VS 0.1F 10F +IN 8 IN-AMP 6 5 OUT SENSE 08529-013 Figure 33. Basic Connection for the AD849x -IN 1 To measure negative temperatures, apply a voltage at the reference pin to offset the output voltage at 0C. The output voltage of the AD849x is VOUT = (TMJ x 5 mV/C) + VREF A filter at the input is recommended to remove high frequency noise. The 1 M resistor to ground enables open thermocouple detection and proper grounding of the thermocouple. The sense pin should be connected to the output pin of the AD849x. Decoupling capacitors should be used to ensure clean power supply voltages on +VS and, if using dual supplies, on -VS, also. A 0.1 F capacitor should be placed as close as possible to each AD849x supply pin. A 10 F tantalum capacitor can be used farther away from the part and can be shared. AD849x 2 3 REF -VS Figure 34. Ambient Temperature Sensor The AD8494 is the best choice for use as an ambient temperature sensor. The AD8495, AD8496, and AD8497 can also be configured as ambient temperature sensors, but their output transfer functions are not precisely 5 mV/C. For information about the exact transfer functions of the AD8494/AD8495/ AD8496/AD8497, see the product page for the AD8494, AD8495, AD8496, or AD8497, or contact an Analog Devices representative. The thermometer mode can be particularly useful for debugging a misbehaving circuit. If the basic connection is not working, disconnect the thermocouple and short both inputs to ground. If the system reads the ambient temperature correctly, the problem is related to the thermocouple. If the system does not read the ambient temperature correctly, the problem is with the AD849x or with the downstream circuitry. Rev. 0 | Page 14 of 16 AD8494/AD8495/AD8496/AD8497 SETPOINT CONTROLLER The AD849x can be used as a temperature setpoint controller, with a thermocouple input from a remote location or with the AD849x itself being used as a temperature sensor. When the measured temperature is below the setpoint temperature, the output voltage goes to -VS. When the measured temperature is above the setpoint temperature, the output voltage goes to +VS. For best accuracy and CMRR performance, the setpoint voltage should be created with a low impedance source. If the setpoint voltage is generated with a voltage divider, a buffer is recommended. 5V +VS 7 MEASURING NEGATIVE TEMPERATURES The AD849x can measure negative temperatures on dual supplies and on a single supply. When operating on dual supplies with the reference pin grounded, a negative output voltage indicates a negative temperature at the thermocouple measurement junction. VOUT = (TMJ x 5 mV/C) + VREF When operating the AD849x on a single supply, level-shift the output by applying a positive voltage (less than +VS) on the reference pin. An output voltage less than VREF indicates a negative temperature at the thermocouple measurement junction. COLD JUNCTION COMPENSATION +IN THERMOCOUPLE 1M 2 8 REFERENCE PIN ALLOWS OFFSET ADJUSTMENT The reference pin can be used to level-shift the AD849x output voltage. This is useful for measuring negative temperatures on a single supply and to match the AD849x output voltage range to the input voltage range of the subsequent electronics in the signal chain. The reference pin can also be used to offset any initial calibration errors. Apply a small reference voltage proportional to the error to nullify the effect of the calibration error on the output. 08529-014 IN-AMP -IN 1 6 5 OUT SENSE AD849x 3 SETPOINT VOLTAGE REF -VS Figure 35. Setpoint Controller Hysteresis can be added to the setpoint controller by using a resistor divider from the output to the reference pin, as shown in Figure 36. The hysteresis in C is THYST VS R1 /(R1 R2) 5 mV/ C 5V +VS 7 COLD JUNCTION COMPENSATION +IN THERMOCOUPLE 1M 2 8 IN-AMP -IN 1 6 5 OUT SENSE R1 1k SETPOINT VOLTAGE AD849x 3 REF -VS R1 1k R2 100k Figure 36. Adding 10 Degrees of Hysteresis A resistor equivalent to the output resistance of the divider should be connected to the sense pin to ensure good CMRR. Rev. 0 | Page 15 of 16 08529-015 AD8494/AD8495/AD8496/AD8497 OUTLINE DIMENSIONS 3.20 3.00 2.80 8 5 3.20 3.00 2.80 PIN 1 IDENTIFIER 1 5.15 4.90 4.65 4 0.65 BSC 0.95 0.85 0.75 0.15 0.05 COPLANARITY 0.10 0.40 0.25 15 MAX 1.10 MAX 0.70 0.55 0.40 091709-A 6 0 0.23 0.13 COMPLIANT TO JEDEC STANDARDS MO-187-AA Figure 37. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters ORDERING GUIDE Model 1, 2 AD8494ARMZ AD8494ARMZ-R7 AD8494CRMZ AD8494CRMZ-R7 AD8495ARMZ AD8495ARMZ-R7 AD8495CRMZ AD8495CRMZ-R7 AD8496ARMZ AD8496ARMZ-R7 AD8496CRMZ AD8496CRMZ-R7 AD8497ARMZ AD8497ARMZ-R7 AD8497CRMZ AD8497CRMZ-R7 1 2 Temperature Range 0C to 50C 0C to 50C 0C to 50C 0C to 50C 0C to 50C 0C to 50C 0C to 50C 0C to 50C 25C to 100C 25C to 100C 25C to 100C 25C to 100C 25C to 100C 25C to 100C 25C to 100C 25C to 100C Package Description 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel Package Option RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8 Branding Y36 Y36 Y37 Y37 Y33 Y33 Y34 Y34 Y3C Y3C Y3D Y3D Y39 Y39 Y3A Y3A Z = RoHS Compliant Part. The AD8494 and AD8496 models are prereleased. (c)2010 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D08529-0-7/10(0) Rev. 0 | Page 16 of 16 |
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