Thermocouple Signal Conditioning Using AD594/AD595 Marcin IN

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Thermocouple Signal Conditioning Using AD594/AD595 Marcin IN

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AN-369 APPLICATION NOTE
Thermocouple Signal Conditioning Using AD594/AD595
Marcin
INTRODUCTION most widely used devices temperature measurement thermocouple. Whether industrial, commercial scientific application, thermocouple offers cost effective solution temperature measurements many environments over wide temperature ranges. Unfortunately, their basic principles often misunderstood resulting serious measurement errors. This application note will review thermocouple fundamentals illustrate circuit designs thermocouple signal conditioning using AD594/ AD595 monolithic BACK BASICS basic principles thermocouple were discovered 1821 Thomas Seebeck. When dissimilar metals joined both ends heated, current will flow. loop broken center, open circuit voltage (the Seebeck Voltage) generated proportional difference temperature between junctions. Therefore, determining temperature measuring junction, reference junction temperature must known.
MEASURING JUNCTION REFERENCE JUNCTION
These combinations have been characterized classified National Institute Standards Technology (formerly National Bureau Standards). table below lists types, composition characteristics more commonly used thermocouples. Table Thermocouple Properties
ANSI Code Alloy Combination Platinum/Rhodium Chromel/Constantan Iron/Constantan Chromel/Alumel Nicrosil/Nisil Platinum/Rhodium Platinum Platinum/Rhodium Platinum Copper/Constantan Maximum Temperature Range +1700°C -200°C +900°C +750°C -200°C +1250°C -270°C +1300°C +1450°C +1450°C -200°C +350°C Output +12.426 -8.824 +68.783 +42.283 -5.973 +50.633 -4.345 +47.502 +16.741 +14.973 -5.602 +17.816
Figure Thermocouple Loop
bath provides well defined temperature reference junction. This become standard reference point thermocouple output voltage temperature tables various metal combinations.
(VT2 MEASURING JUNCTION
voltmeter commonly used measure Seebeck voltage; however, great care must exercised when interconnecting thermocouple. Referring Figure additional junctions, formed connection between thermocouple meter. These junctions produce opposing voltages within thermocouple loop. Using isothermal block point connection keeps these junctions thermal equilibrium produces equal opposite emfs. measured voltage difference potential between measuring junction isothermal block which serves reference junction.
MEASURING JUNCTION
(VT2
Figure Measuring Thermocouple Voltage with Voltmeter
Figure Point Reference
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PRACTICAL THERMOCOUPLE MEASUREMENT most applications, impractical bath reference junction. compensating voltage developed reference junction, point reference eliminated. This performed adding voltage into thermocouple loop, equal opposite that reference junction. circuit that provides cold junction compensation along with amplification open thermocouple detection included AD594/AD595 family thermocouple signal conditioning ICs.
MEASURING JUNCTION VCOMP VCOMP (VCOMP VT2)
Through feedback path, main amplifier maintains balance inputs. event broken thermocouple open circuit device's input, these inputs become unbalanced, fault detected, overload detection circuit drives current limited n-p-n transistor that interfaced alarm. Although these specifically calibrated Type thermocouple, other thermocouple types used with recalibration. connections internal nodes temperature controlled voltages feedback provided perform recalibration. INTERPRETING AD594/AD595 OUTPUT VOLTAGES produce temperature proportional output mV/°C, provide accurate reference junction over rated operating temperature range, AD594/ AD595 gain trimmed factory match transfer characteristics Type thermocouples +25°C. this calibration temperature, Seebeck coefficient, rate change thermal voltage with respect temperature given temperature, 51.70 µV/°C Type thermocouple 40.44 µV/°C Type This corresponds gain 193.4 AD594 247.3 AD595 realize mV/°C output. Although device trimmed output +25°C, input offset error induced output amplifier resulting offsets AD594/AD595 respectively. determine actual output voltage from AD594/AD595, following equations should used:
Figure Cold Junction Compensation
AD594/AD595 CIRCUIT DESCRIPTION Figure block diagram AD594/AD595 thermocouple signal conditioner Type (for AD594) Type (for AD595) thermocouple connected Pins inputs instrumentation amplifier differential stage. This input amplifier contained loop that uses local temperature reference. With also local temperature, point compensation circuit develops voltage equal deficiency locally referenced thermocouple loop. This voltage then applied second preamplifier whose output summed with output input amplifier. resultant output then applied input main output amplifier with feedback gain combined signals. point compensation voltage scaled equal voltage that would produced bath referenced thermocouple measuring temperature. This voltage then summed with locally referenced loop voltage, result being loop voltage with respect point.
-ALM +ALM COMP
AD594 Output (Type Voltage 193.4 AD595 Output (Type Voltage 247.3
where Type voltage taken from thermocouple voltage tables referred zero degrees Celsius. important note that thermocouple's output linear over narrow temperature range. Over wide temperature range, Seebeck coefficient introduces nonlinearity. Linearization provided AD594/ AD595, linearization techniques must performed externally. This entails calculating thermocouple temperature using high order polynomials. National Institute Standards Technology offers tables polynomial coefficients given thermocouple type which used this process.
OVERLOAD DETECT
AD594/ AD595
POINT COMP.
Figure AD594/AD595 Functional Block Diagram
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Table Calculated Errors Various Ambient Temperatures Ambient Temp. +125 AD594C Temp. Rej. Error 4.83 1.98 0.62 0.00 0.62 1.46 2.25 4.90 AD594C Total Error 5.83 2.98 1.62 1.00 1.62 2.46 3.25 5.90 AD594A Temp. Rej. Error 6.83 3.23 1.25 0.00 1.25 2.59 3.75 7.40 AD594A Total Error 9.83 6.23 4.25 3.00 4.25 5.59 6.75 10.40 AD595C Temp. Rej. Error 5.28 2.04 0.62 0.00 0.62 1.38 1.99 3.38 AD595C Total Error 6.28 3.04 1.62 1.00 1.62 2.38 2.99 4.38 AD595A Temp. Rej. Error 7.28 3.29 1.25 0.00 1.25 2.50 3.49 5.88 AD595A Total Error 10.28 6.29 4.25 3.00 4.25 5.50 6.49 8.88
NOTE Temp. Rej. Error components: Difference between actual reference junction point compensation voltage times gain; Offset gain extrapolated from +50°C limits. Total error temp. rej. plus initial calibration error.
OPTIMIZING PERFORMANCE Cold Junction Errors Optimal performance from AD594/AD595 achieved when thermocouple cold junction device thermal equilibrium. Avoid placing heat generating devices components near AD594/ AD595 this produce cold junction related errors. ambient temperature range AD594/AD595 specified from +50°C, cold junction compensation voltage matched best straight line thermocouple's output within this range. Operation outside this range will result additional error. Table shows maximum calculated errors various ambient temperatures. Circuit Board Layout circuit board layout shown Figure (with optional calibration resistors) achieves thermal equilibrium between cold junction AD594/AD595. package temperature circuit board thermally contacted copper printed circuit board tracks under Pins reference junction composed copper-constantan copper-alumel) connection copper-iron copper-chromel) connection thermal equilibrium with
IRON (CHROMEL) -ALM +ALM CONSTANTAN (ALUMEL)
Soldering Proper soldering techniques surface preparation necessary bond thermocouple tracks. Clean thermocouple wire remove oxidation before soldering. Noncorrosive rosin flux used with following solders: tin-5% antimony, tin-5% silver, tin-10% lead. Bias Current Return input instrumentation amplifier AD594/AD595 requires return path input bias current left "floating." thermocouple measuring junction electrically isolated, then should connected power supply common. some applications, tying thermocouple directly common possible. resistor from common will satisfy bias current return path will, however, generate additional input offset voltage bias current flowing through thermocouple must grounded measuring junction small common mode potential present, make connection between Pins Noise Suppression When detecting level output voltage from thermocouple, noise reduction prime concern. Whether internally generated induced radiation from source, noise becomes limiting factors dynamic range resolution. Solving noise problems involves eliminating source and/or shielding. latter more effective when source cannot controlled identified. Noise injected into AD594/AD595 input amplifier when using long length thermocouple. determine this noise path culprit, disconnect thermocouple from AD594/AD595 Pins output voltage AD594/ AD595 will indicate ambient temperature (250 +25°C). noise output (Pin disappears, then shielding input required. Shielded thermocouple wire with shield connected will provide effective noise suppression. output
AD594 AD595
COMP
COMMON
VOUT
Figure Board Layout
AN-369
still exhibits noise, entering power supply. Proper power supply bypassing decoupling will alleviate this condition. Filtering thermocouple input will attenuate noise before amplification. Figure illustrates effective input filter consisting resistor series with capacitor from this ground. offset voltage will result input bias current flowing through resistor. Since input bias current inverting input (Pin varies with input voltage, resistance series with this input would produce input dependent offset voltage. Therefore, highly recommended connect this directly common. addition, capacitor across input terminals increases response time alarm circuit event broken thermocouple. Adding capacitance frequency compensation (Pin rolls bandwidth AD594/AD595 output amplifier thus limiting noise. Without compensation, bandwidth approximately kHz. capacitor connected between Pins reduces point This technique, however, only useful noise does drive input stage into saturation.
-ALM +ALM COMP
CONSTANTAN (ALUMEL) +15V VOUT
OVERLOAD DETECT
AD594/ AD595
POINT COMP.
IRON (CHROMEL)
RCAL 100k
+15V
Figure Optional Calibration
OFFSETTING GAIN CHANGE AD594/AD595 designed produce output with nominal gain mV/°C, other ranges readily possible. zero output temperature changed applying offset voltage magnitude this voltage calculated using equations AD594/AD595 output voltage given thermocouple temperature. Gain changes easily accommodated adding series resistance increase gain paralleling nominal feedback resistor gain reduction. following method illustrates this principle. Select temperature range T1-T2. Based this range, determine output sensitivity (mV/°C) that limits maximum output excursions from (-VS 2.5) (+VS dual supplies from (+VS single supply operation. Calculate average thermocouple sensitivity over selected temperature range: (VT1-VT2)/(T1-T2). Divide desired output sensitivity (mV/°C) average thermocouple sensitivity (mV/°C). This yields gain AD594/AD595. Measure actual feedback resistance between Pins RFB. RFB/193.4 where measured feedback resistance. NOTE: 247.3 AD595 instead 193.4. feedback resistance, REXT 1)(RIN).
OVERLOAD DETECT
AD594/ AD595
POINT COMP.
Figure Input Filtering
TRIMMING CALIBRATION ERROR AD594/AD595, available performance grades, factory trimmed achieve maximum calibration error depending grade. most applications, this range error acceptable; however, adding optional trim circuit shown Figure this error nulled. negative offset approximately injected into trimming potentiometer provides balancing current into thus nulling calibration error.
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IN704A 4.1V CONSTANTAN (ALUMEL) TEMPERATURE OFFSET VOLTAGE VOUT OVERLOAD DETECT CONSTANTAN (ALUMEL) 2N2222 (OPTIONAL GAIN TRIM) 100k
AD594/ AD595
VOUT POINT COMP.
OVERLOAD DETECT
AD711 10mV/
AD594/ AD595
RFDBK POINT COMP.
REXT
-15V
RSETUP 5.11 (4.02 IRON (CHROMEL)
IRON (CHROMEL)
Figure Current-Mode Transmission
Figure Offsetting Gain Change
CURRENT MODE TRANSMISSION many applications, AD594/AD595 located noisy, remote location with output driving long length cable. Under these demanding conditions, current transmission offers better noise immunity eliminates errors cable resistance. circuit shown Figure converts AD594/AD595 output current then converts back voltage control point. feedback voltage forces voltage across RSENSE equal thermocouple voltage. With values shown RSENSE, this produces current output scale factor µA/°C. Note that AD594/ AD595 quiescent current flows through sense resistor, thus limiting minimum measured temperature 16°C. AD711 converts this current back nominal mV/°C control point. Total error based upon AD594/AD595 calibration error match between sense resistor current voltage conversion resistor control point.
(OPTIONAL) RCOMP
TEMPERATURE-TO-FREQUENCY CONVERSION digital output format produced converting AD594/AD595 voltage output frequency. This format only affords noise immunity over long transmission paths also provides information which directly interfaced computer. cost voltage-to-frequency converter, AD654, converts mV/°C voltage output compatible square wave output. shown Figure entire system powered from single supply provides temperature measurements from +300°C. Higher thermocouple temperatures will require higher power supply voltage maintain maximum AD594/AD595 output swing below supply. AD594/ AD595 output voltage connected AD654 input through series resistor produce full-scale current. Capacitor determines full-scale output frequency with maximum usable frequency resulting 0.4% nonlinearity. Other temperature ranges output frequencies achievable. Refer AD654 data sheet additional information.
CONSTANTAN (ALUMEL) OVERLOAD DETECT
(+5V
+VLOGIC FOUT
AD594/ AD595
OSC/ DRIVER (10V)
FOUT POINT COMP. IRON (CHROMEL)
AD654
-15V)
COMMON
Figure Temperature-to-Frequency Conversion
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FAHRENHEIT OUTPUT AD594/AD595 configured produce voltage proportional temperature Fahrenheit scale. Conversion temperature from Celsius Fahrenheit scale involves multiplying degrees Celsius adding degree offset. offset produced injecting nA/°C current into while increasing feedback resistor accommodate gain 9/5. Output calibration follows: With thermocouple disconnected, apply p-p, signal Pins Adjust RGAIN output 3.481 (AD594) 4.451 (AD595). With thermocouple connected measuring 0°C, adjust ROFFSET until output reads ideal transfer function based Fahrenheit output AD594 Output (Type Voltage 348.12 AD595 Output (Type Voltage 445.14 This yields higher output voltage swing over useful range thermocouple therefore, requiring higher power supply voltage maintain maximum output voltage below supply.
IRON (CHROMEL) +20V
AD680
10mV/ 7.5k
ROFFSET
CONSTANTAN (ALUMEL)
RGAIN 9.1k
OVERLOAD DETECT
AD594/ AD595
POINT COMP.
598k
Figure Fahrenheit Output
AVERAGE TEMPERATURE connecting number thermocouples parallel AD594/AD595 input, average junction temperature will measured. shown Figure resistor placed series with side each thermocouple limit current circulating between thermocouple branches. Based thermocouple temperature that either higher lower than mean, positive negative voltage drop will developed.
CONSTANTAN (ALUMEL)
(R/N)
VOUTAVE (T1,
OVERLOAD DETECT
AD594/ AD595
IRON (CHROMEL)
POINT COMP.
ISOTHERMAL REGION
Figure Measuring Average Temperature
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MULTIPLEXED THERMOCOUPLES Multiple thermocouples connected single AD595/AD595 external CMOS analog multiplexer such ADG507A. proper operation, interconnects between thermocouples, multiplexer AD594/AD595 inputs copper held thermal equilibrium isothermal block. shown Figure thermocouple mounted measure temperature well cancel reference junction voltage isothermal block. With multiplexer enabled, Constantan (Alumel)-Copper junction formed thermocouple connection isothermal block series with Copper-Constantan (Alumel) junction formed reference thermocouple connection. This series combination contributes equal opposite voltages since block isothermal. Under this condition, AD594/AD595 internal cold junction compensation compensates reference junction which must remain between +50°C. Note however, that isothermal block convenient temperature location. Unused multiplexer inputs should connected common minimize stray signal pickup. prevent AD594/AD595 inputs from "floating" resulting output saturation, multiplexer permanently enabled connecting enable input REFERENCES Sheingold, Dan, Transducer Interface Handbook, Analog Devices, 1980. 1992 Amplifier Applications Guide, Analog Devices, Pub. G1646-10-4/92. American Society Testing Materials, Manual Thermocouples Temperature Measurement, ASPCN 04-470020-40.
CONSTANTAN (ALUMEL) IRON (CHROMEL)
REFERENCE JUNCTION +15V VOUT
CONSTANTAN (ALUMEL) OVERLOAD DETECT
AD594/ AD595
POINT COMP.
-15V
+15V -15V
ADG507A
ISOTHERMAL REGION
IRON (CHROMEL) ISOTHERMAL BLOCK
PRINTED U.S.A.
Figure Multiplexed Inputs
E1796a-0-7/98

Thermocouple Signal Conditioning Using AD594/AD595 Marcin IN

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