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Precision Temperature Measurement with the ADS1248
Joseph Wu
Senior Applications Engineer
Texas Instruments – Tucson
2009 European FAE Summit, Munich
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2009 European FAE Summit, Munich
• An Overview of Temperature Elements
• The ADS1248 and ADCPro
• Precision Measurements with the ADS1248
Presentation Overview
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2009 European FAE Summit, Munich
What sort of temperature elements can we measure with
the ADS1248?
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2009 European FAE Summit, Munich
• RTD: resistance temperature detector• Positive temperature coefficient• Wire-wound or thick film metal resistor• Manufacturers: Advanced Thermal Products, U.S.
Sensors, Sensing Devices Inc.
Temperature Monitoring - RTD
Source: Advanced Thermal Products, Inc.
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2009 European FAE Summit, Munich
Temperature Monitoring - RTD
a.) Two-wire leadconfiguration
b.) Three-wire leadconfiguration
c.) Four-wire leadconfiguration
PRTD
A
B
PRTD
B
A
C
PRTD
B
A
C
D
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2009 European FAE Summit, Munich
Advantages:• Most Accurate• High linearity over limited temperature range
(-40oC to +85oC)• Wide usable temperature range
Temperature Monitoring - RTD
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2009 European FAE Summit, Munich
Disadvantages: • Limited resistance• Low sensitivity • Lead wire resistance may introduce errors• Requires linearization for wide range• Wire wound RTDs tend to be fragile• Cost is high compared to a thermistor
Temperature Monitoring - RTD
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermocouple
Source: Datapaq
• Thermocouple: temperature element based on two dissimilar metals
• The junction of two dissimilar metals creates an open circuit voltage that is proportional to temperature
• Direct measurement is difficult because each junction will have it’s own voltage drop
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermocouple
Reference (Cold) Junction Compensation
Voltage is proportional to Temperature
• V = (V1 – V2) ~= α(tJ1 – tJ2)
• If we specify TJ1 in degrees Celsius: TJ1(C) + 273.15 = tJ1(K)
• V becomes: V = V1 – V2 = α[(TJ1 + 273.15) – (TJ2 + 273.15)]
= α(TJ1 – TJ2 ) = (TJ1 – 0)
V = αTJ1
Source: Agilent
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermocouple
Advantages:• Self-powered• Simple and durable in construction• Inexpensive• Wide variety of physical forms
• Wide temperature range (-200oC to +2000oC)
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermocouple
Disadvantages:• Thermocouple voltage can be non-linear with temperature• Low measurement voltages• Reference is required• Least stable and sensitive• Requires a known junction temperature
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2009 European FAE Summit, Munich
• Thermistor: Thermally sensitive resistor
• Sintered metal oxide or passive semiconductor materials
• Suppliers – Selco, YSI, Alpha Sensors, Betatherm
Temperature Monitoring - Thermistor
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermistor
Advantages:
• Low cost
• Rugged construction
• Available in wide range of resistances
• Available with negative (NTC) and positive (PTC) temperature coefficients.
• Highly sensitive
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2009 European FAE Summit, Munich
Temperature Monitoring - Thermistor
Disadvantages:
• Limited temperature range: -100oC to 200oC• Highly non-linear response• Linearization nearly always required• Least accurate• Self-heating
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2009 European FAE Summit, Munich
What can we do with the ADS1248 and its EVM?
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2009 European FAE Summit, Munich
ADS1248 Block Diagram
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2009 European FAE Summit, Munich
ADS1248EVM-PDK
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2009 European FAE Summit, Munich
ADS1248EVM Schematic
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2009 European FAE Summit, Munich
ADS1248EVM Layout
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2009 European FAE Summit, Munich
ADCPro with the ADS1248 Plug-in
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
ADS1248 Plug-In
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2009 European FAE Summit, Munich
What type of systems can be put together with the ADS1248?
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2009 European FAE Summit, Munich
2-Wire RTD Measurement
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2009 European FAE Summit, Munich
Advantages:• Simple
• Ratiometric – IDAC current drift is cancelled
• Noise in the IDAC is reflected in both the reference and the RTD
2-Wire RTD Measurement
Disadvantages:• Least Accurate
• Line resistance affects the measurement
• The filter must be removed on the EVM.
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2009 European FAE Summit, Munich
Plug-in:• PGA Gain = 1, Data Rate = 20 • Block Size = 128• AINP = AIN0 < IDAC0• AINN = AIN1• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA• IDAC0 = AIN, IDAC1 = Off• VREF = 1V ≈ (1000uA x 1k)
2-Wire RTD Measurement Setup
Setup:• 2-Wire measurement sensitive to series resistance• R4 and R5 removed on EVM
Board:• RTD: Black, Green: AIN0• RTD: White, Red: AIN1 • Reference Resistor: AIN1 to GND, 1k• Jumper: GND to REF-• Wire: AIN1 to REF+
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2009 European FAE Summit, Munich
Example:• RTD: PT100
• IDAC = 1mA
• RBIAS = 1k
• Each line resistance = 0.5
2-Wire RTD Measurement
We get:• Reference
1mA x 1k = 1V
• ADC Measurement:
1mA x (100 + 0.5+ 0.5)
= 101mV
• Input is within ADC common- mode input range
A PT100 has about a 0.384 change for each 1oC of change
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2009 European FAE Summit, Munich
3-Wire RTD Measurement
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2009 European FAE Summit, Munich
3-Wire RTD Measurement
Advantages:• Simple• Input line resistances cancel • Sensor can be farther away• Ratiometric – IDAC current drift is cancelled
Disadvantages:• IDAC current and drift need to match
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2009 European FAE Summit, Munich
3-Wire RTD Measurement Setup
Plug-in:• PGA Gain = 1, Data Rate = 20• Block Size = 128• AINP = AIN2 < IDAC0• AINN = AIN3 < IDAC1• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA• IDAC0 = AIN, IDAC• VREF = 1V ≈ (1000uA x 1k)
Setup:• 3-Wire measurement far less sensitive to series resistance• Measurement illustrated with 47 of series resistance• Change reference resistor to 499
Board:• RTD: Black, Green: AIN2• RTD: White: AIN3• RTD: Red: AIN5 • Reference Resistor: AIN5 to GND, 499• Jumper: GND to REF-• Wire: AIN5 to REF+
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2009 European FAE Summit, Munich
3-Wire RTD Measurement
Example:• RTD: PT100
• IDAC1 = IDAC2 = 1mA
• RBIAS = 500
• Each line resistance = 0.5
We get:• Reference
(1mA+1mA) x 500 = 1V
• ADC Measurement:
1mA x (100 + 0.5
1mA x 0.5
= 100mV
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2009 European FAE Summit, Munich
3-Wire RTD Measurement
However:• If the IDAC currents or line resistances do not match, there can be errors in cancellation.• ADS1248 IDAC currents are matched to 0.03% typ.• With 1mA IDACs, the mismatch is 0.3A• In previous example, error is 0.3A x 0.5 = .15uV
• The error in line resistance mismatch can be higher in comparison!
A PT100 has about a 0.384change for each 1oC of change
0.384 x 1mA = 384uV
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2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware Compensation
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2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware Compensation
Advantages:• Centers the measurement so that the center temperature is at 0V
• Easier to use a larger PGA gain
Same Benefits and Problems as the typical 3-wire measurement
Disadvantages:• IDAC current mismatch is gained up by RCOMP as well as the line resistance
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2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware Compensation Setup
Plug-in:• PGA Gain = 128, Data Rate = 20• Block Size = 128• AINP = AIN2 < IDAC0• AINN = AIN4 < IDAC1• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA• IDAC0 = AIN, IDAC• VREF = 1V ≈ (1000uA x 1kW)
Setup:• 110 resistor added as hardware compensation• Centers the measurement around 0V so that more gain can be used.
Board:• RTD: Black, Green: AIN2• RTD: White: AIN3• RTD: Red: AIN5• 100 resistor AIN3 to AIN4• Reference Resistor: AIN5 to GND, 499• Jumper: GND to REF-• Wire: AIN5 to REF+
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2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware Compensation
Example:• RTD: PT100
• IDAC1 = IDAC2 = 1mA
• RBIAS = 500
• Each line resistance = 0.5• RCOMP = 100
We get:• Reference
(1mA+1mA) x 500 = 1V
• ADC Measurement (0oC):
1mA x (100 + 0.5)
1mA x (100 + 0.5)
= 0mV
• ADC Measurement (100oC):
1mA x (138.4 + 0.5)
1mA x (100 + 0.5)
= 38.4mV
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2009 European FAE Summit, Munich
4-Wire RTD Measurement
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2009 European FAE Summit, Munich
4-Wire RTD Measurement
Advantages:• Most accurate, line resistances are no longer a problem
• Sensor can be far away
• Ratiometric measurement
• No IDAC drift component
Disadvantages:• Need to use external IDAC pins
• Only two IDAC pins available
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2009 European FAE Summit, Munich
4-Wire RTD Measurement Setup
Plug-in:• PGA Gain = 1, Data Rate = 20 • Block Size = 128• AINP = AIN3, AINN = AIN4• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA• IDAC0 = AIN, IDAC1 = Off• VREF = 1V ≈ (1000uA x 1kW)
Setup:• Return to G=1• 1k reference resistor• Most accurate measurement
Board:• RTD Black: AIN2• RTD Green: AIN3• RTD White: AIN4• RTD Red: AIN5 • Reference Resistor: AIN5 to GND, 1k• Jumper: GND to REF-• Wire: AIN5 to REF+
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2009 European FAE Summit, Munich
4-Wire RTD Measurement
Example:• RTD: PT100• IDAC1 = 1mA• RBIAS = 1k • Each line resistance = 0.5
We get:• Reference
1mA x 1k = 1V• ADC Measurement:
1mA x 100 = 100mV• Error is differential input current times the line resistance
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2009 European FAE Summit, Munich
Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation
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2009 European FAE Summit, Munich
Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation
Advantages:• Thermocouple needs no excitation source
• RTD used for cold junction compensation.
Disadvantages:• Complex
• Requires multiple resources of the ADS1248
• Internal reference used in measuring thermocouple
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2009 European FAE Summit, Munich
Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation Setup
Plug-in:Thermocouple• PGA Gain = 1, Data Rate = 20 • Block Size = 128• AINN = AIN0 < VBIAS, AINP = AIN1• Reference Select = Internal, VREF = 2.5VThree-wire RTD• AINP = AIN2 < IDAC0, AINN = AIN2 < IDAC0• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA, IDAC0, IDAC1 = AIN • VREF = 1V ≈ (2000uA x 499)
Setup:• Two measurements• Thermocouple uses VBIAS, but no IDAC current.• Three-wire RTD setup as before
Board:• Thermocouple: AIN0 to AIN1 • RTD Black, Green: AIN2 • RTD White: AIN3• RTD Red: AIN5 • Reference Resistor: AIN5 to GND, 499• Jumper: GND to REF-• Wire: AIN5 to REF+
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2009 European FAE Summit, Munich
Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation
Example:• Thermocouple: K-type
• RTD: PT100 with 3-wire measurement
We get:• The thermocouple is DC biased with VBIAS
• Measured using internal reference.
• The cold junction uses an 3-wire RTD
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2009 European FAE Summit, Munich
Thermistor with Shunt Resistor Measurement
Thermistor has a nominal 10k response at 25oC
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2009 European FAE Summit, Munich
Advantages:• Inexpensive temperature element
Disadvantages:• Shunt resistor needed to linearize the response
• Requires reference voltage
• Less accuracy, temperature determined by comparison to graph or lookup table
Thermistor with Shunt Resistor Measurement
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2009 European FAE Summit, Munich
Thermistor with Shunt Resistor Measurement
0.00
0.20
0.40
0.60
0.80
1.00
1.20
-100 -50 0 50 100 150
Ambient Temperature (C)
Vth
erm
(V
)
0.00
1.00
2.00
3.00
4.00
5.00
-100 -50 0 50 100 150
Ambient Temperature (C)
Vth
erm
(V)
Without linearization With linearization
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2009 European FAE Summit, Munich
Thermistor with Shunt Resistor Measurement Setup
Plug-in:• PGA Gain = 1, Data Rate = 20 • Block Size = 128• AINP = AIN0 < IDAC0• AINN = AIN1• Reference Select = VREF0• Internal Reference = On• IDAC mag = 1000uA• IDAC0 = AIN, IDAC1 = Off• VREF = 1V ≈ (1000uA x 1k)
Setup:•Similar to 2-Wire measurement sensitive to series resistance• Resistor in parallel with thermistor for linearization• Thermistor nominal value 1k with negative temperature coefficient (NTC)
Board:• Thermistor||Resistor: AIN0 to AIN1 • Reference Resistor: AIN1 to GND, 1k• Jumper: GND to REF-• Wire: AIN1 to REF+
• Note: For the demo, I could only find a 1k NTC thermistor. The parallel resistor is 1k as is RBIAS.
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2009 European FAE Summit, Munich
Thermistor with Shunt Resistor Measurement
• Improved linearity with shunt resistance
• Non-linearity is under 3% when Rshunt equal to the thermistor at the circuits median temperature
• Heavy shunting reduces output
0.00
0.20
0.40
0.60
0.80
1.00
1.20
-100 -50 0 50 100 150
Ambient Temperature (C)
Vth
erm
(V
)
NTC Thermistor has a nominal 10k response at 25oC
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2009 European FAE Summit, Munich
• We’ve covered three temperature elements: The RTD, thermocouple, and the thermistor
• Evaluation with the ADS1248EVM is easy with ADCPro
• There are many ways to connect the ADS1248 up to get a temperature measurement
Conclusions
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2009 European FAE Summit, Munich
Questions?
Comments?
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2009 European FAE Summit, Munich
References
• ADS1248 Datasheet• ADS1148/ADS1248EVM and ADS1148/ADS1248EVM-PDK User's Guide• Agilent Application Note 290 — Practical Temperature Measurements, pub. no. 5965-7822EN• "Sensors and the Analog Interface", Tom Kuehl, Tech Day Presentation• “Developing a Precise PT100 RTD Simulator for SPICE", Thomas Kuehl, Analog ZONE.com, May 2007 • "Example Applications For Temperature Measurement Using the ADS1247 & ADS1248 ADC", Application Note, (to be published)• "2- 3- 4- Wire RDT (PT100 to PT1000) Temperature Measurement", Olaf Escher, Presentation