precision temperature measurement with the ads1248 joseph wu senior applications engineer texas...
TRANSCRIPT
Precision Temperature Measurement with the ADS1248
Joseph Wu
Senior Applications Engineer
Texas Instruments – Tucson
2009 European FAE Summit, Munich
2009 European FAE Summit, Munich
• An Overview of Temperature Elements
• The ADS1248 and ADCPro
• Precision Measurements with the ADS1248
Presentation Overview
2009 European FAE Summit, Munich
What sort of temperature elements can we measure with
the ADS1248?
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.
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
2009 European FAE Summit, Munich
Advantages:• Most Accurate• High linearity over limited temperature range
(-40oC to +85oC)• Wide usable temperature range
Temperature Monitoring - RTD
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
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
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
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)
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
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
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
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
2009 European FAE Summit, Munich
What can we do with the ADS1248 and its EVM?
2009 European FAE Summit, Munich
ADS1248 Block Diagram
2009 European FAE Summit, Munich
ADS1248EVM-PDK
2009 European FAE Summit, Munich
ADS1248EVM Schematic
2009 European FAE Summit, Munich
ADS1248EVM Layout
2009 European FAE Summit, Munich
ADCPro with the ADS1248 Plug-in
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
ADS1248 Plug-In
2009 European FAE Summit, Munich
What type of systems can be put together with the ADS1248?
2009 European FAE Summit, Munich
2-Wire RTD Measurement
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.
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+
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
2009 European FAE Summit, Munich
3-Wire RTD Measurement
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
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+
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
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
2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware Compensation
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
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+
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
2009 European FAE Summit, Munich
4-Wire RTD Measurement
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
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+
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
2009 European FAE Summit, Munich
Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation
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
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+
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
2009 European FAE Summit, Munich
Thermistor with Shunt Resistor Measurement
Thermistor has a nominal 10k response at 25oC
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
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
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.
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
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
2009 European FAE Summit, Munich
Questions?
Comments?
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