Download 3-Wire RTD Measurement - TI E2E Community

Precision Temperature
Measurement with the ADS1248
Joseph Wu
Senior Applications Engineer
Texas Instruments – Tucson
2009 European FAE Summit, Munich
Presentation Overview
• An Overview of Temperature Elements
• The ADS1248 and ADCPro
• Precision Measurements with the ADS1248
2009 European FAE Summit, Munich
What sort of temperature
elements can we measure with
the ADS1248?
2009 European FAE Summit, Munich
Temperature Monitoring - RTD
Source: Advanced Thermal Products, Inc.
•
•
•
•
RTD: resistance temperature detector
Positive temperature coefficient
Wire-wound or thick film metal resistor
Manufacturers: Advanced Thermal Products, U.S.
Sensors, Sensing Devices Inc.
2009 European FAE Summit, Munich
Temperature Monitoring - RTD
C
A
PRTD
B
C
A
A
PRTD
PRTD
B
B
D
a.) Two-wire lead
configuration
2009 European FAE Summit, Munich
b.) Three-wire lead
configuration
c.) Four-wire lead
configuration
Temperature Monitoring - RTD
Advantages:
• Most Accurate
• High linearity over limited temperature range
(-40oC to +85oC)
• Wide usable temperature range
2009 European FAE Summit, Munich
Temperature Monitoring - RTD
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
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
Source: Agilent
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
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
Temperature Monitoring - Thermistor
•
•
•
Thermistor: Thermally sensitive resistor
Sintered metal oxide or passive semiconductor materials
Suppliers – Selco, YSI, Alpha Sensors, Betatherm
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
2-Wire RTD Measurement
Advantages:
Disadvantages:
• Simple
• Ratiometric – IDAC current drift is
cancelled
• Noise in the IDAC is reflected in
both the reference and the RTD
• Least Accurate
• Line resistance affects the
measurement
• The filter must be removed on the
EVM.
2009 European FAE Summit, Munich
2-Wire RTD Measurement Setup
Setup:
• 2-Wire measurement sensitive to
series resistance
• R4 and R5 removed on EVM
Plug-in:
Board:
• 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)
• 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
2-Wire RTD Measurement
A PT100 has about a 0.384
change for each 1oC of change
Example:
We get:
• RTD: PT100
• IDAC = 1mA
• RBIAS = 1k
• Each line resistance = 0.5
• Reference
1mA x 1k = 1V
• ADC Measurement:
1mA x (100 + 0.5+ 0.5)
= 101mV
• Input is within ADC commonmode input range
2009 European FAE Summit, Munich
3-Wire RTD Measurement
2009 European FAE Summit, Munich
3-Wire RTD Measurement
Advantages:
Disadvantages:
• Simple
• Input line resistances cancel
• Sensor can be farther away
• Ratiometric – IDAC current
drift is cancelled
• IDAC current and drift need to
match
2009 European FAE Summit, Munich
3-Wire RTD Measurement Setup
Setup:
• 3-Wire measurement far less
sensitive to series resistance
• Measurement illustrated with 47
of series resistance
• Change reference resistor to
499
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)
2009 European FAE Summit, Munich
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+
3-Wire RTD Measurement
Example:
We get:
• RTD: PT100
• IDAC1 = IDAC2 = 1mA
• RBIAS = 500
• Each line resistance = 0.5
• 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
A PT100 has about a 0.384
change for each 1oC of change
0.384 x 1mA = 384uV
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 0.3A
• In previous example, error is 0.3A x 0.5 = .15uV
• The error in line resistance mismatch can be higher in
comparison!
2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware
Compensation
2009 European FAE Summit, Munich
3-Wire RTD Measurement with Hardware
Compensation
Same Benefits and Problems as
the typical 3-wire measurement
Advantages:
Disadvantages:
• Centers the measurement so that
the center temperature is at 0V
• Easier to use a larger PGA gain
• 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
Setup:
• 110 resistor added as hardware
compensation
• Centers the measurement around
0V so that more gain can be used.
Plug-in:
Board:
• 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)
• 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
We get:
• Reference
(1mA+1mA) x 500 = 1V
Example:
• RTD: PT100
• IDAC1 = IDAC2 = 1mA
• RBIAS = 500
• Each line resistance = 0.5
• RCOMP = 100
2009 European FAE Summit, Munich
• 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
4-Wire RTD Measurement
2009 European FAE Summit, Munich
4-Wire RTD Measurement
Advantages:
Disadvantages:
• Most accurate, line resistances
are no longer a problem
• Sensor can be far away
• Ratiometric measurement
• No IDAC drift component
• Need to use external IDAC pins
• Only two IDAC pins available
2009 European FAE Summit, Munich
4-Wire RTD Measurement Setup
Setup:
• Return to G=1
• 1k reference resistor
• Most accurate measurement
Plug-in:
Board:
• 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)
• 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:
We get:
• RTD: PT100
• IDAC1 = 1mA
• RBIAS = 1k
• Each line resistance = 0.5
• 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:
Disadvantages:
• Thermocouple needs no excitation
source
• RTD used for cold junction
compensation.
• 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
Setup:
• Two measurements
• Thermocouple uses VBIAS, but no
IDAC current.
• Three-wire RTD setup as before
Plug-in:
Board:
Thermocouple
• PGA Gain = 1, Data Rate = 20
• Block Size = 128
• AINN = AIN0 < VBIAS, AINP = AIN1
• Reference Select = Internal, VREF = 2.5V
Three-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)
• 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:
We get:
• Thermocouple: K-type
• RTD: PT100 with 3-wire
measurement
• The thermocouple is DC biased with
2009 European FAE Summit, Munich
VBIAS
• Measured using internal reference.
• The cold junction uses an 3-wire RTD
Thermistor with Shunt Resistor
Measurement
Thermistor has a nominal
o
10k response at 25 C
2009 European FAE Summit, Munich
Thermistor with Shunt Resistor
Measurement
Advantages:
Disadvantages:
• Inexpensive temperature element
• Shunt resistor needed to linearize
the response
• Requires reference voltage
• Less accuracy, temperature
determined by comparison to
graph or lookup table
2009 European FAE Summit, Munich
Thermistor with Shunt Resistor
Measurement
Without linearization
With linearization
1.20
5.00
1.00
4.00
Vtherm (V)
Vtherm (V)
0.80
3.00
2.00
0.60
0.40
1.00
0.00
-100
0.20
0.00
-50
0
50
100
Ambient Temperature (C)
2009 European FAE Summit, Munich
150
-100
-50
0
50
Ambient Temperature (C)
100
150
Thermistor with Shunt Resistor
Measurement Setup
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)
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)
2009 European FAE Summit, Munich
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.
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
1.20
1.00
0.80
Vtherm (V)
NTC Thermistor
has a nominal
10k response
at 25oC
0.60
0.40
0.20
0.00
-100
-50
0
50
Ambient Temperature (C)
2009 European FAE Summit, Munich
100
150
Conclusions
• 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
2009 European FAE Summit, Munich
Questions?
Comments?
2009 European FAE Summit, Munich
References
• ADS1248 Datasheet
• ADS1148/ADS1248EVM and ADS1148/ADS1248EVMPDK 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 DS ADC", Application
Note, (to be published)
• "2- 3- 4- Wire RDT (PT100 to PT1000) Temperature
Measurement", Olaf Escher, Presentation
2009 European FAE Summit, Munich