Precision Temperature Measurement with the ADS1248 Joseph Wu Senior Applications Engineer Texas Instruments – Tucson 2009 European FAE Summit, Munich.

Precision Temperature Measurement with the ADS1248

Joseph Wu

Senior Applications Engineer

Texas Instruments – Tucson

2009 European FAE Summit, Munich

• An Overview of Temperature Elements

• The ADS1248 and ADCPro

• Precision Measurements with the ADS1248

Presentation Overview

What sort of temperature elements can we measure with

the ADS1248?

• 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.

a.) Two-wire leadconfiguration

b.) Three-wire leadconfiguration

c.) Four-wire leadconfiguration

Advantages:• Most Accurate• High linearity over limited temperature range

(-40oC to +85oC)• Wide usable temperature range

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 - 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

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

Advantages:• Self-powered• Simple and durable in construction• Inexpensive• Wide variety of physical forms

• Wide temperature range (-200oC to +2000oC)

Disadvantages:• Thermocouple voltage can be non-linear with temperature• Low measurement voltages• Reference is required• Least stable and sensitive• Requires a known junction temperature

• Thermistor: Thermally sensitive resistor

• Sintered metal oxide or passive semiconductor materials

• Suppliers – Selco, YSI, Alpha Sensors, Betatherm

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

Disadvantages:

• Limited temperature range: -100oC to 200oC• Highly non-linear response• Linearization nearly always required• Least accurate• Self-heating

What can we do with the ADS1248 and its EVM?

ADS1248 Block Diagram

ADS1248EVM-PDK

ADS1248EVM Schematic

ADS1248EVM Layout

ADCPro with the ADS1248 Plug-in

ADS1248 Plug-In

What type of systems can be put together with the ADS1248?

2-Wire RTD Measurement

Advantages:• Simple

• Ratiometric – IDAC current drift is cancelled

• Noise in the IDAC is reflected in both the reference and the RTD

Disadvantages:• Least Accurate

• Line resistance affects the measurement

• The filter must be removed on the EVM.

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+

Example:• RTD: PT100

• IDAC = 1mA

• RBIAS = 1k

• Each line resistance = 0.5

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

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

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+

• IDAC1 = IDAC2 = 1mA

• RBIAS = 500

• Each line resistance = 0.5

(1mA+1mA) x 500 = 1V

• ADC Measurement:

1mA x (100 + 0.5

1mA x 0.5

= 100mV

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

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

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+

• IDAC1 = IDAC2 = 1mA

• RBIAS = 500

• Each line resistance = 0.5• RCOMP = 100

(1mA+1mA) x 500 = 1V

• ADC Measurement (0oC):

1mA x (100 + 0.5)

• ADC Measurement (100oC):

1mA x (138.4 + 0.5)

1mA x (100 + 0.5)

= 38.4mV

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

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+

Example:• RTD: PT100• IDAC1 = 1mA• RBIAS = 1k • Each line resistance = 0.5

1mA x 1k = 1V• ADC Measurement:

1mA x 100 = 100mV• Error is differential input current times the line resistance

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

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+

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

Thermistor with Shunt Resistor Measurement

Thermistor has a nominal 10k response at 25oC

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

-100 -50 0 50 100 150

Ambient Temperature (C)

-100 -50 0 50 100 150

Without linearization With linearization

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.

• 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

-100 -50 0 50 100 150

NTC Thermistor has a nominal 10k response at 25oC

• 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

Questions?

Comments?

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

Precision Temperature Measurement with the ADS1248 Joseph Wu Senior Applications Engineer Texas Instruments – Tucson 2009 European FAE Summit, Munich.

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