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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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Presentation on theme: "Precision Temperature Measurement with the ADS1248 Joseph Wu Senior Applications Engineer Texas Instruments – Tucson 2009 European FAE Summit, Munich."— Presentation transcript:

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

2 An Overview of Temperature Elements The ADS1248 and ADCPro Precision Measurements with the ADS1248 Presentation Overview

3 2009 European FAE Summit, Munich What sort of temperature elements can we measure with the ADS1248?

4 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.

5 2009 European FAE Summit, Munich Temperature Monitoring - RTD a.) Two-wire lead configuration b.) Three-wire lead configuration c.) Four-wire lead configuration PRTD A B B A C B A C D

6 2009 European FAE Summit, Munich Advantages: Most Accurate High linearity over limited temperature range (-40 o C to +85 o C) Wide usable temperature range Temperature Monitoring - RTD

7 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

8 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 its own voltage drop

9 2009 European FAE Summit, Munich Temperature Monitoring - Thermocouple Reference (Cold) Junction Compensation Voltage is proportional to Temperature V = (V1 – V2) ~= α(t J1 – t J2 ) If we specify T J1 in degrees Celsius: T J1 (C) + 273.15 = t J1 (K) V becomes: V = V1 – V2 = α [(T J1 + 273.15) – (T J2 + 273.15)] = α (T J1 – T J2 ) = (T J1 – 0) V = α T J1 Source: Agilent

10 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 (-200 o C to +2000 o C)

11 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

12 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

13 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

14 2009 European FAE Summit, Munich Temperature Monitoring - Thermistor Disadvantages: Limited temperature range: -100 o C to 200 o C Highly non-linear response Linearization nearly always required Least accurate Self-heating

15 2009 European FAE Summit, Munich What can we do with the ADS1248 and its EVM?

16 2009 European FAE Summit, Munich ADS1248 Block Diagram

17 2009 European FAE Summit, Munich ADS1248EVM-PDK

18 2009 European FAE Summit, Munich ADS1248EVM Schematic

19 2009 European FAE Summit, Munich ADS1248EVM Layout

20 2009 European FAE Summit, Munich ADCPro with the ADS1248 Plug-in

21 2009 European FAE Summit, Munich ADS1248 Plug-In

22 2009 European FAE Summit, Munich ADS1248 Plug-In

23 2009 European FAE Summit, Munich ADS1248 Plug-In

24 2009 European FAE Summit, Munich ADS1248 Plug-In

25 2009 European FAE Summit, Munich ADS1248 Plug-In

26 2009 European FAE Summit, Munich ADS1248 Plug-In

27 2009 European FAE Summit, Munich ADS1248 Plug-In

28 2009 European FAE Summit, Munich What type of systems can be put together with the ADS1248?

29 2009 European FAE Summit, Munich 2-Wire RTD Measurement

30 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.

31 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+

32 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 1 o C of change

33 2009 European FAE Summit, Munich 3-Wire RTD Measurement

34 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

35 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+

36 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

37 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 0.3 A In previous example, error is 0.3 A x 0.5 =.15uV The error in line resistance mismatch can be higher in comparison! A PT100 has about a 0.384 change for each 1 o C of change 0.384 x 1mA = 384uV

38 2009 European FAE Summit, Munich 3-Wire RTD Measurement with Hardware Compensation

39 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

40 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+

41 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 (0 o C): 1mA x (100 + 0.5 ) = 0mV ADC Measurement (100 o C): 1mA x (138.4 + 0.5 ) 1mA x (100 + 0.5 ) = 38.4mV

42 2009 European FAE Summit, Munich 4-Wire RTD Measurement

43 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

44 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+

45 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

46 2009 European FAE Summit, Munich Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation

47 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

48 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 = 2.5V 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 ) 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+

49 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

50 2009 European FAE Summit, Munich Thermistor with Shunt Resistor Measurement Thermistor has a nominal 10k response at 25 o C

51 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

52 2009 European FAE Summit, Munich Thermistor with Shunt Resistor Measurement Without linearizationWith linearization

53 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.

54 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 NTC Thermistor has a nominal 10k response at 25 o C

55 2009 European FAE Summit, Munich Weve 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

56 2009 European FAE Summit, Munich Questions? Comments?

57 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


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