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Precision Temperature Measurement with the ADS1248

Joseph Wu Senior Applications Engineer Texas Instruments – Tucson. Precision Temperature Measurement with the ADS1248 . 2009 European FAE Summit, Munich. Presentation Overview. An Overview of Temperature Elements The ADS1248 and ADCPro Precision Measurements with the ADS1248.

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Precision Temperature Measurement with the ADS1248

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

  2. Presentation Overview • An Overview of Temperature Elements • The ADS1248 and ADCPro • Precision Measurements with the ADS1248 2009 European FAE Summit, Munich

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

  4. 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

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

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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

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

  16. ADS1248 Block Diagram 2009 European FAE Summit, Munich

  17. ADS1248EVM-PDK 2009 European FAE Summit, Munich

  18. ADS1248EVM Schematic 2009 European FAE Summit, Munich

  19. ADS1248EVM Layout 2009 European FAE Summit, Munich

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

  21. ADS1248 Plug-In 2009 European FAE Summit, Munich

  22. ADS1248 Plug-In 2009 European FAE Summit, Munich

  23. ADS1248 Plug-In 2009 European FAE Summit, Munich

  24. ADS1248 Plug-In 2009 European FAE Summit, Munich

  25. ADS1248 Plug-In 2009 European FAE Summit, Munich

  26. ADS1248 Plug-In 2009 European FAE Summit, Munich

  27. ADS1248 Plug-In 2009 European FAE Summit, Munich

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

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

  30. 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

  31. 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

  32. Example: RTD: PT100 IDAC = 1mA RBIAS = 1k Each line resistance = 0.5 2-Wire RTD Measurement A PT100 has about a 0.384 change for each 1oC of change • 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 2009 European FAE Summit, Munich

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

  34. Advantages: Simple Input line resistances cancel Sensor can be farther away Ratiometric – IDAC current drift is cancelled 3-Wire RTD Measurement • Disadvantages: • IDAC current and drift need to match 2009 European FAE Summit, Munich

  35. 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 1kW) 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 • 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

  36. Example: RTD: PT100 IDAC1 = IDAC2 = 1mA RBIAS = 500 Each line resistance = 0.5 3-Wire RTD Measurement • 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

  37. 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! 3-Wire RTD Measurement A PT100 has about a 0.384 change for each 1oC of change 0.384 x 1mA = 384uV 2009 European FAE Summit, Munich

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

  39. 3-Wire RTD Measurement with Hardware Compensation Same Benefits and Problems as the typical 3-wire measurement • Advantages: • Centers the measurement so that the center temperature is at 0V • Easier to use a larger PGA gain • Disadvantages: • IDAC current mismatch is gained up by RCOMP as well as the line resistance 2009 European FAE Summit, Munich

  40. 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) 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. • 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

  41. Example: RTD: PT100 IDAC1 = IDAC2 = 1mA RBIAS = 500 Each line resistance = 0.5 RCOMP = 100 3-Wire RTD Measurement with Hardware Compensation • 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

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

  43. 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

  44. 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) 4-Wire RTD Measurement Setup • 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

  45. Example: RTD: PT100 IDAC1 = 1mA RBIAS = 1k Each line resistance = 0.5 4-Wire RTD Measurement • 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

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

  47. 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

  48. 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) 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 • 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

  49. 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

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

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