joseph wu senior applications engineer texas instruments tucson l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Precision Temperature Measurement with the ADS1248 PowerPoint Presentation
Download Presentation
Precision Temperature Measurement with the ADS1248

Loading in 2 Seconds...

play fullscreen
1 / 57

Precision Temperature Measurement with the ADS1248 - PowerPoint PPT Presentation


  • 949 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Precision Temperature Measurement with the ADS1248' - benjamin


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
joseph wu senior applications engineer texas instruments tucson
Joseph Wu

Senior Applications Engineer

Texas Instruments – Tucson

Precision Temperature Measurement with the ADS1248

2009 European FAE Summit, Munich

presentation overview
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
What sort of temperature elements can we measure with the ADS1248?

2009 European FAE Summit, Munich

temperature monitoring rtd
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 rtd5
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

temperature monitoring rtd6
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 rtd7
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

slide8

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

slide9

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

slide10

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

slide11

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
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

slide13

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

slide14

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
What can we do with the ADS1248 and its EVM?

2009 European FAE Summit, Munich

ads1248 block diagram
ADS1248 Block Diagram

2009 European FAE Summit, Munich

slide17

ADS1248EVM-PDK

2009 European FAE Summit, Munich

slide18

ADS1248EVM Schematic

2009 European FAE Summit, Munich

ads1248evm layout
ADS1248EVM Layout

2009 European FAE Summit, Munich

slide20

ADCPro with the ADS1248 Plug-in

2009 European FAE Summit, Munich

slide21

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide22

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide23

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide24

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide25

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide26

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide27

ADS1248 Plug-In

2009 European FAE Summit, Munich

slide29

2-Wire RTD Measurement

2009 European FAE Summit, Munich

slide30
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

slide31

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
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

slide33

3-Wire RTD Measurement

2009 European FAE Summit, Munich

slide34
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

slide35

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

example rtd pt100 idac1 idac2 1ma rbias 500 each line resistance 0 5
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

slide37

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

slide39

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

slide40

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

example rtd pt100 idac1 idac2 1ma rbias 500 each line resistance 0 5 rcomp 100
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

slide42

4-Wire RTD Measurement

2009 European FAE Summit, Munich

slide43

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

slide44

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

example rtd pt100 idac1 1ma rbias 1k each line resistance 0 5
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

slide46

Thermocouple Measurement with 3-Wire RTD as Cold Junction Compensation

2009 European FAE Summit, Munich

slide47

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

slide48

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

slide49

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

slide50

Thermistor with Shunt Resistor Measurement

Thermistor has a nominal 10k response at 25oC

2009 European FAE Summit, Munich

slide51

Thermistor with Shunt Resistor Measurement

  • 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

2009 European FAE Summit, Munich

slide52

Thermistor with Shunt Resistor Measurement

Without linearization

With linearization

2009 European FAE Summit, Munich

slide53

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 1kW)

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 1kW 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 1kW NTC thermistor. The parallel resistor is 1kWas is RBIAS.

2009 European FAE Summit, Munich

slide54

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 25oC

2009 European FAE Summit, Munich

slide55

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
Questions?

Comments?

2009 European FAE Summit, Munich

slide57

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 DS ADC", Application Note, (to be published)

"2- 3- 4- Wire RDT (PT100 to PT1000) Temperature Measurement", Olaf Escher, Presentation

References

2009 European FAE Summit, Munich