RTD and Thermocouple Temperature Sensing using Delta Sigma Converters (ADS1248, ADS1118) Joachim Würker Texas Instruments System Engineer - Precision Analog
Agenda Introduction of the various temperature sensors Thermistor RTD Thermocouple Cold Junction Compensation RTD Measurement Implementations ADS1248 A universal temperature sensing ADC ADS1118 Thermocouple measurement ADS1118 - Evaluationmodule
Types of Temperature Sensors Thermistor RTD Resistance Temperature Device Thermocouple Temp. Sensor IC
Thermistor Basics Material: generally a ceramic or polymer ΔR = k ΔT PTC: k > 0 NTC: k < 0 Linear approximation only applicable over a small temperature range 3 rd order Steinhart-Hart equation usually used for more accurate approximation Advantages Low cost Rugged construction Available in wide range of resistances Available with either negative (NTC) or positive (PTC) temperature coefficients High sensitivity (100Ω/ C) Disadvantages Limited temperature range: -100 C to +150 C Highly non-linear response Excitation required Linearization nearly always required Least accurate Self-heating
Thermistor Resistance vs. Temperature Example of Glass Encapsulated Thermistor R=10kΩ @ 25 C; Tolerance +/-0.2C from 0 C to 70 C 10kΩ @ 25 C Graph obtained using Steinhart-Hart Equation: 1/T= A + B [Ln(R)] + C [Ln(R)] 3 Omega Thermistor Model 55016: A=1.275x10-3 ; B=2.3441x10-4 ; C=8.6482x10-8
RTD Resistance Temperature Detector Resistive Thermal Devices Principle of Operation: Predictable resistance change Mostly made of Platinum linear resistance-temperature relationship chemical inertness Pt100 most common device used in industry Nominal Resistance R=100Ω @ 0 C Sensitivity = 0.385Ω/ C (typ.) Slowly replacing thermocouples in many industrial applications below 600 C due to higher accuracy, stability and repeatability 2-, 3-, 4-wire types Advantages Disadvantages High accuracy: < ±1 C Best stability over time Temperature range: -200 C to +500 C Good linearity Expensive Excitation required Self heating Lead resistance Slow
RTD/Thermistor Sensor Excitation Current Source Bridge Configuration REF200 REF5050 Voltage Reference Shunt V DD R Shunt V Shunt =I EX x R Shunt
Thermocouple Basics Principle of Operation: Thermoelectric / Seeback Effect Any conductor that is subjected to a thermal gradient will generate a voltage between its ends Consists of two dissimilar metal alloys. Most common types: Chromel-Alumel (Type K) Iron-Constantan (Type J) Copper-Constantan (Type T) Source: Omega Engineering
Thermocouple Basics Advantages Wide temperature range (-200 o C to +2000 o C) Only contact temperature measurement device for T >600 o C Self-powered Very fast response, fractions of sec Simple and durable in construction Inexpensive Wide variety of physical forms Disadvantages Thermocouple voltage non-linear with temperature Low measurement voltages (mv) Lowest accuracy: ±2 C Least stable and sensitive Requires a known junction temperature
Thermocouple Types Type Type E Type K Type J Type R Type S Type T Junction Material Nickel -10% Chromium Vs Constantan Nickel -10% Chromium Vs Nickel -5% Aluminum Silicon Iron Vs Constantan Platinum - 13%, Rhodium Vs Platinum (-) Platinum - 10%, Rhodium Vs Platinum (-) Copper Vs Constantan Seebeck Coefficient @20 C 62uV/ C 40uV/ C 62uV/ C 7uV/ C 7uV/ C 40uV/ C Temperature Range -100 C to 1000 C 0 C to 1370 C 0 C to 760 C 0 C to 1450 C 0 C to 1750 C -160 C to 400 C General Application Cryogenic use; nonmagnetic use General purpose Higher Sensitivity High Resistance to oxidation and corrosion, calibration purposes Standard for calibration for the melting point of gold Often used in differential measurements
Thermocouple Seebeck Coefficient Source: Omega Engineering
Thermocouple Cold Junction Compensation (CJC)
Thermocouple Temperature to Voltage Conversion Source: Omega Engineering
Thermocouple - CJC Software Compensation Cu C Software Compensation 1. Measure R T to find T REF and convert T REF to its equivalent reference junction voltage V REF 2. Measure V and add V REF to find V 1 and convert V 1 to temperature T J1 Source: Agilent
Thermocouples - CJC Example V = V 1 - V REF Measure T REF using Temp Sensor: T REF = 26 C Convert T REF to equivalent junction voltage using look up table: V REF = 1.329mV @ 26 C Measure V = 3.961mV with external meter Solve for V 1 : V 1 = V + V REF V 1 = 3.961mV + 1.329mV = 5.269mV Convert V 1 to T J1 from table: 5.269mV corresponds to T J1 = 100 C
Summary Source: Agilent
RTD Measurement How to cope with lead resistance R LEAD 2-, 3-, 4-Wire RTD measurement Absolute vs. ratiometric RTD measurement
Absolute 2-Wire Operation Using internal Reference IDAC1 R LEAD AIN1 Input RTD COM Mux Main ADC R LEAD Level Shift Int Ref Reference Mux Advantages: + Simplest implementation + Only one current source required IDAC2 Disadvantages/Error Sources: - Drift matching IDAC1 and int. Ref - 2x R LEAD
Ratiometric 2-Wire Operation IDAC1 R LEAD AIN1 Input RTD COM Mux Main ADC R LEAD Int Ref Reference Mux Advantages: + No drift mismatch between references + Drift and noise of reference cancel out REFP Disadvantages/Error Sources: - 2x R LEAD - Precision R REF required R REF REFN
Ratiometric 2-Wire Operation PT100 R= 100Ω @ 0ºC R= 139Ω @ 100ºC + 100mV - Lines 1 & 2: model 250 ft. of 16 AWG copper wire - 1mV + 1 Ω Line 1 Line 2 1 Ω 1mA 1mA Printed circuit board + 102mV - ADS1248 Current DAC PGA Third order ΔΣ modulator + 1mV - Input MUX V in 102mV 1mA REFP1 RBIAS 1 KΩ REFN1 AVSS V RT D 100mV + 1V - V in_span ( 139 100 ) 1mA 0.039V V in V RT D Error 100 5.128 % V in_span
Ratiometric 3-Wire Operation AIN1 = IDAC1 (R LEAD + RTD + R LEAD + R REF ) COM = IDAC2 (R LEAD + R LEAD + R REF ) AIN1 COM = IDAC1 RTD R LEAD AIN1 IDAC1 IDAC2 Input RTD COM Mux Main ADC R LEAD Int Ref Reference Mux Advantages: + R LEAD compensation Disadvantages/Error Sources: - Drift matching IDAC1 and IDAC2 - Precision R REF required - R LEAD need to match R LEAD R REF REFP REFN
Ratiometric 4-Wire Operation IDAC1 R LEAD AIN2 AIN1 Input Mux RTD R LEAD R LEAD COM Main ADC R LEAD Int Ref Reference Mux Advantages: + Most accurate implementation + R LEAD compensation + R LEAD do not need to match + No drift mismatch between references REFP R REF REFN Disadvantages/Error Sources: - Precision R REF required
ADS1248 A complete temperature sensing solution
Why use a ΔΣ- instead of SAR-Converter 50/60Hz line suppression Better noise performance for DC applications High resolution (up to 31Bit) No active anti-aliasing filter required Good for slow signals Lower cost Lower power Small size Integration: PGA Current sources Sensor burn out detection Temp sensor
ADS1247, ADS1248 24-Bit, Complete Temperature Measurement ADC Device Features: 2/4 Differential or 3/7 Single-Ended True Bipolar ±2.5V or Unipolar 5V Max Data Rate 2kSPS Low Noise PGA: 40nV @ G = 128 50/60Hz Simultaneous Rejection Mode (20SPS) On-Chip Integration: Low Drift Internal Reference (10 ppm/ Max) Dual Matched Current DACs (50 1500 μa) Oscillator, Temp Sensor, Burnout Detect 4/8 GPIO s Ultimate Temperature Sensor Measurement Solution Most Flexible Front End for a Wide Range of Industrial Sensors High Integration Without Compromising Performance Scalable Solutions Temperature Management RTDs, Thermocouples, Thermistors Flow/Pressure Measurement Industrial Process Control EVM ADS1248EVM
Ratiometric 3-Wire RTD measurement Using ADS1248
Ratiometric 3-Wire RTD measurement Hardware Compensation RCOMP is chosen such that it is equal to the RTD resistance at the middle of the temperature measurement range Smaller input voltage range Higher PGA Gain can be used to make use of entire full sale range Higher effective resolution
Ratiometric 3-Wire RTD measurement Choosing RCOMP to achieve fully differential input PT100 R= 100Ω @ 0ºC R=120Ω @ 51ºC R= 139Ω @ 100ºC + 120mV - 1mA Lines 1, 2 & 3 model 250 ft. of 16 AWG copper wire - 1mV + 1 Ω Line 1 Line 2 1 Ω - 1mV + 1mA 1mA RCOMP 120Ω - 120mV + Printed circuit board + 0mV - ADS1248 Input MUX Current DAC PGA Current DAC Third order ΔΣ modulator 2mA Line 3 1 Ω 2mA 2mA REFP1 RBIAS 650Ω REFN1 AVSS + 2mV - + 1.3V -
Eliminating IDAC mismatches Sample 1 AVDD IEXC1 IEXC2 REFP1 REFN1 VREFOUT VREFCOM DVDD RTD AVDD Vref Mux Internal Reference GPIO Bias Generator System Monitor PGA 24 bit 24/16 bit ΔΣ ΔΣ Modulator Modulator 1 st order Oscillator Interface and Digital Filter Self Calibration CSn DOUT/DRDYn DIN SCLK DRDYn START RESETn AVSS CLK DGND ADS1248 allows any configuration of IDAC on any channel IDACs can be used to implement a correlated double sampling of the inputs Kind of chopping
Eliminating IDAC mismatches Sample 2 AVDD IEXC1 IEXC2 REFP1 REFN1 VREFOUT VREFCOM DVDD RTD AVDD Vref Mux Internal Reference GPIO Bias Generator System Monitor PGA 24 bit 24/16 bit ΔΣ ΔΣ Modulator Modulator 1 st order Oscillator Interface and Digital Filter Self Calibration CSn DOUT/DRDYn DIN SCLK DRDYn START RESETn AVSS CLK DGND 0.5 x (Sample 1 + Sample 2) mismatch-corrected result
Thermocouple Measurement
A common and important use of thermocouples by engineers!
Thermocouple configuration 3-wire RTD used for CJC K-type Ref. Junct. @ 0ºC ΔV= 0mV @ 0ºC ΔV= 4.096mV @ 100ºC Line 1 Printed circuit board 1kΩ 1μF ADS1248 VBIAS Line 2 15.9 Hz 10μF 1kΩ 1μF PGA Third order ΔΣ modulator PT100 Line 3 Line 4 Input MUX Current DACs Line 5 REFP1 RBIAS 500Ω REFN1 AVSS Isothermal block + 1V -
ADS1118 Low Power Solution for Thermocouple Measurements with CJC
ADS1118 World s Smallest 16-bit ADC, 0.5 C (max) Accurate Temp Sensor Complete set of integrated functions: Four multiplexed analog inputs PGA (Gain: x0.33, x0.5, x1, x2, x4 or x8) Precision ADC with data rates from 8 to 860 SPS Internal temperature sensor (0.5 C max) Serial SPI Interface Small and versatile Low supply current: 150uA typ QFN 2.05mm x 1.55mm x 0.4mm Supply 2.0V 5.5V Temperature Measurement Battery Pack Portable Instrumentation Industrial Process Control Gas Monitoring Embedded ADC Upgrade ADS1118 EVM A single ADS1118 can perform data acquisition of multiple signals from a wide variety of sensors. Small package that readily senses ambient temperature to perform cold junction compensation in thermocouple applications. Its size and low power consumption makes the ADS1118 a great device for portable applications where extended battery life is critical AINP0 AINP1 AINP2 AINP3 M U X Gains: 2/3,1,2,4,8,16 PGA Internal Reference 16-bit ADC Oscillator SPI Interface Temp Sensor Tiny QFN(RUG) or MSOP(DGS) Package CSn DIN DOUT/DRDYn SCLK VDD GND
Temperature Sensing IC Solutions TMP102 TMP112 ADS1018 ADS1118 Resolution 0.0625 0.0625 0.125 0.03125 Max Error 2 C 0.5 C 1 C 0.5 C Speed 40SPS 40SPS 3300SPS 860SPS Size SOT-DRL 1.6x1.6x0.5 SOT-DRL 1.6x1.6x0.5 QFN-RUG 2x1.5x0.4mm QFN-RUG 2x1.5x0.4mm Power 23uW 23uW 500uW 500uW uw/conv 0.575uW/Conv 0.575uW/Conv 0.15uW/Conv 0.581uW/Conv Interface I2C I2C SPI SPI 1k Price $0.75 $0.85 $0.99 $2.25
ADS1118 Application Example with Thermocouples Cold (Reference) Junction Isothermal Block Temp Sensor for CJC - ADS1118 ideal low power solution for Thermocouples with reference junction compensation - Internal Temperature Sensor absolute accuracy is 0.5 C at 25 C - Programmable Gain Amplifier (PGA) improves resolution
ADS1118 Thermocouple measurement Input Impedance - Low Pass filter considerations Consider device input impedance effect in measurement accuracy Keep resistor values of RC low pass filters smaller than <5kΩ for direct thermistor measurements to prevent Linearity/Gain Errors
ADS1118 Thermocouple measurement Low Pass filter considerations Common Mode Noise Filter BAD * Mismatch in C cm ( C) will cause asymmetric e n attenuation; amplify at V in Diff + Common Mode Noise Filter GOOD * As long as C diff 10 C cm, mismatch in C cm ( C) will be attenuated to insignificant levels
ADS1118 Thermocouple measurement Estimation of Accuracy/Resolution Accuracy/Resolution of ADC Accuracy of Thermocouple Accuracy of Reference Junction Temperature Sensor
ADS1118 Thermocouple measurement Estimation of Accuracy Example using K-type TC at 150 C Using FS ±0.256V; LSB = (0.256V*2) / 2 16 = 7.81uV Thermocouple Voltage at 150 C = 6.138mV Estimation of ADC voltage conversion absolute accuracy at 150 C: Total Error = [(offset) 2 + (gain error) 2 + (INL) 2 + (Noise) 2 ] 1/2 Offset Error = ±3 LSB = ±23.43uV Gain Error = ±0.03%*6.138mV = ±1.84uV INL Error = ±1 LSB = ±7.81uV Noise @128SPS = 7.81uVp-p Gain Error due to RC Resistors = ±0.0037% * 6.138mV = ±0.227uV This yields an accuracy error = ±25.97uV or ±(25.97uV / 40uV/ C) = ±0.649 C Total Accuracy Error= [(IC Temp Sensor) 2 + (ADC Error) 2 +(Thermocouple Error) 2 ] 1/2 Total Accuracy Error= [(0.5 C) 2 + (0.649 C) 2 +(2.2 C) 2 ] 1/2 Total Accuracy Error = ±2.34878 C @ 150 C
ADS1118 Thermocouple measurement Estimation of Resolution ( C/bit) Example using K-type TC Using FS ±0.256V Calculation of ADC Resolution: Noise = ~8uVp-p @ ±0.256 FS, 128SPS SNR (db) = 20*log 10 (Full-Scale / Noise) SNR (db) = 20*log 10 ((0.256*2 V) / 8uV) = 96.12dB SNR (db) = 6.02 N + 1.76 where N = Noise Free Bits Noise Free Bits N = (96.12 1.76)/6.02 = 15.67 Resolution (Volts) = 0.256 V *2 / 2 15.67 = 24.35uV Assuming Seebeck Coefficient of 40uV/ C for K-type Resolution = 24.35uV / 40 uv/ C = 0.244 C/bit (Noise Free)
ADS1118 Thermocouple measurement Experiment using Thermal Bath Experimental Result: Max Error = 0.173 C Min Error = -1.251 C Calculated absolute Accuracy: ±2.347 C ±2.2 C error due to Thermocouple error
ADS1115 and ADS1118 Power Savings Using Single-Shot Mode Using single-shot mode results in great power savings on applications that require only periodic conversions. Device automatically enters a low power shut down after a conversion. Continuous Mode Single Shot Mode
ADS1115 and ADS1118 Power Savings Using Single-Shot Mode Using single-shot mode results in great power savings on applications that require only periodic conversions. Device automatically enters a low power shut down after a conversion. Sampling Rate Mode Duty Cycle Power Consumption Total Samples per Second 8 SPS Continuous Mode 100 % 300uW @ 2V Supply 8 32 SPS Single-Shot Mode 25 % 75uW @ 2V Supply 8 860 SPS Single-Shot Mode 1 % ~3uW @ 2V Supply ~8 128 SPS Continuous Mode 100 % 300uW @ 2V Supply 128 860 SPS Single-Shot Mode 15 % 46uW @ 2V Supply 128
ADS1118 Evaluationboard
ADS1118 EVM Demo Thermocouple Temperature Measurement Detected Cold Junction Temperature with On-Board Temp Sensor K-Type Temperature Measurement without Junction Compensation K-Type Temperature Measurement with Reference Junction Compensation K-Type Temperature Measurement Constant at 25 C
Links Product Folders: ADS1248 http://www.ti.com/product/ads1248 ADS1118 http://www.ti.com/product/ads1118 Order samples and EVM s: http://focus.ti.com/general/docs/buy.tsp?dcmp=tihead ertracking&hqs=other+ot+hdr_b_buy