RTD and Thermocouple Temperature Sensing using Delta...

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

Thermocouple

Thermistor

RTD Resistance

Temperature

Device

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

• 3rd order Steinhart-Hart equation usually used for more accurate approximation

Low cost

Rugged construction

Available in wide range of resistances

Available with either negative (NTC) or

positive (PTC) temperature coefficients

High sensitivity (100Ω/°C)

Advantages

Limited temperature range: -100°C to +150°C

Highly non-linear response

Excitation required

Linearization nearly always required

Least accurate

Self-heating

Disadvantages

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

High accuracy: < ±1°C

Best stability over time

Temperature range: -200°C to +500°C

Good linearity

Advantages

Expensive

Excitation required

Self heating

Lead resistance

Slow

Disadvantages

RTD/Thermistor Sensor Excitation

Current Source

Voltage Reference

REF200

Bridge Configuration

Shunt

REF5050 VDD

VShunt=IEX x RShunt RShunt

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

Wide temperature range (-200oC to +2000

oC)

Only contact temperature measurement

device for T >600oC

Self-powered

Very fast response, fractions of sec

Simple and durable in construction

Inexpensive

Wide variety of physical forms

Advantages

Thermocouple voltage non-linear with

temperature

Low measurement voltages (mV)

Lowest accuracy: ±2°C

Least stable and sensitive

Requires a known junction temperature

Disadvantages

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; non-

magnetic 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


Thermocouple Cold Junction Compensation (CJC)

Thermocouple Temperature to Voltage Conversion


Thermocouple - CJC Software Compensation

Software Compensation

1. Measure RT to find TREF and convert TREF to its

equivalent reference junction voltage VREF

2. Measure V and add VREF to find V1 and convert

V1 to temperature TJ1 Source: Agilent

Cu

C

Thermocouples - CJC Example

• V = V1 - VREF

• Measure TREF using Temp Sensor:

TREF = 26°C

• Convert TREF to equivalent junction voltage

using look up table:

VREF = 1.329mV @ 26°C

• Measure V = 3.961mV with external meter

• Solve for V1:

V1 = V + VREF

V1= 3.961mV + 1.329mV = 5.269mV

• Convert V1 to TJ1 from table:

5.269mV corresponds to

TJ1 = 100°C

Summary

Source: Agilent

RTD Measurement

How to cope with lead resistance RLEAD

– 2-, 3-, 4-Wire RTD measurement

Absolute vs. ratiometric RTD measurement

Absolute 2-Wire Operation Using internal Reference

Main ADC

IDAC1

RTD

Int Ref Reference Mux

Input

Mux

RLEAD

RLEAD

AIN1

COM

IDAC2

Level Shift

Advantages:

+ Simplest implementation

+ Only one current source required

Disadvantages/Error Sources:

- Drift matching IDAC1 and int. Ref

- 2x RLEAD

Ratiometric 2-Wire Operation

Main ADC

IDAC1

RTD

RREF


Input

Mux

RLEAD

RLEAD

REFP

REFN

AIN1

COM

Advantages:

+ No drift mismatch between references

+ Drift and noise of reference cancel out


- 2x RLEAD

- Precision RREF required


Third order

ΔΣ modulatorPGA

- 1mV +

1 Ω

Line 1

1mA

+ 1mV -

+102mV

- Line 2

1 Ω

PT100R= 100Ω @ 0ºC

R= 139Ω @ 100ºC

+100mV

-

Input

MUX

ADS1248

1mA

Current

DAC

1mA

+ 1V -

REFP1 REFN1AVSS

Printed circuit

boardLines 1 & 2: model

250 ft. of 16 AWG

copper wire

RBIAS

1 KΩ

Vin 102mV

VRT D 100mV

Vin_span 139 100( ) 1 mA 0.039V

ErrorVin VRT D

Vin_span

100 5.128 %


Main ADC

IDAC1 IDAC2

RTD

RREF


Input

Mux

RLEAD

RLEAD

RLEAD

REFP

REFN

AIN1

COM

Advantages:

+ RLEAD compensation


- Drift matching IDAC1 and IDAC2


- RLEAD need to match

AIN1 = IDAC1 (RLEAD + RTD + RLEAD + RREF)

COM = IDAC2 (RLEAD + RLEAD + RREF)

AIN1 – COM = IDAC1 ∙ RTD


Main ADC

IDAC1

RTD

RREF


Input

Mux

RLEAD

RLEAD

RLEAD

RLEAD

REFP

REFN

AIN2

AIN1

COM

Advantages:

+ Most accurate implementation

+ RLEAD compensation

+ RLEAD do not need to match

+ No drift mismatch between references



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

EVM

• Ultimate Temperature Sensor Measurement Solution

• Most Flexible Front End for a Wide Range of Industrial Sensors

• High Integration Without Compromising Performance

• Scalable Solutions

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

• Temperature Management

• RTDs, Thermocouples, Thermistors

• Flow/Pressure Measurement

• Industrial Process Control

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

Third order

ΔΣ modulatorPGA

- 1mV +

1 Ω

Line 1

1mA

+0mV

- Line 2

1 Ω

PT100R= 100Ω @ 0ºC

R=120Ω @ 51ºC

R= 139Ω @ 100ºC

+120mV

-

Input

MUX

ADS1248

1mA

Current

DAC

2mA

+ 1.3V -

REFP1 REFN1AVSS

Printed circuit

boardLines 1, 2 & 3 model

250 ft. of 16 AWG

copper wire

RBIAS

650Ω

Current

DAC

+ 2mV -

Line 3

1 Ω 2mA

2mA

- 1mV +

1mA

- 120mV +

RCOMP

120Ω

Eliminating IDAC mismatches Sample 1

Internal

Reference

Oscillator

Interface

and

Digital Filter

GPIO

CSn

DIN

SCLK

DRDYn

CLK DGND

AVDD VREFOUT VREFCOM

24 bit

ΔΣ

Modulator

1st order

PGA 24/16 bit

ΔΣ

Modulator

Vref Mux

REFP1 REFN1

Bias

Generator

System

Monitor

Self

Calibration

AVSS

DOUT/DRDYn

AVDD

START

RESETn

IEXC1 IEXC2 DVDD

RTD

• 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

Internal

Reference

Oscillator

Interface

and

Digital Filter

GPIO

CSn

DIN

SCLK

DRDYn

CLK DGND

AVDD VREFOUT VREFCOM

24 bit

ΔΣ

Modulator

1st order

24/16 bit

ΔΣ

Modulator

Vref Mux

REFP1 REFN1

Bias

Generator

System

Monitor

Self

Calibration

AVSS

DOUT/DRDYn

AVDD

START

RESETn

IEXC1 IEXC2 DVDD

RTD

PGA

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

Third order

ΔΣ modulatorPGA

Line 3

Line 4PT100

Input

MUX

ADS1248

Current

DACs

+ 1V -

REFP1 REFN1AVSS

Printed circuit

board

RBIAS

500ΩLine 5

VBIASLine 1

Line 2

Isothermal

block

K-typeRef. Junct. @ 0ºC

ΔV= 0mV @ 0ºC

ΔV= 4.096mV @ 100ºC

10μF

1μF

1μF1kΩ

1kΩ

15.9

Hz

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

CSn

DOUT/DRDYn

VDD

GND

SCLK

16-bit

ADC PGA

Oscillator

Internal Reference

M

U

X

Gains:

2/3,1,2,4,8,16

DIN

Temp Sensor

AINP1

AINP0

AINP3

AINP2

SPI

Interface

Tiny QFN(RUG) or MSOP(DGS) Package

• 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

ADS1118 EVM

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

- 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

Temp Sensor for CJC

Cold (Reference) Junction Isothermal Block

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

Common Mode Noise Filter

Diff + Common Mode Noise Filter

BAD * Mismatch in Ccm (∆C) will

cause asymmetric en

attenuation; amplify at Vin

GOOD * As long as Cdiff ≥ 10∙Ccm,

mismatch in Ccm (∆C) will

be attenuated to

insignificant levels

ADS1118 Thermocouple measurement Low Pass filter considerations

ADS1118 Thermocouple measurement Estimation of Accuracy/Resolution

Accuracy of

Thermocouple Accuracy of Reference Junction

Temperature Sensor

Accuracy/Resolution of ADC

ADS1118 Thermocouple measurement Estimation of Accuracy

• Example using K-type TC at 150°C Using FS ±0.256V;

LSB = (0.256V*2) / 216 = 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*log10(Full-Scale / Noise)

– SNR (dB) = 20*log10((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 / 215.67 = 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

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

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.

ADS1118

Evaluationboard

ADS1118 EVM Demo Thermocouple Temperature Measurement

K-Type Temperature Measurement with

Reference Junction Compensation

K-Type Temperature Measurement

without Junction Compensation

Detected Cold Junction Temperature

with On-Board Temp Sensor

K-Type Temperature Measurement Constant at 25°C

Links

• Product Folders:

– ADS1248

http://www.ti.com/product/ads1248

– ADS1118


• Order samples and EVM’s:

– http://focus.ti.com/general/docs/buy.tsp?DCMP=TIHead

erTracking&HQS=Other+OT+hdr_b_buy



http://focus.ti.com/general/docs/buy.tsp?DCMP=TIHeaderTracking&HQS=Other+OT+hdr_b_buy

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RTD and Thermocouple Temperature Sensing using Delta...

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