TMP006 Customer Training

Applications Team

Sensing Products

ThermopileThermocouple

-

+

Heat Absorber Cold Junction

What is a Thermocouple / Thermopile

Thot

Thot

Thot

Thot

TcoldThot

Tcold

+

-

Heat Absorber

Heat Sink

Vout Vout

Vout = G x (Thot – Tcold)

G: Seebeck Constant

Vout = n x G x (Thot – Tcold)

G: Seebeck Constant

Heat Transfer (1): Conduction

• Conduction: Heat is transferred through direct touch.– Example: Touching a hot cup of coffee

Heat Transfer (2): Convection

• Convection: Heat is transferred by a moving liquid or gas.– Example: Fan cooling

Heat Transfer (3): Radiation

• Radiation: Heat is transferred by waves (photons)– Example: The warmth you feel from the sun– The amount of energy radiated is dependent on the absolute (Kelvin)

temperature of the object.

Device Principle of Operation

Object

PCB

TMP006

Infrared Radiation

TMP006 at its core is a heat sensor. It converts the heat being transferred to a voltage.

PCB Recommended Layout

• The sensor at it’s core is a heat sensor and is sensitive to conduction and convection in addition to radiation.

• Recommended PCB layout isolates the sensor from the rest of the PCB

Via Connecting to Circuit

Copper Dot

Thermal Break

GND

TMP006 Conduction & Convection Offsets

• The sensor, is a heat sensor at its core. It will detect heat from all three sources: conduction, convection, and radiation.

• The signal created by heat conduction and convection is relatively small in a setup with the recommended PCB layout but it is not zero.

• Calibration is needed to cancel out remaining offsets

Case

Field of View Consideration (1)

• For best performance it is recommended that the diameter of object is 4 times the distance to the TMP006 sensor.

• d = h – 250µm

• object size ≥ 4×d

PCB

TMP006

distance = d

object size ≥ 4d

h

Field of View Consideration (2)

• For best performance it is recommended that the diameter of object / opening is 4 times the distance to the TMP006 sensor.

• d = h – 250µm

• Opening size ≥ 4×d

PCB

TMP006

distance = d

opening size ≥ 4d

h

Case/Lens

heat sink

Emissivity Considerations

• Not all objects emit the same amount of energy at the same temperature.

• Emissivity is the ratio of energy radiated by an emitter to the theoretical ideal emitter. Emissivity is always less than one.

• Since the emissivity may change for different systems, then the system gain will need to be calibrated.

TMP006 Equation

2210 1 refdierefdie TTaTTaSS

2210 refdierefdieos TTbTTbbV

22 osobjosobjobj VVcVVVf

44 )(

S

VfTT obj

dieobj

System independent: No need to calibrate

Models voltage offset due to conduction & convection. System dependent & needs calibration

Needs calibration. Depends on field of view and emissivity of object

System independent: No need to calibrate

Transient Correction Basics

• Rth Thermal resistance of thermopile

• Cth Thermal capacitance of thermopile

Thermopile

RthCth

Thot Tcold

• The hot junction is isolated from the rest of the sensor by the sensor’s thermal

resistivity

• When the sensor’s cold junction temperature is changed through heat conduction

from the outside, the hot junction follows, but is delayed by a time constant

• This effect can be modeled with a first order RC filter that delays the hot junction

relative to the sensor substrate temperature

Thot

Tcold

+

-

Heat Absorber

Heat Sink

Vout

Transient Error Example

Thot

Tcold

Thot

Tcold

time time

Tem

pera

ture

Tem

pera

ture

Transient Correction Basics (2)

/ C) (V.CRSeebeckα

t

TV

CRSeebeckt

TV

CRt

TSeebeckV

-thth

ColdErr

ththCold

Err

ththCold

Err

sec10962 4

Alpha is a constant which corresponds to the multiplication of Seebeck constant and the thermal time constant

A temperature gradient is created on the thermopile during transients that is proportional the slope of the transient itself.This gradient generates a voltage error through the Seebeck effect.

Effect of Transient Correction

Wit

h t

ran

sien

t c

orr

ecti

on

Wit

ho

ut

tran

sien

t co

rrec

tio

n

Coefficient Calibration

• Coefficients that get calibrated are: S0, b0, b1 & b2

• Before calibrating the coefficients, transient correction should be applied to the sensor’s output voltage to cancel out the transient effects.

• At this point in time it is recommended that the customer send us the data of their system and we will provided a custom set of “b” coefficients for optimum accuracy.

• An automatic calibration software is being developed at this point and should be available for customer use in the near future

Setting Up The System For Calibration

• For calibration: the data from the reference: Tobj as well as the TMP006 data: Tdie & Vout need to be collected simultaneously.

Object

PCB

TMP006

Reference Temperature Sensor (Example: TMP112)

Glued to object using thermal epoxy

Calibrating S0

221

41

42

120

2210

121241

42

21

21

21

2

2242

42

1

1141

41

2210

2210

1

1

if

1

REFdieREFdieobjobj

objobj

REFdieREFdie

objobjobjobjobjobj

ososos

diedie

osobjdieobj

osobjdieobj

REFdieREFdieos

REFdieREFdie

TTaTTaTT

VVS

TTaTTaS

VV

S

VVTT

SSS

VVV

TT

S

VVTT

S

VVTT

TTbTTbbV

TTaTTaSS

To be able to accurately calibrate S0 you need at least two points with the same Tdie values but different Tobj values.More than one measurement is recommended to reduce the effect of noise

Calibrating “b” Coefficients

• The residual offset based on the calibrated S0 value is calculated according to the equation above.

• The calculated offset is drawn versus (Tdie – Tref) and a second order curve fit for the data is done to calculate the b0, b1 and b2 coefficients.

44

44

dieobjobjos

osobjdieobj

TTSVV

S

VVTT

2210 REFdieREFdieos TTbTTbbV

2nd order curve fit(use excel)Coefficients of curve fit are b0, b1 and b2

Vos

= V

obj –

S(T

obj4

– T

die4 )

Tdie – Tref

Importance of the Number and Range of Points

• The larger the number of points the lower the effect of noise.

• The further apart the points are the better the correction.

Actual device behavior

Calibrated curve based on only middle three points

Points collected don’t exactly match the actual curve due to noise

x-axis

y-ax

is

Error at the extreme points

Default Versus Calibrated CoefficientsD

efau

lt c

oef

fici

ents

Cal

ibra

ted

co

effi

cien

ts