Ipyroelectric Detectors - InfraTec GmbH

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Infrarotsensorik und Messtechnik

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Pyroelectric detectors Introduction

The radiation sensitive key components of InfraTec‘s detectors are single crystalline lithium tantalate

(LiTaO3) elements formed as a very thin plate capacitor. Lithium tantalate is a pyroelectric crystal whose

ends become oppositely charged when heated. Although this unique effect was already known about in the

ancient world and was given the name pyroelectric in 1824 by Brewster, even though the broad application in

infrared detectors was introduced in the early 1970s. Nowadays due to its simple but robust construction and

its perform ance the pyroelectric detect or is one of the most widely-used thermal infrared detectors.

In fig. 1the individual stages of the transformation from infrared radiation to an electrical signal is

represented. Via a window or IR filter with a transmission rate of ττFthe radiation arrives at the pyroelectric

element. The radiation flux ΦΦSis absorbed and causes a change in temperature ∆∆TPin the pyroelectric

element. The thermal to electrical conversion is due to the pyroelectric effect by which the temperature

change ∆∆TPalters the charge density on the electrodes. An electrical conversion often follows in which, for

example, an electrical signal is created by a preamplifier or impedance converter.

Fig. 1: Conversion stages of the pyroelectric infrared detectors

∆∆TP∆∆QP∆∆uS

thermal to electrical

conversion electrical

conversion

thermal

conversion

ττF∆Φ∆ΦS

1. Thermal conversion

Within this chain thermal conversion is the basis for a high res ponsivity and a high signal to noise rat io,

through which a high temperature change ∆∆TPis the object ive.

Fig. 2 represents a simplified therm al model and in fig. 3 the equivalent electrical circuit is depicted. The

radiation sensitiv e element is charact erised by the absorption r ate αα, the heat capacity HPand the ther mal

conductance GTto its surroundings which is represented by a heat sink with a given temperature TA.

ASHP

∆TP

τF∆ΦS

heat sink

Fig. 2: Simplified thermal model Fig. 3: Equivalent electrical circuit

τFα∆ΦS

1/jωHP

1/GT

∆TP

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Pyroelectric detectors Introduction

22 PT

PHG

Tω

ατ+Φ

=∆

( )

Tωτ

ατ +

⋅

=∆

=τ

the temperature diff erence results in

or for sinusoidal agitation in the steady state

Using the therm al time constant (1)

(2)

(3)

For significant temperature differences to occur the product ατατFhas to be as near to 100% as possible. This

can especially be achieved by the use of an absorption layer. The heat capacity value HPhas to be low. For

this the thickness tPof the radiation-sensitive element has to be very low. Compromises are necessary as

the required reduction in the thermal conductance GTis opposed by the increase of the thermal time

constant.

2. Thermal to electrical conversion

The thermal to electrical conversion is due to the pyroelectric effect and is proportional to the temperature

rate and the detector ‘s surface area:

pAi P

SP ∆

=(4)

( )

SP G

pAi ωτ

ατ

ϖ+

⋅

=(5)

The frequency dependence on the temperature change portrays the typical low pass characteristics. The

corner frequency fTresults fr om the therm al time constant according to equation (6)

fπτ

=(6)

and has the value of 1Hz. Below the corner frequency the temperature change attains a saturation value of

513µK. Above the corner frequency, the pyroelectric current, however, attains a saturation value of

approximately 2.2pA.

For sinusoidal agitation and considering equation (3) the result for the rms value of the pyroelectric short

circuit current iPis as follows:

Fig. 4 represents the frequency dependence on the temperature change and the short circuit current of a

typical pyroelectr ic detector at an incident radiation flux of 1µW.

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100

1000

0,001 0,01 0,1 110 100 1000

Fre que ncy [Hz]

TP [µK]

100

1000

10000

iP [fA]

temperature change

pyroelectric current

Pyroelectric detectors Introduction

Fig. 4: Frequency dependence on tem peratur e change and short circuit current of a pyroelectric element

3. Electrical conversion

3.1. Responsivity

This extremely low current, supplied by a high-impedance source has to be converted by a preamplifier with

a high-impedance input. There are two alternatives available: voltage mode and current mode. The voltage

mode can be implemented using a voltage follower and the current mode using an inverting operational

amplifier as seen in fig. 5.

Voltage Mode

1/jωCPRGuS

Current Mode

1/jωCPuS

1/jωCfb

Rfb

Fig. 5: Alternative preamplifier circuits

τF=α=1

ΦS=1 µW

GT=1.95 mW/K

HP=310µWs/K

τT=159 ms

tP=25 µm

AS=4 mm²

cP=3.1 J/cm³/K

p=17 nC/cm²/K

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0,01

0,1

100

0,001 0,01 0,1 110 100 1000

Fr e quenc y [Hz]

uS [mV]

100

1000

10000

100000

RV [V/W]

v oltage mode c urr ent mode

(

ωτ

E CM

) ² = 1

(

ωτ

) ² = 1

(

ωτ

E VM

)² = 1

Pyroelectric detectors Introduction

The signal voltage and the responsivity for both modes can be defined respectively using the same equation:

( )

[ ]

( )

[ ]

2/1

SSFS R

pAu ϖτϖτ

ϖατ ++

Φ=

( )

[ ]

( )

[ ]

2/1

Rϖτϖτ

ϖατ ++

PGE

GCR

RR =

fbfbE

where

(7)

(8)

(9)

(10)

is valid for the voltage mode

High megohm resistors may be necessary for both current and voltage mode to achieve a high signal voltage

and responsivity but the feedback capacitance Cfb is kept considerably lower than the capacitance of the

pyroelectric chip CP. Therefore the electrical time constant ττEis considerably lower for the current mode and

the signal voltage above the electric al corner frequency is considerably higher than for the voltage mode. Fig.

6 illustrates the frequency dependence of both modes for typical detectors based on the results represented

in fig. 4.

for the current mode.

Fig. 6: Comparison of the fr equency dependencies of signal voltage/ responsivit y for voltage and curr ent mode

and

ΦS=1 µW

CP=62 pF

Cfb=0.68 pF

RG=Rfb=24 G Ω

τE VM =1.5 s

τE CM =16 ms

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Pyroelectric detectors Introduction

3.2. Specific Detectivity

The noise sources of the pyroelectric chip and the preamplifier limit the detectable radiation flux or the signal

to noise ratio. As a measure of the signal to noise ratio the specific detectivity is frequently used with the

infrared detectors:

uSA

2/1

*=(11)

where is the effective noise value which is related to a noise band width of 1Hz at the preamplifier output

(voltage noise density). In table 1 the individual noise sources as well as the appendant specific detectivity

components are summarised. The reciprocal quadratic superposition of the components results in the

specific detectivity D*. The frequency dependence and the resulting specific detectivity for a typical

pyroelectric detector, with the already utilised parameters, is portrayed in fig. 7. It is irrelevant whether the

detector is being used in the current or voltage mode as the sam e noise sources are operating in both cases.

Table 1: noise sources and the appendant specif ic detectivit y

In an ideal pyroelectric detector the heat exchange due to radiation between the pyroelectric chip and its

surroundings acts as the only unavoidable noise source. This so-called temperature noise determines the

component D*Trespectively the theoretically highest possible specific detectivity of a pyroelectric

detector operated at room temperature:

WHzcmD

max 108.1 ⋅=

∗(12)

In a typical pyroelectric detector the other noise sources are considerably higher. Whilst the components D*I

and D*Rare dominant at low frequencies, the specific detectivity is considerably influenced by D*Uat high

frequencies. At a medial fr equency range D*Dis dominant.

noise sources components of the specific detectivi ty D*

temperature noise

2/1

*4













FT GkT

Dατ

nyquist noise of the megohm

resistor

( )

[ ]

2/1

FR t

Dϖτ

ωτ

ατ +













′

tanδnoise of the pyroele ctric

element

( ) ( )

[ ]

2/1

tan T

PPP

FD tkT

Dϖτ

ωτ

ϖε

δε

ατ +













′

current noise of the preamplifier

( )

[ ]

2/1

FI t

Dϖτ

ωτ

ατ +

′

voltage noise of the preamplifier

( )

[ ]

( )

[ ]

2/1

FU t

Dϖτ

ωτ

ατ +

′′

′

fbIP

RRRR

CCCC

modecurrent

////

′++=

′

GIP

IP RRRR

CCC

modevoltage

////

′+=

′

In an ideal pyroelectric detector the heat exchange due to radiation between the pyroelectric chip and its

surroundings acts as the only unavoidable noise source. This so-called temperature noise determines the

component D*Trespectively the theoretically highest possible specific detectivity of a pyroelectric

detector operated at room temperature:

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1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

1.0E+11

0.001 0.01 0.1 110 100 1000

Frequency [Hz]

D* [c mHz

1/2

/W]

D*T

D*U

D*D

D*I

D*R

Reihe1

Pyroelectric detectors Introduction

Fig. 7: Frequency response of the various components and the res ulting specif ic detectivity of a typical

pyroelectric detector

4. Voltage mode

4.1. General information

Due to its simplicity the voltage mode is the most commonly used operating mode for pyroelectric detectors.

The following restrictions have to be considered regarding the layout of the amplifier and signal conditioning

unit:

ØThe signal voltage of a pyroelectric voltage mode detector usually includes very low-frequency parts

(mHz) caused by 1/f characteristics.

The cut-on fr equency of the amplifier ‘s high pass should not be too low.

ØThe gate resist or (load resistor) should have a resistance of at least 10 GΩfor high performance.

The best solution for the protection of high impedance components against humidity, which would

cause current leakage, is the integration of these inside tr ansistor style housing. Pyroelectric detectors

should not be used without integrated impedance preamplifiers in high performance applications.

ØThe output signal of volt age mode detectors corresponds to the time-integral of the IR radiation.

This behaviour suppresses fluctuations effectively. Sinusoidal signals, however, are phase-shifted by

90°by this electrical lowpass filt er (f>fT).

input-referred current noise: in=0.5 fA/Hz1/2

input-referred voltage noise: en=6 nV/Hz1/2

input preamplifier capacitance: Ci=2 pF

input preamplifier resistance: Ri=10TΩ

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Pyroelectric detectors Introduction

4.2. Circuit diagram

In the simplest case the preamplifier is formed as a JFET source follower. The gate resistor and the JFET

are integrated into the detector housing. The resis tor in the source line is placed outside the detector housing

(see fig. 8). The high signal to noise ratio and the low temperature dependence, as well as the simplicity of

the circuitry, are the reason for this widespread use.

1≤

≈Sfs

Sfs Rg

v(13)

The gain for these circuits results from the transconductance of the JFET in the operating point and the

source resistance:

Fig. 8: Basic circuits for the voltage mode

The demand for a high source resist ance or a small drain current can be deduced fr om this. See below for an

example equation for a gain of at least 0.8 (IDSS = saturation drain current):

1.0

≤

DSS

II(14)

However the demand for a low output resist ance limits the increase of the source resistanc e necessary for a

gain near to the value of 1. The source resistance should not be over 100kΩat drain voltages up to 15 volt s.

A constant current source can be used as an alternative as this possesses a very high inner resistance. Next

to a gain value of approximately 1 the temperature dependence of the transconductance is simultaneously

suppressed and therefore the temperature stability of the gain is improved. See fig. 9 for suggestions

concerning the operation of the source follower.

The JFET used by InfraTec represents a IDSS with a characteristic value of 1mA. The recommended drain

current values for the operation of the detectors are between 10 and 100µA.

LIM-314LIM-114

Output

+5V LIM-214

uncompensated serial compensatedparallel compensated

Output

+5V

Output

+5V

RSRSRS

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resistor to negative

supply

47k

+15V

resistor to

ground

Output

470k

30k

10k

BC849C

20µA current

source

with green

LED

-3 ... -15V

20µA current

source

with

n-channel

JFET

-3 ... -15V

47k 2N4117

Pyroelectric detectors Introduction

Fig. 9: Voltage mode detectors alternative circuitry for low-noise drain current supply

Remarks

•higher dynam ic inner resistance

•greater tuning range than in (1),

symmetrical tuning possible

•negative supply voltage

•lo wer temperature drift

•2 active and 2 passive components

•very high dynamic inner resistance

•very low noise

•greater tuning range than in (1),

symmetrical tuning possible

•negative supply voltage

•greater temperature drift than in (3)

•lo w current and and gain

dependency as in (1)

•a greater tuning range than in (1),

symmetrical tuning possible

•negative supply voltage

•the simplest circuit

•high drain current and gain

dependency on operat ing point

•low dynamic

•asymmetrical tuning

For the design of the drain current supply circuitry please note the following:

•The noise optimum for the JFET used in the InfraTec detect or lies at 20µA.

•Pyroelectric detectors generate a DC offset in temperature ramps, which defines the large signal

behaviour and can lead to significant changes in the gain of the source follower. Uncompensated

standard detectors portray a positive offset shift. In comparison compensated detectors approximately

portray a ten-fold lower shift, which, dependent on the symmetry between the active and the

compensat ing element, can be positiv e or negative.

•This occurr ing effect, taking place exclusively in the temperature ramps, can be minim ised at the expense

of a higher noise, using a lower electrical time constant (available for all types on demand).

•The integrated current sources available, for example the LM 134 from NSC, worsen the signal to noise

ratio or are expensive (REF200 from Burr-Brown / TI).

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OPA 2604 - dual OpAmp TI (Burr-Brown)

1µF to signal

conditioning unit

(channel 2)

+15V

LIM-222

47k

-15V

10µF

100k

47k

OPA 2604

2N4117

4M7

to signal

conditioning unit

(channel 1)

OPA 2604

2µ2 47k

1µF

47p

-15V

100nF

(1)

(2)

4M7

47p

47k

2µ2

Pyroelectric detectors Introduction

Fig. 10: Circuitry for alow voltage supply preamplifier for the NDIR gas analysing sensor (3,3V Lithium

battery supply; 2 -15Hz; gain of 60) using TO18 detectors with electrically isolated housing

Fig. 11: Circuitry for ahigh precision preamplifier for the NDIR gas analysing sensor (±15V bipolar supply; 5

-200Hz; gain of 100) using dual colour detectors

4.3. Wiring suggestions

The electronic components shown in figures 10 and 11 which are connected to the pyroelectric detector

considerably determine noise and large-signal response. However, low cost OpAmps can be used due to the

high signal level of pyroelectric detectors in comparison to thermopiles. The best results are achieved using

low-noise amplifiers , which have been developed for high quality audio applications.

180k

10µF

33k

1µF

LMV751- National Semiconductor

LMV751

75k

470nF

LMV751

4,7M

2,2nF

100nF

to signal

conditioning

unit

1µF

+3,3V

100nF

180k

LIE-216

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Pyroelectric detectors Introduction

5. Current Mode

5.1. General information

Current mode pyroelectric detectors are not as widely available as voltage mode ones, the most probable

reason for this being that elementary pyroelectric detectors are mass produced for light switches and motion

detectors. Due to the complexity of the preamplifier circuit its use was limited to very few applications. At

InfraTec we can supply a wide spectrum of current mode detectors which makes it less complicated to

include detectors for gas and fir e detection.

5.2. Circuit diagram

Øtransistor style housing containing only the

pyroelectric element (thermal compen-

sation element also possible)

ØTO housing containing the pyroelectric

element (also with thermal compensation

element), JFET and feedback resistor (an

additional feedback capacitor of several

picofarad is also possible). The integrated

feedback–capacitor prevents so-called

gain peaking.

Øtransistor style housing containing the

pyroelectric element (incl. thermal

compensation) and a complete current

voltage converter with low input bias

current OpAmp.

Fig. 12: Four alternative pyroelectric detectors suitable for

current mode

Ground=Case

-V

Out

LME-341

Source

LME-300

Case

Ground =

RDrain

Feedback

Case

Ground

Output

Ground

Case

Output

LME-501 LIE-215

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Pyroelectric detectors Introduction

5.3. Suggestions for wiring

The following examples are supposed to inspire one to consider the curr ent mode as a reasonable

alternat ive to the classic voltage mode based on the most modern available components.

Fig. 13: Circuitry for current mode (0.2 -25Hz; 1V/nA) of the pyroelectric detectors LME-301 and LME-501

Fig. 14: Circuitr y for current mode (typical 31,000 V/W) of the pyroelectric detectors LME-300 and LME-500

OpAmps with a low voltage noise should be used. Disadvantages of the circuitry depicted in figs. 13 and 14

include:

•EMC problems due to parasitic capacities

•permanent voltage offset across the pyroelect ric element due to VGS of the JFET

•IGSS of the JFET determines the level and tem peratur e dependence of the current noise

These disadvant ages can be avoided by integr ation of the OpAmp into the detector housing.

+1µF

OPA 129 - TI (Burr-Brown)

33nF

OPA 129

-15V

to signal

conditioning unit

1GOhm

33nF

2.2pF

+15V

LME-501

270k

-V = -9V

Ground

OPA277

-Out

+V = +9V

OPA 277 - TI (Burr-Brown)

33nF

24GOhm

0.47pF

LME-300

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Pyroelectric detectors Introduction

Modern low power OpAmps with low current and voltage noise ensure the same signal to noise rat io as in a

simple JF ET source follower, however due to the considerably lower electrical time constant it allows a

considerably higher responsivity. The advantages of the integrated current mode detector are that:

•although there is a very high responsivity (RV>100,000 V/ W) there is also a high stability

•there is a very low output off set together with a very low offset temperature drift

•there is very low current noise together with a lower temperature drift of the specific detectivity at low

frequencies (f<100Hz)

•there is a low electr ical time constant and therefore a short warm-up phase and fast recovery time

•there is no signal loss when using the parallel compensation in thermal compensated detectors

Fig. 15: Circuitr y for single, dual or quad current mode detectors LME-235, LME-335, LIM -262, LMM-242 or

LMM-244 directly coupled with an ADC

LME-335

SHDN

Ground

V+ = 5V

100G

0.2pF

47k

4k7

V- = -5V

SHDN

2MAX 195

(16-BIT ADC)

VDD

VSS REF

SCLK

DOUT Serial

Interface

4.096V

MAX 4251

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