designing a low-cost, high-accuracy infrared thermometer

Application ReportDesigning a Low-Cost, High-Accuracy InfraredThermometer

ABSTRACT

An infrared (IR) thermometer can measure temperature without contact, which can help ease the spread of acontact infection. Highly-accurate, non-contact temperature measurement is one such necessity that is neededeverywhere to identify persons at risk. This design demonstrates a low-cost, high-accuracy IR thermometersolution capable of sensing accurate temperature up to a distance of 10 cm. The design uses a 24-bit sigma-delta ADC and an ultra-low-noise signal chain to provide small offset and drift with temperature. The designimplements a complete solution with a low-cost MCU, LCD interface, and GPIO controls. The design operatesfrom a 3-V AA battery down to 2.2 V and has < 1 μA of total standby current consumption making it ideal forbattery-powered systems.

Table of Contents1 Introduction.............................................................................................................................................................................22 Key Specifications and Features.......................................................................................................................................... 23 System Design Challenges and Part Selection................................................................................................................... 3

3.1 Design Challenge #1: Accuracy of Medical Measurement.................................................................................................33.2 Design Challenge #2: Maximize the Battery Power of the System.................................................................................... 43.3 Design Challenge #3: Quickly Generate a Measurement.................................................................................................. 4

4 IR Thermometer: Hardware Design.......................................................................................................................................54.1 Low-Noise Signal Chain Design.........................................................................................................................................64.2 System Power Generation and Management.................................................................................................................... 74.3 Microcontroller Section and LCD Display...........................................................................................................................74.4 Power ON, Automatic LCD Backlight Circuit and EEPROM.............................................................................................. 9

5 Software.................................................................................................................................................................................106 Test Results...........................................................................................................................................................................11

6.1 Board Images................................................................................................................................................................... 116.2 Test Setup for Evaluating the Thermometer.....................................................................................................................126.3 Test Procedure................................................................................................................................................................. 12

7 Test Results...........................................................................................................................................................................137.1 Current Consumption....................................................................................................................................................... 137.2 Electrical Noise................................................................................................................................................................ 137.3 Thermal Measurements................................................................................................................................................... 13

8 Conclusion............................................................................................................................................................................ 149 References............................................................................................................................................................................ 14

TrademarksAll trademarks are the property of their respective owners.

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SBOA501 – NOVEMBER 2020Submit Document Feedback

Designing a Low-Cost, High-Accuracy Infrared Thermometer 1

Copyright © 2020 Texas Instruments Incorporated

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1 IntroductionAn IR thermometer can measure temperature without contact, which can help ease the spread of a contactinfection. A high-performance analog-to-digital converter (ADC) is required to sample high-precision signalscollected by the analog infrared temperature sensor – typically, a single pixel thermopile sensor. Thethermometer provides temperature readings by placing the sensor near the object without actually touching it. Itprovides portable and instant on-the-spot readings to screen patients and gauge the health of a person while stilllimiting exposure risk of the tester.

This application note describes in detail the complete design of an IR-based thermometer for patient monitoring.The design is implemented using a thermopile infrared sensor which produces an analog voltage proportional tothe temperature difference between the object and the ambient. Figure 1-1 shows the system-level blockdiagram of the IR thermometer. The key advantage of this architecture is that it provides very high performanceat a very low cost.

MSP430I2040

AMP

TLV2333

Diode ORing

AMP

TLV2333Load Switch

TPS22919

Analog

Infrared

Sensor

Thermopile: Object temperature

Thermistor: Ambient temperature

EEPROM

(Optional)

Battery

Voltage

Monitoring

Boost

TPS61023

Ideal Diode

LM66100

Reverse Polarity Protection

Up to 32 segment display

PWM

PWM

GPIO

ISR

GPIO

I2C

GPIOs (12

SPI (optional for display)

ENABLE

BatteryBuzzer

LCD Backlight

Power ON/OFF

C/F

OUT1

OUT2

BATT+3.3 V

ADC

ADC

ADC

System Interface

ADC Measurement

Power

Figure 1-1. System-Level Block Diagram of the IR Thermometer Design

The heart of the system is the MSP430I2040, 16-MHz metering AFE with four 24-bit sigma-delta ADCs, two 16-bit timers, 16KB flash, and 1KB RAM, which provides exactly the adequate resources to implement thetemperature measurement. It controls the complete system, in terms of power management, small signalmeasurement, digitization, and user interface such as display, input buttons, buzzer, and so forth.

2 Key Specifications and FeaturesThe thermometer has the following specifications and features:

• High temperature accuracy up to ±0.2°C with distance up to 10 cm can be obtained by precise calibration ofthe sensor (this design does not incorporate calibration)

• Object measurement range of 30°C to 45°C with an ambient temperature range of 16°C to 40°C (supportsfrom 0°C to 50°C)

• Supports up to 32-segment LCD using the GPIO pins, additional support of I2C and SPI based smart LCDs• °C/°F temperature display, changeable using a push button• Automatic turn ON/OFF of backlight LED; battery reverse polarity protection

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3 System Design Challenges and Part SelectionFor a high-level overview, read the technical article How to design an infrared thermometer quickly.

3.1 Design Challenge #1: Accuracy of Medical MeasurementA high level of precision for each measurement is required, down to ±0.2°C. While the signal from thermopile isvery small (less than a few mV), the variation with temperature is even smaller and can be < 10 µV/°C for somesensors. Figure 3-1 shows a reference curve of one such sensor showing thermopile output versus objecttemperature. Precisely and accurately measuring such a signal in a noisy environment is a challenge.

Ambient Temperature = 25°C

Vactive: output range for 30°C ± 45 °C @ 45 °C ambient

Vactive = 1.75 ± 0.4 = 1.35 mV

Savg = = 83.75 µV/°C

x Sensor output voltage

range for the desired temperature range Vsen

x The average sensitivity is

83.8 uV/C in the range of interest

~0.4 mV S30 = 80 µV/°C

~1.75 mV

Vsen = 4 mV

Range where maximum accuracy is required

Actual data from the sensor data sheet shall be used for LUT creation

S45 = 87.5 µV/°C

Reference Curve

Sensor: TS318-11C55

87.5 + 80

2

Figure 3-1. Typical Reference Curve of a Thermopile Sensor Showing Example Calculations and Rangeof Measurement

3.1.1 TI Solution: Pre-amplification Using LNA to Remove Effect of ADC Offset and Drift

TI precision op amps have extremely low offset voltage and temperature drift. This allows small voltage signalsto be amplified.

TLV333:

• Ultra-low input offset voltage, 15 μV (max) optimized to measure low voltage signals• Low offset voltage drift 0.02 μV/°C (max)

The pre-amplification stage is necessary to gain the signal and must have extremely low offset and offset drifterrors in the system. If the signal is fed directly to the ADC of the MSP, the offset and drift parameters of theADC will cause the error to be very high.

www.ti.com System Design Challenges and Part Selection




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3.2 Design Challenge #2: Maximize the Battery Power of the SystemThe batteries of the thermometer need to last for 1,000+ measurements and it must minimize current draw whenturned off.

3.2.1 TI Solution: TI has High-Efficiency DC/DC With Ultra-low IQ, MSP430I2040 With Ultra-low lpmCurrent Consumption.

TPS61023:• < 100-nA ultra-low shut down current• < 21-μA quiscent current• > 83% efficiency at 10-μA load• > 90% efficiency at 1-mA to 2-A load

Power Optimization using MSP430I2040: The enable of the regulated 3.3-V supply is controlled by anMSP430 which only turns on the system supply whenever the user wants to take thermal measurements.

3.3 Design Challenge #3: Quickly Generate a Measurement

3.3.1 TI Solution: MSP430 Family

The MSP430 family of products combines an ADC, LCD driver, and processing into one chip ensuring thatresults are quickly calculated.

MSP430I2040:

• Integrated 24-bit, sigma-delta analog-to-digital converters• Internal low-noise reference to minimize power supply noise which can affect the measurement

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4 IR Thermometer: Hardware DesignA voltage is developed across a metal if the ends are placed at different temperatures. This phenomenon iscalled the Seebeck Effect. Two dissimilar metals connected at one end (called a thermocouple) will also producea voltage difference between them due to the Seebeck effect. Modern technology makes it possible to producethermopile sensors consisting of hundreds of thermocouples in a very small area.

Figure 4-1 shows such a temperature sensor with four leads coming out of it. Two leads are for the thermopileoutput where the potential due to the Seebeck effect is generated and another two leads are for the thermistorleads which are used for ambient temperature measurement.

Figure 4-1. Temperature Sensor Composed of Thermopile Sensor and Thermistor

Figure 4-2 explains the reason for using two sets of sensors. To know the temperature of the object, one mustknow the voltage developed when IR light falls on one end of the thermopile sensor. The voltage (ΔV) will beproportional to the temperature difference (ΔT). Knowing ΔT (using LUT corresponding to ΔV) and ambienttemperature TCOLD (in Figure 4-2), the object temperature can be determined.

V1

Hot Junction Cold Junction Voltage

100°C 0°C 4 mV

150°C 50°C 4 mV

200°C 100°C 4 mV

V2 HOT COLD

HOT COLD

V S T T

VT T

S

'

'

Must be

known

Voltmeter

¨V = V1 ± V2 = 4.0 mV

+

±

If the ambient temperature and the voltage difference due to the Seebeck effect is known, the object temperature can be calculated.

Figure 4-2. Object Temperature Calculation

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4.1 Low-Noise Signal Chain DesignThe signal output of the thermopile sensor TP+ and TP– is fed to a low offset drift op amp with a gain of 201.The signal is offset by VREF. The differential signal is fed to the 24-bit ADC on channel 0 of the MSP430I2040 fordigitization.

As illustrated in Figure 3-1, designing the front end requires that the range of the input must be calculated.Assume that the thermopile signal (Vtc) varies between –1 mV to 3 mV as shown by the red marking (Vsen) inFigure 3-1 to cover the range of object temperatures with ambient variation, which gives Equation 1:

tcV 1mV to 3mV 4 mV range (1)

This signal is amplified using one channel of the TLV2333 device with a maximum offset of 15 μV. The gain isgiven by Equation 2:

3

OUT tc REF1 tc REF1

8

RV 1 V V 201 V V

R

§ · u u u¨ ¸¨ ¸© ¹ (2)

This VOUT is sent to the ADC in differential mode as shown in Figure 4-3. The ADC input will be varying from201 × Vtc which will be 201 mV to 603 mV (for best performance the signal range should be in ±928 mV).

The system requires a DC voltage shift to the input to have sufficient offset to have a negative signal as well(when ambient is higher than the object temperature, the thermopile output will be negative). This is done byusing another channel of the TLV2333 which serves two purposes. First, it provides a buffer for the thermistorvoltage measurement, the output is also used as a reference or DC offset for the thermopile measurement.

Note that the input impedance of the ADC in the MSP430I2040 is of the order of 200 kΩ. The typical thermistoroutput will vary from 35 kΩ at 50°C to 331 kΩ ay 0°C (from the TS318-11C55 data sheet). To measure thesignal, a buffer between the thermistor and the ADC channel is absolutely necessary.

The thermistor voltage is measured using a resistor divider followed by the buffer which is used from the secondop amp in the TLV2333. Equation 3 calculates the VREF range:

REF1 NTCV ( R ) u

19

NTC 17 19

3.3R

R R R (3)

Equation 3 shows that VREF1 varies between 383 mV (for RNTC 35 kΩ) to 993 mV (for RNTC 331 kΩ). Thevalue of RNTC at 25°C is approximately 100 kΩ, which translates to VREF1 of 550 mV.

A

Thermopile Sensor

TP+1

NTC2

TP-3

GND4

PreliminaryU1

TS318-11C55

TP+ TP-

NTC+ NTC-

GND

102k

0.1%

R19

GND

35V1uF

C14

VREF1

VREF1

Thermistor measurement and reference voltage bias generation

TP+

VREF1

Vtc = TP+-TP-Vout = (1+ Rx/Ry) x Vtc + VREF1Vtc = -1mV to 3 mV

200k

0.1%R3

1.00k

0.1%R8

VREF1 - 200mV < Vout < VREF1 + 600mV

GND

V_TP+V_TP+

VREF1

GND

V_TP-

ain Stage

1.0k

R9

1.0k

R14

1.0k

R21

GND

VREF1_ADC

0.1µFC20

0.1µFC22

GND

100

R47

5

6

7B

U3B

TLV2333IDGKR

2

3

1A

U3A

TLV2333IDGKR

12 4

3

S3434153017835

3V3

47.0kR59

DNP

GND

0.1%

1.00MR1

NTC+

Divider; can be used to modify the VREF value to get in range of ADC

0R2

35V1uF

C9

35V1uF

C4

35V1uF

C7

35V1uF

C10

35V1uF

C16

35V1uF

C1

Differential Signal: to be digitized

Thermopile Amplifier

Figure 4-3. Low-Noise Small Signal Chain Implementation

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4.2 System Power Generation and ManagementThe system requires a battery input source whose voltage can vary from 2 V to 3 V. The LM66100 providesbattery reverse polarity protection to the system. This battery voltage is fed to a boost device (TPS61023) togenerate a regulated 3.3-V system voltage. To conserve power drain from passive and other devices, the 3.3 Vis controlled by a BOOST_EN signal. When boost is not enabled, only the MCU, MSP430I2040, gets power fromthe battery and is running in LPM4.5 mode. The battery voltage is ORed with 3.3 V, when the system is active,3.3 V takes over.

The same pin of the MCU is like a PWM which serves multiple functionalities, to Enable boost, run the buzzerwith PWM. Note that the peak detector circuit keeps the Enable HIGH during PWM.

1

2

J1

PEC01DAAN

GND

GND

2.2uH

L1

GND

3V3

3

.3V

Supply GenerationUltra low Iq and shutdown current

Reverse polarity pro tection

33µFC6

0.1µFC2

IN1

GND2

CE3

N/C4

ST5

OUT6

U8

LM66100DCKR

GND

FB1

EN2

VIN3

GND4

SW5

VOUT6

U9

TPS61023DRL

GND

1.1pFC5

100k

0.1%

R36

453kR35

22µF

C3

22µF

C25

0

R30

T_PLUS

BOOST_EN

BOOST_EN_SIG

GND

0.1µFC26

100kR52

BOOST_EN 3

1

2

Q1 BC857C-7-F

680R28

1

2

NC3

SMT-0540-T-2-R

LS1

SMT-0540-T-2-R

GND

100kR20

100k

R26

D3

1N4148X-TP

PWM Signal fro m MCU

Regulated 3.3V System Vo ltage Generation

Peak detectorBuzzer Alarm

Figure 4-4. System Power Management and Protection

4.3 Microcontroller Section and LCD DisplayThe heart of the system is the MSP430I2040 device which controls multiple peripherals and completemanagement in terms of power, display, measurements, timing, and so forth. The MCU in standby mode sleepsin LPM4.5 mode with a current system draw of < 1 µA and 3.3-V rail cutoff while essentially removing excessivepower loss.

Upon detection of button press (S1) on P2.2, the system wakes up and turns on the power for variousfunctionalities. When in sleep mode, the battery provides power to the MCU and when 3.3 V is on, it takes overdue to the ORing diode D4. Figure 4-5 shows the schematic of the microcontroller section.

A0.0+1

A0.0-2

A1.0+3

A1.0-4

A2.0+5

A2.0-6

A3.0+7

A3.0-8

VREF9

AVSS10

ROSC11

DVSS12

VCC13

VCORE14

RST/NMI/SBWTDIO15

TEST/SBWTCK16

P1.0/UCA0STE/MCLK/TCK17

P1.1/UCA0CLK/SMCLK/TMS18

P1.2/UCA0RXD/UCA0SOMI/ACLK/TDI/TCLK19

P1.3/UCA0TXD/UCA0SIMO/TA0CLK/TDO/TDI20

P1.4/UCB0STE/TA0.021

P1.5/UCB0CLK/TA0.122

P1.6/UCB0SCL/UCB0SOMI/TA0.223

P1.7/UCB0SDA/UCB0SIMO/TA1CLK24

P2.0/TA1.0/CLKIN25

P2.1/TA1.126

P2.2/TA1.227

P2.3/VMONIN28

P2.4/TA1.029

P2.5/TA0.030

P2.6/TA0.131

P2.7/TA0.232

PAD33

MSP430I2041TRHBR

U4

TEST

50V1100pF

C11

SPY-BI-WIRE Interface

Change connector if required

Connector for 32 Segment LCD Display

AVSS

AVSS

RESET

GND

GND

BOOST_EN_SIG

V_TP+V_TP-

GND

PGA Gain =1

Vref_ADC = 1.158V (internal)

V_TP+ - V_TP- = -1.158V to +1.158V

For performance, V_TP+ - V_TP- = -928 mV to +928 mV6.3V10µF

C8

GND

SEG7

SEG0

SEG1

SEG2

SEG3

SEG4

SEG5

GND

3V3

47.0kR4

47.0kR5

47.0kR6

47.0kR7

47.0kR13

47.0kR12

47.0kR11

47.0kR10

COM3

COM2

COM1

SEGx lines are operated at 2 levels VCC and GND

COMx lines are operated at 3 levels VCC, VCC/2 and GND

47.0k

AVSS

ESD Protecttion

0 ohm resistor

0.1µFC12

37.5CLoB

ample

Temperature

Low Battery

TP1

TP_SM_1MM

33.0

R31

0R440R450R46

5

4

1

2

3

6

7

8

9

10

11

12

J4

M22-2511205

Modify accor ding to LCD design

VREF1_ADC

V_BATT_ADCin

0.1µFC19

0R16

0.47µF

C13

0R40

4

1

2

3

J2

PTT-104-01-L-S

SEG0

SEG1

SEG2

SEG3

SEG4

SEG5

SEG6

COM3

COM0

COM2

COM1

TSTCOM0

Wake up from LPM4.5

0R33ON/OFF

0R320R34

0R39

0R370R38

SEG6I2C_SCL

0R41SEG7

TP2TP_SM_1MM

0R420R430R500R51

SEG6

SEG7

I2C_SDA

I2C_SDA

I2C_SCL

1

3

2

D4

BAT54C-7-F

3V

3

BATT_PLUS

MCU_VCC

22kR53

GND

0

R17

3V

4

Diode OR-ing for MCU power

ADC Measurements

ON/OFF: W akeup fro m LPM4.5

LCD GPIOs

LCD GPIOs & I2C

Figure 4-5. Microcontroller Section Implementation





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Software LCD: The design uses onboard GPIOs to implement the LCD display functionality. It has 12 GPIOs (8SEG and 4 COM pins) to support up to 32 LCD segments. The implementation is done in software and isavailable through TI for evaluation. For detailed information on similar implementation, see Software Glass LCDDriver Based on MSP430 MCU.

Especially for one-third bias LCD driving using GPIOs, the TIDA-00848 reference design from TI that shows analternative, patent-pending solution can be used with any TI MCU without an on-chip display module. Using afew resistors and GPIO control software, the LCD drive functionality was implemented on a CC1310 WirelessMCU. Users can implement the same architecture using the MSP430I2040. One additional PWM pin is needed.The ON/OFF pin can be used in this case (multi-purpose), since after turn on, the ON/OFF pin has no use whilethe system is working. It can be configured into a PWM pin to support the display. Small hardware modificationsare required, contact TI on E2E for more details.

Figure 4-6. Example Implementation Using Same ON/OFF pin as PWM Output

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4.4 Power ON, Automatic LCD Backlight Circuit and EEPROMThe system wakes up when the S1 button is pressed which goes to pin 2.2 of the MSP430I2040 and this wakesup the MCU from LPM mode. Simultaneously, the load switch TPS22919 enables the backlight power and theLCD backlight turns on for a specified time interval set by the time constant of the RC network C17 and R23.Figure 4-7 shows the corresponding circuitry.

Battery Vol tage Measurement

V+8

V-4

U3C

TLV2333IDGKR

12 4

3

S2434153017835

GND

BATT_PLUS

VSS2

SDA3

SCL1

WP5

VCC4

U5

AT24C02C-STUM-T

GND

35V1uF

C15

1

2

J3

PEC01DAAN

2.2kR25

Slave Address : Ah

2.2kR24

35V1uF

C18

I2C_SCL

I2C_SDA

GND

GND

I2C EEPROM

12 4

3

S1434153017835

4.70kR22

GND

GND

GND

Backlight Power for 10s

35V1uF

C17

Baclinght LED connector

Remove and directly connect across LED

Green

12

D1LTST-C190GKT

LED for test

ON/OFF

10MR23

ON/OFF switch to be used in conjunction with Backlight ON/OFF

47.0kR27

.8V

17.4k

C/F selection

D2

1N4148X-TP

0.1µFC21

33.0

R48

1.0k

R49

0.1µFC23

GND

V_BATT_ADCin

NC1

NC2

IO13

GND4

IO2RST TST

GND

ESD Protection

TST

IN1

GND2

ON3

N/C4

QOD5

OUT6

U7

TPS22919DCKR

35V1uF

C24GND

680R54

100

3V3

3V3

3V3

100

R56MCU_VCC

0R57DNP

Other subsytems

Increase divider resistors to reduce current consumption

Figure 4-7. Power ON/OFF and LCD Backlight Circuit

The battery voltage is monitored on the second channel of the ADC and the same is also used as a selectionswitch between C/F displays of temperature. The same ADC channel is periodically sampled and button pressand battery voltage is measured using a resistor divider network. The I2C EEPROM (optional) is provided forstoring sensor data and other system parameters such as calibration data, sensor constants, and so forth. ESDprotection on the Spy-Bi-Wire interface is also placed.





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5 SoftwareFigure 5-1 shows the software flow diagram of the code implemented in the IR thermometer design. Each step inthe diagram is self-explanatory. Download project collateral and source code discussed in this application reportfrom http://www.ti.com/lit/zip/sboa501. The software does not include any calibration routine and it must also bemust implemented by the customer. The software implements a look up table for the TS-308-11C55 sensor fromTE connectivity and implements very basic calculations of the temperature.

Initialize MSP430I2040 DCO Clock

Battery Low?

Initialize LCD

Initialize 24-bit

Sigma-Delta ADC

Initialize Timers

Enable Booster Convert

Beep Piezo

'LVSOD\³/R%´RQ/&'

On/Off Button? Restart 15 second timeout

Switch between °F and °C

Start Capturing Temperature data

Calculate Object temperature

(compensation routine)

Display Temperature on LCD

Timeout?

Shutdown

Button 2?

Check Buttons

Yes

No

Yes

Yes

No

No

Yes

Figure 5-1. Software Flow Diagram

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6 Test Results

6.1 Board ImagesFigure 6-1 and Figure 6-2 show the board images of front side (right) and back side including the mountedsensor (left). The dimension of the board is 29 mm × 30.84 mm.

Figure 6-1. Front Side Board ImageFigure 6-2. Back Side Board Image

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6.2 Test Setup for Evaluating the ThermometerFigure 6-3 shows the test setup which is used to evaluate the IR thermometer design. To simulate the objectplaced at a distance, a Fluke 4180 IR calibrator is used to vary the object temperature. A high-precision 61/2 digitmultimeter and voltmeter are used to measure the input current and ADC voltages, respectively. The experimentwas performed in the lab ambient temperature of in the the range 19°C to 23°C

Precision Voltmeter

(measure voltage at ADC channels)

1. Thermopile Voltage

2. Thermistor Voltage

MSP430I2040

Input Connector

LCD Connector

Multimeter

(measure input current)

Black Body

Fluke 4190 IR

Calibrator

Thermopile Sensor

DC Power

Supply

Up to 3 V

Backlight

Power

Black Body is placed back at a

distance of 5 cm from the the

Thermopile sensor to simulate

object. (Black body accuracy: 0:5 °C)

Figure 6-3. Test Setup to Evaluate the Thermometer

6.3 Test ProcedureTo evaluate the performance of the IR thermometer board, two categories of tests were conducted:

1. Electrical Noise in the system without the thermopile sensor mounteda. Instead of a thermistor, a known resistor is placedb. Instead of a thermopile, a high-precision DC supply (order of mVs) is placed

2. Thermal measurements with the thermopile sensor mounted on the board as described in the Test Setupsection

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7 Test Results

7.1 Current ConsumptionActive current – 36 mA (complete system running including LCD)

Standby – 0.1 μA (with R27 Removed), 47.6 μA (with R27 placed); modify divider R27 accordingly to achieve thedesired current

7.2 Electrical NoiseTable 7-1 shows the circuit level test results without a sensor to show low noise in the signal chain. The noise isthe measure of the maximum ADC reading fluctuations with known values of thermistor resistor and knownthermopile input voltage (0.1 mV–1 mV).

Table 7-1. System Noise ResultsNoise (mV) Resulting Temp Error (°C) Allowable Noise for ±0.2°C (mV)

–0.395 –0.023 6.914

7.3 Thermal MeasurementsTable 7-2 shows the data collected at 36 °C (black body temperature). The following calculations are done byfollowing the compensation described by Thermopile Sensor for Contactless Temperature – TE connectivity. Inthe table, VTP is the thermopile voltage (gained), VNTC is the thermistor voltage, TSEN is the ambienttemperature and TOBJ is the calculated object temperature.

Table 7-3 shows the data obtained at various set point temperatures of the black body. The data is taken at30°C, 36°C, 37°C, and 40°C. The columns show average, maximum, and minimum values obtained doing asimilar experiment as shown in Table 7-2. The last two columns show the deviation from the average value.

Table 7-2. Temperature Data for Black Body set at 36°CReadings Black Body

Set TempMeasured Calculations

Black BodyTemp

VTP (mV) VNTC (mV) TSEN (°C) TOBJ (°C) SCALING(30\31.7)

1 36 35.7 186.58 576.17 23.79262688 38.44000253 36.13360238

2 36 35.8 185.53 576.64 23.75020865 38.32693104 36.02731518

3 36 35.7 189.4 577.53 23.6698454 38.52675126 36.21514619

4 36 35.8 187.58 577.75 23.64997237 38.37995567 36.07715833

5 36 35.5 186.6 576.96 23.72131993 38.37608767 36.07352241

6 36 35.8 187.03 576.77 23.73847341 38.42219174 36.11686024

7 36 35.8 190.28 578.34 23.59666097 38.52179452 36.21048685

8 36 35.6 189.64 578.45 23.58671905 38.46747604 36.15942748

9 36 35.7 191.22 579.76 23.46825916 38.47037531 36.16215279

10 36 35.7 189.59 578.04 23.6237713 38.49792268 36.18804732

11 36 35.7 187.31 577.02 23.71590256 38.42129665 36.11601885

12 36 35.7 188.25 577.45 23.67707117 38.45212682 36.14499921

13 36 35.7 190.51 578.23 23.60660211 38.54715792 36.23432844

14 36 35.5 187.57 578.82 23.55327225 38.29063466 35.99319658

15 36 35.8 187.66 577.81 23.64455191 38.38063967 36.07780129

16 36 35.7 190.18 577.99 23.62828912 38.54374163 36.23111713

17 36 35.8 186.46 576.69 23.74569523 38.38852466 36.08521318

18 36 35.8 188 577.74 23.65087576 38.41045416 36.10582691

19 36 35.7 187.35 577.1 23.70867902 38.41750379 36.11245356

20 36 35.8 185.93 576.07 23.80165018 38.40231967 36.09818049

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Table 7-2. Temperature Data for Black Body set at 36°C (continued)Readings Black Body

Set TempMeasured Calculations

Black BodyTemp

VTP (mV) VNTC (mV) TSEN (°C) TOBJ (°C) SCALING(30\31.7)

21 36 35.8 183.33 574.23 23.96756299 38.37036272 36.06814096

22 36 35.8 182.54 573.53 24.52925434 38.8284961 36.49878633

23 36 35.7 188.45 574.98 23.89996203 38.67059541 36.35035969

24 36 35.7 186.56 576.03 23.80525932 38.45016222 36.14315249

25 36 35.7 187.51 576.27 23.78360293 38.49747329 36.18762489

26 36 35.6 191.88 577.56 23.66713562 38.69947706 36.37750843

27 36 35.8 185.19 575.51 23.85216866 38.39626997 36.09249378

28 36 35.8 188.04 576.21 23.78901738 38.53990008 36.22750608

29 36 35.8 188.98 576.62 23.75201398 38.5724176 36.25807255

30 36 35.6 190.41 578.01 23.62648201 38.55833052 36.24483069

Average 36 35.72 187.85 577.01 23.73343019 38.4755791 36.16704436

Table 7-3. Temperature Data Collected at Various Set Points of Black BodyBlack BodyTemperature

TOBJ (avg) TOBJ (max) TOBJ (min) TOBJ (max) – TOBJ (avg) TOBJ (min) – TOBJ (avg)

30 29.75545216 30.09368583 29.51515572 0.338233667 – 0.240296446

36 36.16704436 36.49878633 35.99319658 0.331741975 – 0.173847779

37 37.40582747 37.82919839 37.05760415 0.423370918 – 0.348223324

40 40.62476939 40.8565339 40.32927866 0.231764509 – 0.295490737

8 ConclusionThe Fluke BB at a given temperature has 0.2°C (±0.1) maximum deviation as per the specification. Our resultsshow a deviation of more than 0.5°C (Max – Max). There could be several reasons for a deviation greater thanthe black body.

• Data mentioned does not include any kind of sensor calibration. For example, thermal oil bath calibration atmultiple points would increase the accuracy significantly.

• Sensors picking up reflections from other sources. (the tests in the laboratory could not be performed inperfectly isolated settings).

9 References• Texas Instruments, Software Glass LCD Driver Based on MSP430 MCU Application Report• Thermopile Sensor for Contactless Temperature – TE connectivity

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