designing a low-cost, high-accuracy infrared thermometer
TRANSCRIPT
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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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.
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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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