temperature sensing considerations
TRANSCRIPT
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Emmy Denton
Temperature Sensor Applications
Texas Instruments
March 17, 2015
Learn How to Measure Body Temperature
Accurately and Cost Effectively
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Overview and challenges ofthermometry solutions
Principles behindIC temperature sensors
Comparison of differenttypes of sensors
System implementation usingIC temperature sensor
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There are several technical challenges for measuring body
emperature
RTD
THERMISTOR
ICTHERMOCOUPLE
IR
37.00°C
Accuracy
T
T
T
T
T
T
http://www.amazon.com/Universal-Indoor-Outdoor-2-Inch-11381/dp/B0015ZSW86/ref=sr_1_4?s=home-garden&ie=UTF8&qid=1426203984&sr=1-4&keywords=clock
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There are a variety of body locations that have been used
Locations Target
Accuracy
Application
Pulmonary artery catheter “Golden
Standard”
Critically ill – blood flow
Sublingual 0.1C Home/hospital
Rectal 0.1C Home/hospital
Superficial temporal artery 0.1C Home/hospital
Ear (ympanic) 0.2C Home/hospital
Telemetry pill (Intestinal) 0.1C Athletics (heat stress)
Wrist 0.5C Fitness
Axillary (armpit) 0.5C Home
Forehead (NFC or LCD sticker) 1C Child/infant dispensable home
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Most accurate sensing methods are internal to the body o
n a body cavity
T
T
http://upload.wikimedia.org/wikipedia/commons/e/ea/Pulmonary_artery_catheter_english.JPG
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kin temperature - How many sensors do you actually ne
o measure core temperature?
22 25 28 31 34
25
28
30
34
37
Feet
Hand
Torso
Head
Rectal
S k i n T e m p e r a t u r e
( ° C )
Ambient Temperature (°C)
T
T
T
You regulate your core temperature
by modulating your skin temperature
through sweat and blood perfusion.
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Complicating the matter further, there are a variety of
emperature sensor types
Criteria
Temp Sense IC Thermistor RTD Thermocouple IR Temp Sen
Temp Range -55°C to +150°C -100°C to
+500°C
-240°C to
700°C
-267°C to
+2316°C
-100°C to
+500°C
Accuracy Meets
requirements
Depends on
calibration
Meets
requirements
Depends on cold
junction
compensation
Depends o
calibration
inearity Best Least Better Better Better
ensitivity Better Best Less Least Less
Circuit
implicity
Simplest Simpler Complex Complex Simple to
Complex
Power Lowest Low High High Medium
Cost $ $-$$$ $$$ $$ $$
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The principles behind IC temperature sensors are simply
based on the temperature coefficient of a base emitter
unction forward voltage drop
S
FF
IIln
qkTVVF
IFSlope ≈ -2mV/°C
VF1 V F2 =kT
qln
J 1
J 2VF1
IF1
VF2
IF2Slope ≈ 240 µV/°C
Compensates for IS
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Error
Sources
Challenges of output impedance
10-bit
ADC
M
U
X
RBias
Vth
Thevenin
Equivalent
Resistance?
Rth
+
-
VDD
Error
Sources
THERMISTOR t°
0
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Thermistor vs LMT70
1
Stainless SteelProbe Assembly
LMT70
DSBGA 4-bump
(0.8mm x 0.8mm)
Thermistor
LMT70
Response time of an IC temperature sensor is slightly
better than a thermistor
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PCB material and layout can affect thermal response time
2
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MT70 requires less processor resources or analog signal
processing than RTDs or thermistors
3
LMT70 is a single ended measurement RTD requires differential measurement
with 3 or 4 wire kelvin connections
LMT70
MSP430
TAO
GPIO2
P2.3
ADCM
U
X
GPIO1
2.2V to 3.6V
Coin CellBattery
LMT70
TAO
GPIO2
T_ON
T_ON
VDD
VDD
P2.5_VREF1.5V
Vref
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MT70 has excellent accuracy
over a wide range of -55°C to +150°C
4
LMT70 accuracy using LUT linear interpolation
Meets 0.36°C over wide range!
RTD accuracy curves
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MT70 beats IEC Class AA RTDs from 10°C to 150°C
5
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What is the system implementation using a semiconducto
emperature sensor?
MCU
Display/UI
Connectivity
NFC
WiFi
BT/BLE
Digital/Analog
Temp Sensor
Power
Management
Memory
Other Sensors
6
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Use a digital sensor if your MCU excludes an ADC that
provides the necessary performance
7
1.6mm x 1.2mm
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ADC error sources include INL, DNL, offset and gain error
LMT70
MSP430
TAO
P2.3
ADCM
U
X
GPIO1
2.2V to 3.6V
Coin Cell
Battery
T_ON
VDD
P2.5_VREF 1.5V
Vref
8
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ADC error sources can be calibrated using calibration
methods
9
LMT70
MSP430
TAO
P2.3
ADCM
U
X
GPIO1
2.2V to 3.6V
Coin CellBattery
T_ON
VDD
P2.5_VREF 1.5V
Vref
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Over a narrow temperature range you can improve the
MT70’s accuracy using a single point calibration
0
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Analog or digital temperature sensors provide an answer
or varying system resources and accuracy requirements
1
High-accuracy, low-power, digital temperature sensor with SMBus™ and two-wire serial interface in SOT5
±0.1°C accuracy over an ultra-wide temperature range using analog sensor and integrated 12-bit ADC
LMT70
MSP430
TAO
P2.3
ADCM
U
X
GPIO1
2.2V to 3.6V
Coin Cell
Battery
T_ON
VDD
P2.5_VREF 1.5V
Vref
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Technical challenges and IC solutions for measuring body
emperature accurately and cost effectively
2
T
T
T
T
TT
LMT70
MSP430
TAO
P2.3
ADCM
U
X
GPIO1
2.2V to 3.6V
Coin CellBattery
T_ON
VDD
P2.5_VREF 1.5V
Vref
Accuracy
http://www.amazon.com/Universal-Indoor-Outdoor-2-Inch-11381/dp/B0015ZSW86/ref=sr_1_4?s=home-garden&ie=UTF8&qid=1426203984&sr=1-4&keywords=clock
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Order a new LMT70 evaluation board and check out its
0.1°C accuracy
www.ti.com/sensing
ti.com/tool/lmt70evm
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