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Texas Instruments SC Temp Sensors Challenge Precision RTDs and Thermistors in Build Automation (Rev. A) Application notes
(1) (2) (3)
Application Report
SNAA267A – April 2015 – Revised May 2019
Semiconductor Temperature Sensors Challenge Precision
RTDs and Thermistors in Building Automation
Thomas Kuglestadt
ABSTRACT
Standalone semiconductor sensors have rarely been considered for implementation into sensor probes or
assemblies due to their larger geometries. However, advances in process technology and design have led
to new, tiny sensor structures with almost linear transfer functions.
In order to provide system designers with this new low-cost alternative to precision temperature
measurement, this application report discusses the TMP117, ±0.1°C accurate digital temperature sensor
and the LMT70 temperature sensor, whose footprint is less than 1 mm2, while its parametric performance
challenges the accuracy of RTDs at cost levels lower than those of thermistors.
1
2
3
4
5
6
7
8
9
Contents
Introduction ................................................................................................................... 2
Resistance Temperature Detectors (RTDs) .............................................................................. 2
Thermistors (NTCs).......................................................................................................... 4
Semiconductor Temperature Sensors .................................................................................... 5
The TMP117 .................................................................................................................. 6
The LMT70 ................................................................................................................... 7
Accuracy and Calibration ................................................................................................... 8
4-to-20 mA Temperature Transmitters .................................................................................. 11
References .................................................................................................................. 11
List of Figures
1
Resistance – Temperature Curve for PT100............................................................................. 2
2
Accuracy Classes for RTDs ................................................................................................ 3
3
R-T Curve Comparison Between Thermistors and RTD ............................................................... 4
4
Accuracy Comparison Between Precision RTDs and Standard and Precision Thermistors
5
6
7
8
9
10
11
(1)
(2)
(3)
......................
Principle of Eliminating IC and IS ...........................................................................................
VPTAT Temperature Sensor ..................................................................................................
Accuracy Chart for TMP117 and RTD ....................................................................................
Simplified Block Diagram ...................................................................................................
Circuit Examples for Noise-Free and Noisy Environments .............................................................
LMT70 Accuracy Over Temperature ......................................................................................
Sensor Output Characteristic Versus Typical Characteristic...........................................................
5
6
6
7
7
8
9
9
12
Two-Point Calibration Requires Offset Adjustment (left), Gain Adjustment (middle) to Yield the Final
Transfer Function (right) ................................................................................................... 10
13
Sensor Accuracy After Initial 2-Point Calibration and Further Fine-Tuning ......................................... 10
14
High Precision Temperature Transmitter With 4-to-20 mA Output .................................................. 11
15
Low-Cost Temperature Transmitter With 4-to-20 mA Output
........................................................
11
SMBus is a trademark of Intel.
I2C is a trademark of Philips Semiconductor.
All other trademarks are the property of their respective owners.
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1
Introduction
1
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Introduction
Temperature measurement applications in building automation and here, in particular, commercial airconditioning use a wide variety of temperature sensors, such as thermocouples, resistance temperature
detectors (RTDs), and measurement resistors with negative temperature coefficient also known as NTC
thermistors.
High temperature applications, such as flame detection in boiler systems, that approach temperatures up
to 1000 degrees require the use of thermocouple or RTDs.
The majority of temperature-sensing applications measuring refrigerant, water, and air temperatures are
limited to a range from 0°C to 100°C (32F to 212F).
This temperature range is commonly monitored by temperature probes using RTDs and thermistors, with
the RTDs being perceived as the more accurate and stable, but also more costly, and the thermistor as
the low-cost alternative with wider resistance tolerance over temperature and larger resistance drift over
time.
2
Resistance Temperature Detectors (RTDs)
RTDs are considered to be amongst the most accurate temperature sensors available. In addition to high
accuracy, they offer excellent stability, repeatability, and high immunity to electrical noise.
They are most commonly made using platinum (Pt) because it follows a very linear resistance-temperature
relationship in a repeatable manner over a large temperature range.
RTDs can be flat film for low temperature applications or wire-wound for higher temperature applications.
Flat film detectors are manufactured by placing a fine layer of platinum wire onto a ceramic substrate. The
element is then coated in epoxy or glass, which provides protection. They are a cheaper alternative to
wire-wound detectors and have a fast response time, however, they offer less stability and have a lower
temperature range than their wire-wound counterparts.
Wire-wound detectors consist of a length of fine coiled platinum wire wrapped around a ceramic or glass
core. They are relatively fragile and are often supplied with a sheath for protection. They have greater
accuracy over a wider temperature range than flat film detectors, however, they are more expensive.
DIN/IEC 60751 is considered the worldwide standard for platinum RTDs. For a PT100 RTD, the standard
requires the sensing element to have an electrical resistance of 100.00 Ω at 0°C and a temperature
coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0°C and 100°C.
The resistance-to-temperature relation is defined for temperature ranges above and below 0°C via:
RT = R0 [(1 + AT ) + BT 2 ]
for T ³ 0°C
2
3
RT = R0 [(1 + AT ) + BT + CT (100 - T )] for T < 0°C
with : A = 3.9083·10-3 , B = - 5.775·10-7 , = C = - 4.183·10-12
(1)
140
Resistance - Ω
135
130
125
120
115
110
105
100
0
10
20
30
40
50
60
70
80
90
100
Temperature - o C
Figure 1. Resistance – Temperature Curve for PT100
For the small temperature range from 0°C to 100°C, the resistance temperature curve is almost linear.
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Resistance Temperature Detectors (RTDs)
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There are four tolerance classes specified in DIN/IEC751 and two more tolerance classes used in the
industry that have not been standardized yet:
CLASS
TOLERANCE °C
Class AA =
± (0.1 + 0.0017·|T|)
Class A =
± (0.15 + 0.002·|T|)
Class B =
± (0.3 + 0.005·|T|)
Class C =
± (1.2 + 0.005·|T|)
1/3 Class B
± 1/3 (0.3 + 0.005·|T|)
1/10 Class B
± 1/10 (0.3 + 0.005·|T|)
Accuracy - o C
4.5
4.0
Class AA
3.5
Class A
3.0
Class B
2.5
Class C
2.0
1.5
1.0
0.5
0.0
-200 -100
0
100
200
300
400
500
600
Temperature - o C
Figure 2. Accuracy Classes for RTDs
These tolerance classes also represent the interchangeability of a detector. Should a detector become
damaged, good interchangeability assures that the replacement sensor delivers the same readings under
the same conditions as the predecessor.
Another important criterion for selecting a temperature sensor is the long term stability. Great stability
produces little output signal drift over time, thus reducing the frequency of costly calibrations. Depending
on the application requirement, today’s RTDs can provide long-term drifts from as little as 0.003°C/year up
to 0.01 and 0.05°C/year.
Often times RTDs are considered the most precise elements amongst temperature sensors. In order to
convert the change in resistance of an RTD into a sensible output signal, a current source that drives a
constant current through the sensing element is commonly used, thus creating a temperature dependent
voltage across the RTD.
This method bares two sources for measurement errors.
First, the current through the RTD causes a certain amount of self heat that adds to the sensing elements
temperature, thus falsifying the actual measurement reading. Therefore, in order to minimize the impact of
self heating, currents in the range of 500 μA to 1 mA maximum are recommended.
The second error source is the voltage drop across long measurement leads particularly in PT100
applications. Here, voltage divider action between lead resistance and RTD can significantly reduce the
measured output voltage at the signal amplifier input, yielding a false temperature reading. To minimize
the impact of lead resistance, the leads must either be short when using a 2-wire RTD, or the RTD itself
must accommodate lead-compensation wires, as provided in 3-wire and 4-wire RTD designs.
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Thermistors (NTCs)
3
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Thermistors (NTCs)
Thermistors are made from mixtures of powdered metal oxides. Recipes are closely guarded secrets of
the various thermistor manufacturers. The powdered metal oxides are thoroughly mixed and formed into
the shape needed for the thermistor's manufacturing process. The formed metal oxides are heated until
the metal oxides melt and turn into a ceramic. Most thermistors are made from thin sheets of ceramic cut
into individual sensors. The thermistors are finished by putting leads on them and dipping them into epoxy
or encapsulated in glass. The most prevalent types of thermistors are glass bead, disc, and chip
configurations.
NTC thermistors exhibit a decrease in electrical resistance with increasing temperature. Depending on the
materials and methods of fabrication, they are generally used in the temperature range of -50°C to 150°C,
and up to 300°C for some glass-encapsulated units. The resistance value of a thermistor is typically
referenced at 25°C (abbreviated as R25). For most applications, the R25 values are between 100 Ω and
100 kΩ.
The resistance versus temperature (R/T) characteristic of the NTC thermistor is a nonlinear, negative,
exponential function. Several interpolation equations are available that accurately describe the R/T curve.
The best known is the Steinhart-Hart equation:
3
1 T = A + B × ln R + C × (ln R )
(2)
Coefficients A, B, and C are derived by calibrating at three temperature points and then solving the three
simultaneous equations. The uncertainty associated with the use of the Steinhart-Hart equation is less
than ± 0.005°C for 50°C temperature spans within the 0°C-260°C range, so using the appropriate
interpolation equation or lookup table in conjunction with a microprocessor can eliminate the potential nonlinearity problem.
The relatively large change in resistance from the NTC thermistor versus temperature, typically on the
order of -3%/°C to -6%/°C, provides an order of magnitude greater signal response than RTDs.
1000
Thermistors
R25 = 100 Ω
R25 = 1 kΩ
R25 = 10 kΩ
Resistance – Ω
800
Platinum RTD
R0 = 100 Ω
600
400
200
0
0
50
100
150
o
Temperature – C
Figure 3. R-T Curve Comparison Between Thermistors and RTD
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0.50
0.45
0.40
Accuracy - oC
0.35
0.30
0.25
0.20
0.15
Class AA
0.10
Class A
0.05
Std -Therm
HP -Therm
0.00
-60
-40
-20
0
20
40
60
80
100
120
140
160
Temperature - oC
Figure 4. Accuracy Comparison Between Precision RTDs and Standard and Precision Thermistors
Another important feature of the NTC thermistor is the high degree of interchangeability that can be
offered at a relatively low cost. Interchangeability describes the degree of accuracy or tolerance to which a
thermistor is specified and produced, and is normally expressed as a temperature tolerance over a
temperature range. For example, disc and chip thermistors are commonly specified to tolerances of
±0.1°C and ±0.2°C over the temperature ranges of 0°C to 70°C and 0°C to 100°C. Interchangeability
helps the systems manufacturer to reduce labor costs by not having to calibrate each instrument/system
with each thermistor during fabrication or use in the field.
The small dimensions of thermistors make for a very rapid response to temperature changes. This feature
is particularly useful for temperature monitoring and control systems requiring quick feedback.
As a result of improvements in technology, NTC thermistors are better able to handle mechanical and
thermal shock and vibration than other temperature sensors.
The R/T characteristic and R25 value of a thermistor are determined by the particular formulation of
oxides. Over the past 10 years, better raw materials and advances in ceramics processing technology
have contributed to overall improvements in the reliability, interchangeability, and cost-effectiveness of
thermistors.
4
Semiconductor Temperature Sensors
Semiconductor temperature sensors, such as the LMT70, are manufactured using semiconductor
technology which allows these devices to be produced efficiently and inexpensively. It also allows these
devices to have properties designed to easily interface with many other types of semiconductor devices,
such as amplifiers, power regulators, buffer output amplifiers, and microcontrollers for signal conditioning,
monitoring, and display purposes.
These sensors offer high accuracy and high linearity over an operating range of about –55°C to +150°C.
Semiconductor temperature sensors make use of the temperature dependent relationship between a
bipolar junction transistor (BJT) base-emitter voltage and its collector current:
VBE =
æI ö
kT
× ln ç C ÷
q
è IS ø
where
•
•
•
•
k is Boltzmann's constant
T is the absolute temperature
q is the charge of an electron
IS is a current related to the geometry and the temperature of the transistor
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5
The TMP117
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Because of the non-linear temperature dependency of IS, many sensor designs use proportional-toabsolute-temperature (PTAT) circuits to eliminate the temperature impact of IC and IS all together. Figure 5
shows the simplified principle. Here, the difference between the base-emitter voltage of a single transistor
and the base-emitter voltage of n in parallel connected transistors is used as a linear, temperaturedependent output. This principle is applied in the so-called Brokaw cell that can be either used to create a
temperature independent bandgap voltage, or a PTAT sensor circuit (see Figure 6).
One Transistor
n Transistors
IC
IC
Qn
Q1
VBE
VBE =
VN
kT
q
ln
IC
VN =
IS
∆VBE = VBE −VN = T ⋅
kT
q
ln
IC
n ⋅ IS
k ⋅ ln( n)
q
Figure 5. Principle of Eliminating IC and IS
VIN
R
R
I2 = I 1
VBandgap
Q1
Qn
Vn
ΔVBE
VBE
R2
VPTAT
R1
VPTAT = T .
k ln( n)
q
. 2R 1
R2
Figure 6. VPTAT Temperature Sensor
The voltage, ΔVBE = VBE - VN, appears across resistor R2. Therefore, the emitter current in Q2 is ΔVBE / R2.
The servo loop of the op amp and the resistors, R, force the same current to flow through Q1. The Q1 and
Q2 currents are equal and are summed and flow into resistor R1. The corresponding voltage developed
across R1 is proportional to absolute temperature (PTAT).
The bandgap cell reference voltage, VBandgap, appears at the base of Q1 and is the sum of VBE(Q1) and VPTAT.
VBE(Q1) is complementary to absolute temperature (CTAT), and summing it with VPTAT causes VBandgap to be
constant over temperature. This circuit is the basic band-gap temperature sensor, which has been widely
used in semiconductor temperature sensors.
5
The TMP117
The TMP117 is a high-precision digital temperature sensor which provides a 16-bit result with a resolution
of 0.0078 °C and an accuracy of up to ±0.1°C across the –20°C to +50°C temperature range with no
calibration. The TMP117 is I2C™ and SMBus™ interface compatible, has a programmable alert function,
and can allow up to four devices on a single bus. The overall accuracy of the TMP117 across its operating
range is given in Table 1.
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Table 1. TMP117 Accuracy Specification
TEMPERATURE RANGE
ACCURACY
–20°C to +50°C
±0.1°C
–40°C to +100°C
±0.2°C
–55°C to +150°C
±0.3°C
The accuracy of the TMP117 versus an RTD is plotted in Figure 7 across the operating temperature range
of –55°C to +150 °C. It is evident looking in the Figure 7 that the TMP117 with no calibration has the same
or better accuracy as an RTD Class-AA sensor. Note that this is the raw accuracy of the two devices and
that the final system layout has a minor effect on the TMP117 and a major effect on the accuracy of an
RTD sensor due to a number of parameters like the choice of ADC, layout of signal traces, and
component tolerances, and so forth.
Figure 7. Accuracy Chart for TMP117 and RTD
6
The LMT70
However, recent advances in process technology and test methodology made it possible to produce
modern semiconductor temperature sensors, such as the LMT70. This device uses a much smaller design
structure while providing superior accuracy of less than ± 0.15°C at 25°C. Also the tiny 0.9-mm x 0.9-mm
package allows for the use in small sensor probes. All of these advantages come at a fraction of the cost
of competing devices.
The sensing element of the LMT70 consists of stacked BJT base emitter junctions that are biased by a
current source. The output of the sensing element is buffered by a precision amplifier whose class AB
push-pull output stage can easily source and sink currents of up to 3 mA.
The amplifier output connects to an output switch that is turned on and off by the digital control input
T_ON (see Figure 8). This switch allows for the multiplexing of multiple sensors on one signal line.
VDD
T_ON
TAO
LMT70
GND
Figure 8. Simplified Block Diagram
The left diagram of Figure 9 shows the most simple sensor interface. For a regulated, low-noise supply in
combination with a light load, such as an analog-to-digital converter (ADC) with internal input buffer, it is
possible to use the LMT70 without additional external components.
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Accuracy and Calibration
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3.3 V
VDD T_ON
LMT70
TAO
CB
100 nF
ADC
3.3 V
VDD T_ON
LMT70
RF
TAO
3 kΩ
GND
GND
CF
10 nF
ADC
Figure 9. Circuit Examples for Noise-Free and Noisy Environments
For a noisy supply and ADCs without internal buffer, a supply bypass capacitor of CB = 100 nF is
recommended. This capacitor filters supply noise and also provides sufficient supply current during ADC
switching cycles.
In very noisy environments, an additional R-C low-pass filter can be implemented. While the LMT70 is
capable of driving heavy capacitive loads of up to 1 nF without a series resistor, larger capacitance
requires a decoupling series resistor to maintain internal loop stability of the device.
Typically the R-C time constant of the external filter is significantly larger than the ADC internal time
constant made up by the multiplexer input resistance (~ 5 kΩ) and sampling capacitance (~ 10 pF). This
affects ADC sampling frequency. The time required to charge the sampling capacitor to n-bit accuracy is
half the period of the sampling frequency (assuming a 50% duty cycle) and is given by using Equation 4:
æ 1 ö
t = t × ln çç
÷÷
è 2n ø
where
•
•
where, τ is the external filter’s time constant
n is the ADC resolution in bits
(4)
Some data converters are limited in their range of sampling frequency and specifying the maximum
external resistance to maintain accuracy. Check the device-specific ADC data sheet for this information
since it impacts the external filter component values.
7
Accuracy and Calibration
The LMT70 is trimmed and calibrated during production. Its accuracy over the temperature range from
-50°C to 140°C is better than 0.3°C, thus challenging even class AA RTDs. These are, however, the
minimum and maximum accuracy limits that narrow down further to less than 0.15°C at 25°C. Actual
characterized components lie within an even narrower band of ± 0.15°C across the entire temperature
range.
For easy implementation of a controller look-up table, the LMT70, LMT70A ±0.1°C Precision Analog
Temperature Sensor, RTD and Precision NTC Thermistor IC Data Sheet lists the minimum, typical, and
maximum sensor output voltages in single degree steps for the entire temperature range. This data sheet
also provides suggestions on how and when to apply linear, quadratic, or cubic polynomials to achieve the
rated accuracies.
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0.60
0.50
0.40
Max Limit
Accuracy (°C)
0.30
0.20
0.10
0.00
–0.10
–0.20
–0.30
Min Limit
–0.40
–0.50
–0.60
–60 –40 –20
0
20
40
60
80
100 120 140 160
DUT Temperature (°C)
Figure 10. LMT70 Accuracy Over Temperature
For higher accuracies, it is possible to apply further calibration. The following example assumes a sensor
whose output voltage-over-temperature characteristic crosses the typical characteristic taken from the
look-up table in the LMT70, LMT70A ±0.1°C Precision Analog Temperature Sensor, RTD and Precision
NTC Thermistor IC Data Sheet (see Figure 11).
The most commonly applied and inexpensive calibration techniques are single-point and dual-point
calibrations. In the case of a single-point calibration, the user usually has only one reference temperature
available, such as an ice-bath that allows you to compare your sensor against it. This type of calibration
however, only allows for an adjustment of the offset between sensor and reference at 0°C.
V (mV)
T (o C)
VTyp (mV)
VS (mV)
V Max(0)
0
1097.774
1098.611
V Typ(0)
10
1046.647
1047.245
20
995.051
995.461
30
943.227
943.499
40
891.179
891.330
50
838.883
838.883
60
786.360
786.189
70
733.608
733.243
80
680.654
680.065
90
627.490
626.646
100
574.117
572.940
VS
V Min(0)
(T)
VT
yp(
T)
2
VTyp(T) = a ·T + b·T + V Typ(0)
V Max(100)
2
VS(T) = G·a ·T + G·b·T + V S(0)
with G =
ΔVS
VS(100) – VS(0)
=
ΔVTyp
VTyp(100) – VTyp(0)
V Typ(100)
V Min(100)
0
10
20
30
40
50
60
70
80
90
100
T (o C)
Figure 11. Sensor Output Characteristic Versus Typical Characteristic
However, depending on the sensor characteristic, adjusting the offset at cold temperatures can lead to
increased deviation at higher temperatures, which makes single-point calibration questionable (see the left
diagram of Figure 12).
In order to achieve high accuracy, it is necessary to also adapt the slope of the typical characteristics to
the slope of the sensor, which is known as gain adjustment. Because a slope is defined by two
coordinates in the voltage-temperature diagram, a second reference temperature, such as the boiling point
of water at 100°C, is required to determine the second sensor output voltage.
A calibration that applies offset and gain adjustment to a sensor characteristic is known as two-point or
dual-point calibration.
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T)
VTy
=a2
·T
V(
T)
+b
·T
p(T)
+V
Output Voltage
S(0
)
V
Increased
Offset(100)
initial
Offset(100)
S(
T)
0
100
Temperature
=a2
·T
V
S(
T)
+b
·T
+
V
VS
(0)
S(
T)
ΔVS VS(100) – VS(0)
G=
=
ΔVT VT(100) – VT(0)
0
100
Temperature
=
G·
a2
·T
Output Voltage
V(
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Output Voltage
ΔVS
ΔVT
Offset(0)
Accuracy and Calibration
+
G·
b
·T
+
V
S(
0)
0
Temperature
100
Figure 12. Two-Point Calibration Requires Offset Adjustment (left),
Gain Adjustment (middle) to Yield the Final Transfer Function (right)
In this example, the typical output characteristic of the LMT70 in the LMT70, LMT70A ±0.1°C Precision
Analog Temperature Sensor, RTD and Precision NTC Thermistor IC Data Sheet ) (see Figure 11) is
approximated by the least-square fitting method. In this case, use a second order polynomial of the form:
VTyp(T ) = a × T 2 + b × T + c
(5)
Then, the output voltages of the actual sensor are measured at 0°C and 100°C.
For a first offset adjustment, VS(0) is used to replace c in Equation 5.
V(T ) = a × T 2 + b × T + VS(0)
(6)
Then, the ratio of the actual sensor slope to the typical sensor slope, also known as Gain, is computed
using Equation 7.
G=
DVS VS(100) - VS(0)
=
DVT VT (100) - VT (0)
(7)
The a and b coefficients are then multiplied with G to yield Equation 8.
VS(T ) = G × a × T 2 + G × b × T + VS(0)
(8)
To determine the sensor accuracy, Equation 8 is solved for temperature and calculated for each of the
sensor output voltages, VS, listed in Figure 11.
T(V ) =
S
-b - b 2 - 4 a (VS(0) - VS(T ) )
(9)
2a
Figure 13 shows the differences between the calculated and actual temperatures of the sensor
accuracies.
0.004
0.035
0.030
Accuracy – oC
0.025
Offset & Gain adjustment (initial)
0.020
Offset & Gain adjustment (fine tuned)
0.015
0.010
0.005
± 0.01oC
0.000
-0.005
-0.010
0
10
20
30
40
50
60
70
80
90
100
Temperature – oC
Figure 13. Sensor Accuracy After Initial 2-Point Calibration and Further Fine-Tuning
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Figure 13 shows best accuracy at 0°C. This is due to the offset compensation at this temperature. By
lowering the initial offset, the curve can be adjusted symmetrically around zero. Because of the lower
resolution of a second order polynomial across a wide temperature range, the application of additional
finer gain adjustment has an impact on accuracy by yielding levels of up to ± 0.01°C across the entire
temperature range.
8
4-to-20 mA Temperature Transmitters
In building automation, the distance between a sensor and its control processor unit can reach up to
several hundreds of yards. The most reliable method of transmitting sensor data across noisy environment
is the 4-to-20 mA current loop due to its high noise immunity. In order to maintain high accuracy during
transmission, high resolution data converters are used. Figure 14 shows the simplified schematic of a high
precision data acquisition system with current-loop output.
V S1
100 nF
DVDD
GPIO1
SCLK
DIN
DOUT
GPIO/IRQ
GPIO2
INT
DVSS
V S1
10 μ 10 n
MSP430
V S1
V S1
100 nF
VDD
T_ON
100 nF
AVDD
100 kΩ
TAO
47 n
LMT70
100 nF
GND
DVDD
AIN0
AIN1
ADS1220
AIN2
AIN3
AVSS
DGND
TPS7A4901
OUT
IN
18 k
FB
GND
10 μ
EN
10 k
24 Vdc
V S1
3 x 100 nF
V S1
100 nF
GND
100 nF
VD VA
C1 C2 C3
CSB
BASE
SCLK
SDO
DAC161S997
SDI
ERRB
OUT
ERRLVL
COMD COMA
CS
SCLK
DOUT
DIN
DRDY
CLK
R RCV
Figure 14. High Precision Temperature Transmitter With 4-to-20 mA Output
For inexpensive sensor transmitters with lower accuracy requirements, the circuit in Figure 15 can be
applied. The operational amplifier (OPA317) converts the negative slope of the sensor output into a
positive slope and also provides gain and offset adjustment. The 4-20 mA transmitter (XTR117) converts
the sensor output current into the appropriate loop current.
VREG
R1
VDD
V+
Rin
24 Vdc
B
IIN
Q1
VS
R2
GND
VO
IRET
RRCV
COM
R
99 R
RG
R1
E
40 µA
VOH = RL ⋅ 160 µA + VOL
RF
OPA317
LMT70
VOL
RL =
XTR117
RF
T_ON TAO
GND
V-Reg
RG
R2
=
=
V OH – V OL
V SH – V SL
VCC(1 + RF / RG)
VOL + VSH ⋅ RF / RG
−1
IO
Figure 15. Low-Cost Temperature Transmitter With 4-to-20 mA Output
9
References
•
•
Texas Instruments, LMT70, LMT70A ±0.1°C Precision Analog Temperature Sensor, RTD and
Precision NTC Thermistor IC Data Sheet
Texas Instruments, RTD Class-AA Replacement With High-Accuracy Digital Temperature Sensors in
Field Transmitters Application Report
SNAA267A – April 2015 – Revised May 2019
Submit Documentation Feedback
Semiconductor Temperature Sensors Challenge Precision RTDs and
Thermistors in Building Automation
Copyright © 2015–2019, Texas Instruments Incorporated
11
Revision History
www.ti.com
Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (April 2015) to A Revision .......................................................................................................... Page
•
•
•
12
Added the TMP117 to the abstract. ..................................................................................................... 1
Revised app report for clarity. ........................................................................................................... 1
Added section about the TMP117 device. ............................................................................................. 6
Revision History
SNAA267A – April 2015 – Revised May 2019
Submit Documentation Feedback
Copyright © 2015–2019, Texas Instruments Incorporated
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