Texas Instruments | Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor | Application notes | Texas Instruments Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor Application notes

Texas Instruments Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor Application notes
Application Report
SNOA969 – November 2017
Replacing Resistance Temperature Detectors with the
TMP116 Temp Sensor
Lakeishia Brown
ABSTRACT
The TMP116 is a low-power, high accuracy local digital temperature sensor. In this application note we
compare the usage of the TMP116 with that of a platinum RTD (resistance temperature detector). RTDs
are commonly used for their high accuracy, but here we show that over the temperature range supported
by the TMP116, the TMP116 provides greater and more consistent accuracy at lower power, cost and
complexity.
1
2
3
4
Contents
Resistance Temperature Detector (RTD) Introduction ..................................................................
TMP116 Local Temperature Sensor Introduction ......................................................................
RTD and TMP116 Comparison ............................................................................................
Conclusion ....................................................................................................................
1
2
2
5
List of Figures
1
A 2-wire RTD Connection Block Diagram ................................................................................ 3
2
A 3-wire RTD Connection Block Diagram ................................................................................ 3
3
A 4-wire RTD Connection Block Diagram
4
The TMP116 Block Diagram ............................................................................................... 4
5
Comparing the Accuracy of an RTD to the TMP116 .................................................................... 5
...............................................................................
4
List of Tables
Trademarks
All trademarks are the property of their respective owners.
1
Resistance Temperature Detector (RTD) Introduction
A resistance temperature detector (RTD) is a passive circuit element whose resistance increases as
temperature increases. They are generally constructed using platinum, copper, or nickel, and one major
advantage of RTDs is that they support a wide span of temperature, ranging from -200°C to +850°C. The
accuracy limits of an RTD are defined by the class, or grade, of the RTD. The industry standard for
platinum RTDs include Class B, Class A, 1/3 DIN, and 1/10 DIN with Class B being the least accurate,
and 1/10 DIN being the most accurate. However, for each class, the accuracy varies as the temperature
changes.
The characteristics of platinum, copper, or nickel determine the linear approximation of resistance vs
temperature within the 0°C to 100°C temperature range. The platinum RTD is known for its strong linearity
and repeatability characteristic. Given that the TMP116 carries similar characteristics, it is a suitable
replacement for the platinum RTD. Platinum, which is generally the most commonly used metal in RTDs,
has an alpha value of α=0.003925 Ω/Ω/°C. Each metal carries its own alpha value.
The following equation is used to find α:
R100qC R0qC
a
y 100qC
R0qC
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TMP116 Local Temperature Sensor Introduction
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where
•
•
R0°C is element resistance at 0°C
R100°C is element resistance at 100°C
(1)
Using the alpha value, the resistance change (∆R) with response to the temperature change (∆T) can be
determined.
The following equation is used to determine the resistance change:
∆R=αRo ∆T
where
•
•
•
α is Temperature Coefficient of Resistance (TCR) in Ω/Ω/°C
Ro is the nominal sensor resistance at 0°C
∆T is the temperature change from 0°C in °C
(2)
Because α is small, the change in resistance for small temperature changes is also small which is why the
RTD solutions require additional gain and high resolution analog to digital converters. As Equation 2
shows, the output is linear to the resistance, and therefore RTDs are offered with different resistance
values such as the PT100 and PT1000 platinum RTDs with a resistance of 100Ω and 1000Ω respectively.
2
TMP116 Local Temperature Sensor Introduction
The TMP116 temperature sensor operates from -55°C to +150°C with high accuracy across the entire
temperature range. It contains I2C and SMBus-interface communication, as well as an integrated
EEPROM memory. There is no calibration required for the TMP116, and minimal current is consumed
which minimizes self-heating. The TMP116 is typically found in applications with a heavy focus on high
accuracy.
A BJT transistor is used to measure the temperature of the TMP116. The base and the collector of the
BJT are connected which allows it to act as a diode, and therefore utilizes the diode equation, shown in
Equation 3, to internally determine the temperature of the device.
§ nkT ·
'VBE ¨
¸ ln r
© q ¹
where
•
•
•
•
•
•
η is the ideality factor
k is the Boltzmann's constant
T is the temperature in Kelvin
q is the charge of the electron
r=Ic1/Ic2 is the ratio of the excitation currents
ΔVbe is the change in the voltage across the base to the emitter
(3)
To find the change in the voltage across the base and the emitter, two excitation currents are applied to
the BJT. One current produces one voltage value, and the other current produces the second voltage
value. The difference between both voltages is then measured by the internal analog-to-digital converter,
and converted to the temperature of the device.
3
RTD and TMP116 Comparison
3.1
Resistance Temperature Detector (RTD)
The RTD device contains a 2-wire, 3-wire, or 4-wire connection. The more wires that exist within the RTD,
the lower the resistance errors the RTD signal-chain will have. The RTD errors are explained more in
details below.
A 2-wire connection RTD experiences a voltage drop across the wires due to the resistance of the wires
as the excitation currents are applied. The voltage drop is included in the total voltage calculated within
the ADC, and therefore, it produces an error in the calculation of the temperature proportional to the
voltage drop. The 2-wire connection RTD is shown below in Figure 1.
2
Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor
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V+
ISOURCE
RL
+
VMEAS
PGA
û ADC
±
RL
RBIAS
Figure 1. A 2-wire RTD Connection Block Diagram
A 3-wire RTD, shown in Figure 2, also experiences a voltage drop across the wire. However, the third wire
connected to the RTD cancels out the voltage drop from the calculation as long as the excitation currents
are equal, and the wires are the same length.
V+
ISOURCE1
V+
ISOURCE2
RL
+
VMEAS
RTD
PGA
û ADC
±
RL
RL
RBIAS
Figure 2. A 3-wire RTD Connection Block Diagram
A 4-wire connection RTD does not have any resistance errors due to the current not passing through the
wires in which the voltage is being measured. This diagram is shown in Figure 3.
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RTD and TMP116 Comparison
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V+
ISOURCE
RL
RL
+
VMEAS
PGA
û ADC
±
RL
RBIAS
RL
Figure 3. A 4-wire RTD Connection Block Diagram
A typical RTD signal-chain design uses the following components: a programmable gain amplifier (PGA),
a high resolution analog-to-digital converter (ADC), and a reference bias resistor. The voltage drop across
the RTD is extremely small, and therefore requires a PGA to increase the output signal prior to sampling
the voltage with the ADC. The high resolution ADC is used to convert the voltage signal to a digital signal.
The system utilizes an ADC of high resolution to maintain the required precision. The reference bias
resistor is used as a voltage divider in the 2-wire and 4-wire RTD, and as a ratio for the ADC in a 3-wire
RTD.
Once the resistance value is sampled, the temperature is determined in the microcontroller using a lookup table (LUT), or a best fit equation based on temperature coefficient parameters provided by the
manufacture.
3.2
TMP116 Local Temp Sensor
The TMP116 device is fully integrated and requires no external integrated circuit components. In Figure 4,
a detailed view of the TMP116, shows the ADC integration as well as all other required blocks. The output
of the TMP116 is the measured temperature and no additional functionality is required at the
microcontroller.
Temperature
GND
ALERT
1
2
3
Diode
Temp.
Sensor
Control
Logic
6
'6
A/D
Converter
Serial
Interface
5
Config.
and Temp.
Register
4
OSC
EEPROM
SCL
SDA
V+
ADD0
Figure 4. The TMP116 Block Diagram
4
Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor
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The temperature is also calculated internally, and then displayed on a Graphical User Interface (GUI) after
it is received by the microcontroller.
3.3
TMP116 Accuracy vs RTD Accuracy
The RTD accuracy varies depending on the RTD class, as well as the temperature. However, over the
operating range of the TMP116, the TMP116 has better accuracy.
1.4
1.2
RTD: Class A
RTD: Class B
TMP116
Accuracy (qC)
1
0.8
0.6
0.4
0.2
0
-50
-30
-10
10
30
50
70
90
Temperature (qC)
110
130
150
D001
Figure 5. Comparing the Accuracy of an RTD to the TMP116
The graph in Figure 5, shows that the TMP116 device has significantly better accuracy than the Class B
RTD. In addition, when compared to the Class A RTD, the TMP116 accuracy is also better over most of
the TMP116 operating temperature range. From 30°C to 125°C, the TMP116 outperforms the Class A
RTD by up to 10°C.
4
Conclusion
The TMP116 device is lower in cost, requires a much less complex design, and offers improved accuracy
over most of the temperature range. These characteristics make it an interesting alternative to RTDs when
accurate temperature measurements are needed in the -55°C to 125°C range.
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