Texas Instruments | DN403 -- CC112x/CC120x On-Chip Temperature Sensor (Rev. D) | Application notes | Texas Instruments DN403 -- CC112x/CC120x On-Chip Temperature Sensor (Rev. D) Application notes

Texas Instruments DN403 -- CC112x/CC120x On-Chip Temperature Sensor (Rev. D) Application notes
Application Report
SWRA415D – October 2012 – Revised October 2018
CC112X/CC120X On-Chip Temperature Sensor
Bjarte Nystøyl and Torstein Ermesjø
ABSTRACT
This application report provides the necessary information to use the temperature sensor of the CC112X
and CC120X families. The temperature sensor is based on a proportional to absolute temperature (PTAT)
current from a bandgap cell fed to a resistor to generate a PTAT voltage. It is possible to read out the
temperature information as an analog voltage on a general-purpose input/output (GPIO) pin.
1
2
Contents
Analog Readout .............................................................................................................. 2
References ................................................................................................................... 6
List of Figures
1
GPIO1 Voltage vs Temperature ........................................................................................... 3
2
Temperature Error Due to Different Temperature Coefficients After Single-Point Calibration
3
....................
Typical GPIO1 Measurements vs Supply Voltage ......................................................................
4
5
List of Tables
1
Register Settings for Temperature Sensor ............................................................................... 2
2
Typical Temperature Sensor Parameters
................................................................................
2
Trademarks
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1
Analog Readout
1
Analog Readout
1.1
Operation
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The temperature sensor is activated using the register settings of Table 1, which makes the GBIAS output
a single-ended voltage measurement on GPIO1.
Table 1. Register Settings for Temperature Sensor
Register
Value
IOCFG1
0x80
ATEST
0x2A
ATEST_MODE
0x0C
GBIAS1
0x07
Setting IOCFG1 to 0x80 configures the GPIO1 pad into analog mode (digital GPIO input and output is
disabled). The remaining registers set up the ATEST (analog test) module to output the temperature value
as a PTAT voltage on the GPIO1.
1.2
Temperature Sensor Parameters
General Information
Value
Temperature sensor fitted from
Unit
-40 to +85
°C
Effect of supply voltage deviance
1.17
mV/VDD-V
Effect of supply voltage deviance
0.44
°C /VDD-V
Changes in the supply voltage affect the voltage of the GPIO pin, and the supply voltage must be stable in
order to get accurate temperature sensor readings.
Table 2. Typical Temperature Sensor Parameters
Technical Information
Value
Unit
Typical output voltage @ 0°C
727.42
mV
Typical output voltage @ 25°C
793.73
mV
Temperature coefficient
2.6598
mV/°C
Typical output voltage @ 0°C
728.55
mV
Typical output voltage @ 25°C
794.78
mV
Temperature coefficient
2.6733
mV/°C
Typical output voltage @ 0°C
730.62
mV
Typical output voltage @ 25°C
796.94
mV
Temperature coefficient
2.6773
mV/°C
VDD - 2 V
VDD - 3 V
VDD - 3.6 V
2
CC112X/CC120X On-Chip Temperature Sensor
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Analog Readout
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1.3
Calibration
As seen in Figure 1, the CC112X/CC120X temperature sensor voltage is highly linear, but for some
devices there is an offset in the GPIO1 voltage from the typical (average) value that could potentially give
an error of up to ±10°C in the temperature reading. In order to ensure accurate temperature sensor
measurements, the sensor must be calibrated. There are two simple approaches depending on the
required accuracy level: single- and two-point calibration.
0.900
0.850
0.800
GPIO1 Voltage
Highest
0.750
Average
Lowest
0.700
0.650
0
10
20
30
40
50
Temperature (°C)
Figure 1. GPIO1 Voltage vs Temperature
1.4
Single-Point Calibration
This is a simple and fast approach that can be applied for applications targeting approximately ±1°C
accuracy within a limited temperature range around the temperature used for the single-point calibration,
or approximately ±2°C accuracy across the -40°C to +85°C temperature range.
1.4.1
Performing Single-Point Calibration
The calibration should be performed at the center of the temperature range in which the device will
operate. A given temperature, T, will be given as:
T = T CALIBRATION +
•
•
•
•
(V MEASURED - V CALIBRATION )
tC
(1)
TCALIBRATION is the temperature when the calibration is performed
tc is the temperature coefficient for the given supply voltage (see the typical temperature parameters in
Table 2)
VMEASURED is the voltage of the GPIO1 pin at a given temperature
VCALIBRATION is the GPIO1 voltage at the calibration temperature
Performing a single-point calibration removes the error caused by the device-specific voltage offset seen
in Figure 1. The temperature reading accuracy is then limited by the accuracy of the individual
temperature coefficients as the typical temperature coefficient is used in Equation 1.
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Analog Readout
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Figure 2 shows the maximum error in the temperature reading when using the lowest and highest
temperature coefficients out of 30 devices from different processing corners.
• Approximately ±2°C accuracy is possible across the -40°C to +85°C temperature range with singlepoint calibration and using the typical temperature coefficient in Table 2.
• Approximately ±1°C accuracy is possible across the temperature range defined by TCALIBRATION ± 25°C
with single-point calibration and using the typical temperature coefficient in Table 2.
Error
(°C)
2
Lowest
temperature
coefficient
1
0
-40
-20
0
20
40
60
80
Highest
temperature
coefficient
-1
-2
Temperature (°C)
Figure 2. Temperature Error Due to Different Temperature Coefficients After Single-Point Calibration
1.4.2
Single-Point Calibration Example
A CC112X/CC120X device is operated using at 3 V supply voltage. The temperature coefficient is typically
2.673 mV/°C and for each degree Celsius increase in temperature the GPIO1 voltage increases by 2.673
mV.
The device is calibrated at room temperature (25°C), and the GPIO1 voltage is measured to be 793.0 mV.
After changing the temperature, the GPIO1 voltage is measured to be 830.0 mV. This corresponds to a
temperature T of:
T = 25°C +
(830 mV - 793 mV )
2.673 mV / °C
(2)
T = 25°C + 13.84°C = 38.84°C
1.5
Two-Point Calibration
If the application requires better accuracy than given by the single-point calibration, a two-point calibration
must be used to correct for chip-to-chip variations in the temperature coefficients. As the sensor is highly
linear, a two-point calibration will ensure high accuracy across the full temperature range of the chip.
1.5.1
Performing Two-Point Calibration
Choose two calibration temperatures more than 10°C apart, called T0 and T1, and set the reference
voltage (VDD) to what it will be in the final product.
NOTE: Changes in the voltage supply will influence the temperature sensor output.
Measure the output from the GPIO1 pin (V0 and V1) at the corresponding temperatures.
The temperature coefficient has a typical value of 2.673 mV/°C. The exact coefficient (tc) for a given
device is calculated as:
V -V
tc = 1 0
T1- T
0
4
(3)
CC112X/CC120X On-Chip Temperature Sensor
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Analog Readout
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Using the exact coefficient, the measured voltage of the GPIO1 pin (VMEASURED), the temperature (T0) and
the GPIO1 voltage (V0) of the first calibration, the temperature, T, can be found as:
T =T0 +
1.5.2
(V MEASURED - V 0)
tC
(4)
Two-Point Calibration Example
A CC112X/CC120X device is operated using a 3 V supply voltage, and will have a typical temperature
coefficient of 2.673 mV/°C.
The device is calibrated at two temperatures: 0°C and 25°C (T0 and T1). The respective GPIO1 voltages
are measured to be 743.379 mV and 808.312 mV (V0 and V1). The exact temperature coefficient tc is
given as:
tc =
808.312 mV - 743.379 mV
= 2.5973 mV / °C
25°C - 0°C
(5)
At a given temperature T, the GPIO1 voltage is measured to be 921.465 mV. This corresponds to:
T = 0°C +
(921.465 mv - 743.379 mv )
= 68.57°C
2.5973 mv / °C
(6)
NOTE: Single-point calibration at 25°C, using the typical tc of 2.673 mV/°C, would in this case give a
temperature reading of 67.33°C, which would have an error of 1.24°C.
1.6
Change in Supply Voltage (VDD)
As seen in Figure 3, the voltage measured on the GPIO1 pin depends on the supply voltage. Changing
the supply voltage affects the measured voltage on the GPIO1 pin by typically 1.17 mV/V. This means that
if the supply voltage is decreased by 1 V, the voltage measured at the GPIO1-pin is typically 1.17 mV
lower.
0.950
GPIO1
Voltage
(V)
0.900
0.850
VDD 2V
VDD 3V
VDD 3.6V
0.800
0.750
0.700
0.650
0.600
-40
10
Temperature (°C)
60
Figure 3. Typical GPIO1 Measurements vs Supply Voltage
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5
References
2
References
1.
2.
3.
4.
6
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High Performance RF Transceiver for Narrowband Systems Data Sheet
High Performance Low Power RF Transceiver Data Sheet
Ultra-High Performance RF Narrowband Transceiver Data Sheet
CC112X/CC1175 Low-Power High Performance Sub-1 GHz RF Transceivers/Transmitter User's Guide
CC112X/CC120X On-Chip Temperature Sensor
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from C Revision (June 2014) to D Revision .................................................................................................... Page
•
•
Update was made in the Abstract of this document. ................................................................................. 1
Removed Section 2. ...................................................................................................................... 1
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Revision History
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