```Application Report
SBAA073B – April 2002 – Revised November 2017
Measuring Temperature With the
Saeid Jannesari and Jim Todsen
ABSTRACT
measurements. Included among the analog features is a diode inside the input MUX. Coupled with the
high-resolution, analog-to-digital converter (ADC), the diode provides a convenient means of measuring
temperature. This application report discusses some of the considerations and techniques in using this
diode. The last section presents some measurement data to help illustrate the performance that can be
expected.
1
2
3
Contents
Measuring Temperature With a Diode .................................................................................... 2
How To Use the ADS1216 Diode.......................................................................................... 5
Results ........................................................................................................................ 7
List of Figures
1
IDIODE vs VDIODE ................................................................................................................. 2
2
VDIODE vs Temperature ....................................................................................................... 3
3
Slope of VDIODE vs Temperature ............................................................................................ 3
4
Block Diagram of ADS1216 Input Circuitry ............................................................................... 5
5
Error vs Temperature, Single Measurement Technique ................................................................ 7
6
Error vs Temperature, Differential Measurement Technique
..........................................................
7
List of Tables
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Measuring Temperature With a Diode
1
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Measuring Temperature With a Diode
To understand how to use the diode to measure temperature, it helps to briefly review some of the key
equations. For a good, detailed description of diode operation, see The PN Junction Diode by Gerold W.
Neudeck. The current through a diode (IDIODE) can be approximated by Equation 1:
IDIODE
IS e
VDIODE
nVT
where
•
•
•
•
VT
IS is a constant that depends on the area of the diode and its temperature among other things.
VDIODE is the forward voltage across the diode.
n is a constant usually close to 1.
VT is the thermal voltage given by Equation 2
(1)
KT
q
where
•
•
•
K is Boltzman’s constant.
T is the absolute temperature (in kelvin).
q is the charge of an electron.
(2)
Figure 1 shows the typical plot of IDIODE vs VDIODE (the ADS1216 diode was used in this and all other plots).
1000
IDIODE (µA)
800
600
400
200
0
0.0
0.2
0.4
0.6
0.8
VDIODE (V)
Figure 1. IDIODE vs VDIODE
2
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Measuring Temperature With a Diode
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Now consider the temperature-dependent terms of Equation 1. Equation 2 gives VT dependence as a
proportion to the absolute temperature. The other temperature-dependent term, IS, roughly doubles every
5°C. Together, these two terms produce a net change in the voltage across the diode of approximately –2
mV/°C for a diode biased with a constant current. This relationship can be used to measure temperature
by simply measuring the voltage across the diode; just bias the diode with a constant current and measure
the diode’s voltage. Figure 2 shows the diode voltage vs temperature for 2 μA and 10 μA bias currents.
0.9
V DIODE (V)
0.8
0.7
IDIODE = 10 µA
0.6
IDIODE = 2 µA
0.5
0.4
±
±
±
10
30
50
70
90
Temperature (°C)
Figure 2. VDIODE vs Temperature
In practice, there are a few issues to consider when using this technique. First, manufacturing process
variations between diodes create subsequent variations in the diode voltages. The ADS1216 has a 6-σ
statistical variation in diode voltage that exceeds 5 mV. In other words, if a large sample of ADS1216
diodes was taken, and each of them biased with the same current at the same temperature, and
measured the voltage across the diodes, the minimum and maximum readings would differ by more than 5
mV. To remove this uncertainty and the errors in the derived temperature measurement that would result,
each diode must be calibrated to determine the voltage at a known temperature.
In addition to variations in diode voltage, there are variations in the slope of this voltage; that is, the
change in voltage with respect to temperature. When measuring temperature, think of the room
temperature variations as an offset error and the slope variations as a gain error. To correct for the slope
error, a second calibration point is needed at a different temperature.
Finally, the slope of the diode voltage versus temperature is not perfectly linear. Figure 3 shows the
derivative of VDIODE for the 2-μA biased diode from Figure 2. Notice that the slope increases as the
temperature increases. This nonlinearity will cause errors even when the diode voltage versus
temperature is calibrated at two points.
Slope of V DIODE (mV/°C)
±
±
±
±
±
±
±
±
±
10
30
50
70
90
Temperature (°C)
Figure 3. Slope of VDIODE vs Temperature
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Measuring Temperature With a Diode
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To devise an improved scheme for measuring temperature, consider what happens when rearranged
Equation 1 to give the diode voltage as a function of bias current, as shown in Equation 3:
§I
·
nVT ln ¨ DIODE ¸
VDIODE
(3)
Assuming the diode is biased at two different currents, I1 and I2, and measure the resulting differential
voltage. The resulting voltage is derived using Equation 3, and is shown as Equation 4:
'VDIODE
V2
V1
§I ·
§I ·
nVT ln ¨ 2 ¸ nVT ln ¨ 1 ¸
I
(4)
Now combine the natural log functions and cancel the common terms so that Equation 3 becomes
Equation 5:
'VDIODE
§I ·
nVT ln ¨ 2 ¸
(5)
or Equation 6:
'VDIODE DT
where
•
D
α is given in Equation 7
(6)
K §I ·
n ln ¨ 2 ¸
(7)
Notice that ΔVDIODE is proportional to absolute temperature. Measuring ΔVDIODE allows the temperature to
be directly determined. IS drops out of α in Equation 7 so the variations in this parameter are of no
concern. If calibration is required, now only one reading is needed to find the slope of Equation 6 (α).
Finally, the absolute bias currents drop out of Equation 5, leaving only the ratio of currents. This ratio of
currents reduces the requirements on the biasing circuitry. The next section describes how to use the
ADS1216 onboard current digital-to-analog converters (IDACs) to bias the diode. Removing the sensitivity
to absolute bias currents allows a low tolerance RDAC resistor to be used to set the current. See the
4
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How To Use the ADS1216 Diode
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2
How To Use the ADS1216 Diode
Figure 4 shows a block diagram of the relevant circuitry inside of the ADS1216 when using the onboard
diode. The switches that connect the diode are controlled by the MUX register. See the ADS1216 data
sheet for more details on the registers. Writing FFH to this register closes switches SDP, SDN, and SDI
and opens all the switches to the input pins AIN0-AINCOM (S0P, S0N ... S7P, S7N, SCP, SCN). With the
closing of SDI, IDAC1 output connects to the diode allowing IDAC1 to bias the diode during the
temperature measurements. Notice that IDAC1 output always remains connected to the output pin.
Remember to account for the effects of any circuitry on this pin when measuring temperature. If possible,
leave IDAC1 output pin disconnected when using the diode.
SOP
AIN0
SON
SIP
AIN1
SIN
S2P
AIN2
S2N
S3P
AIN3
S3N
+
S4P
AIN4
To Buffer, PGA,
S4N
±
S5P
AIN5
S5N
SDN
SDP
SDI
IDAC1
S6P
AIN6
S6N
S7P
AIN7
S7N
SCP
AINCOM
SCN
IDAC1
Figure 4. Block Diagram of ADS1216 Input Circuitry
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How To Use the ADS1216 Diode
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To measure temperature using Equation 6 from the previous section, first prepare the ADC. For the best
results, set the programmable gain amplifier (PGA) to 1, enable the buffer, and perform a self-calibration.
After the ADC is ready, configure the diode by writing FFh to the MUX register. If possible, use a 3-V
digital supply to reduce the ADS1216 digital power dissipation that causes self-heating. Now apply the first
current bias by setting IDAC1 value; 2 μA works well. You can use other values, just make sure to keep
the currents low (< 30 μA) to minimize errors from IR drops. After setting IDAC1, allow the ADC digital
filter to settle, then read the first data. Additional readings can be averaged to reduce the noise, though if
the decimation ratio is above 300, additional readings are probably not necessary. Repeat this process for
the second current bias: set IDAC1 to the new current level (10 μA works well); wait for the filter to settle,
then read the second data. The measurement is now complete. Use the two data readings in Equation 6
to calculate temperature. For the ADS1216, α is typically 7000 K/V. Calibration can be used to find a more
accurate value. To calibrate, measure a known temperature and use Equation 6 to find α. Remember,
Equation 6 uses absolute temperature in kelvins (K). Subtract 273 to convert to °C.
Steps:
1. Set PGA = 1 and enable the buffer
2. Self-calibrate
3. Connect the diode: write FFh to the MUX register
4. Set IDAC1 to 2 μA
5. After the digital filter has settled, read data (VD1)
6. Set IDAC1 to 10 μA
7. After the digital filter has settled, read data (VD2)
8. Calculate temperature as shown in Equation 8 (for best results, determine α for each ADS1216):
1
T(K)
VD2 VD1
D
6
(8)
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Results
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Results
An ADS1216 was placed in an oven with a calibrated thermometer mounted nearby as the temperature
was swept from –40°C to +85°C. Two different techniques were used to measure temperature.
Afterwards, the error of each technique was measured.
For the first technique (single measurement technique), the diode was biased at 2 μA and the voltage
across the diode was measured. The data was calibrated at two temperatures, –10°C and +50°C. Figure 5
shows the absolute error.
0.5
Error (°C)
0
±
±
±
±
±
±
10
30
50
70
90
Temperature (°C)
Figure 5. Error vs Temperature, Single Measurement Technique
For the second technique (differential measurement technique), the differential voltage measurement in
Equation 6 was used, as described in the previous section. The data were calibrated at 25°C to give α =
6971°C/V. Figure 6 shows the absolute error.
Error (°C)
0.5
0
±
±
±
±
10
30
50
70
90
Temperature (°C)
Figure 6. Error vs Temperature, Differential Measurement Technique
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