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TMP17 Datasheet(PDF) 4 Page - Analog Devices

Part No. TMP17
Description  Low Cost, Current Output Temperature Transducer
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TMP17 Datasheet(HTML) 4 Page - Analog Devices

   
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TMP17
REV. 0
–4–
THEORY OF OPERATION
The TMP17 uses a fundamental property of silicon transistors
to realize its temperature proportional output. If two identical
transistors are operated at a constant ratio of collector current
densities, r, then the difference in base-emitter voltages will be
(kT/q)(ln r). Since both k, Boltzmann’s constant, and q, the
charge of an electron, are constant, the resulting voltage is
directly Proportional To Absolute Temperature (PTAT). In the
TMP17 this difference voltage is converted to a PTAT current
by low temperature coefficient thin film resistors. This PTAT
current is then used to force the total output current to be
proportional to degrees Kelvin. The result is a current source
with an output equal to a scale factor times the temperature (K)
of the sensor. A typical V-I plot of the circuit at 125
°C and the
temperature extremes is shown in Figure 6.
Factory trimming of the scale factor to 1
µA/K is accomplished
at the wafer level by adjusting the TMP17’s temperature
reading so it corresponds to the actual temperature. During
laser trimming the IC is at a temperature within a few degrees of
25
°C and is powered by a 5 V supply. The device is then
packaged and automatically temperature tested to specification.
FACTORS AFFECTING TMP17 SYSTEM PRECISION
The accuracy limits given on the Specifications page for the
TMP17 make it easy to apply in a variety of diverse applica-
tions. To calculate a total error budget in a given system it is
important to correctly interpret the accuracy specifications, non-
linearity errors, the response of the circuit to supply voltage
variations and the effect of the surrounding thermal environ-
ment. As with other electronic designs external component
selection will have a major effect on accuracy.
CALIBRATION ERROR, ABSOLUTE ACCURACY AND
NONLINEARITY SPECIFICATIONS
Two primary limits of error are given for the TMP17 such that
the correct grade for any given application can easily be chosen
for the overall level of accuracy required. They are the calibra-
tion accuracy at
25
°C, and the error over temperature from
40
°C to 105°C. These specifications correspond to the
actual error the user would see if the current output of a
TMP17 were converted to a voltage with a precision resistor.
Note that the maximum error at room temperature or over an
extended range, including the boiling point of water, can be
directly read from the specifications table. The error limits are a
combination of initial error, scale factor variation and non-
linearity deviation from the ideal 1
µA/K output. Figure 2
graphically depicts the guaranteed limits of accuracy for a
TMP17GS.
The TMP17 has a highly linear output in comparison to older
technology sensors (i.e., thermistors, RTDs and thermo-
couples), thus a nonlinearity error specification is separated
from the absolute accuracy given over temperature. As a
maximum deviation from a best-fit straight line this specification
represents the only error that cannot be trimmed out. Figure 8
is a plot of typical TMP17 nonlinearity over the full rated
temperature range.
0.2
0.1
0
0.1
0.2
TEMPERATURE – C
40
105
25
0
25
70
TYPICAL NONLINEARITY
Figure 8. Nonlinearity Error (TMP17)
TRIMMING FOR HIGHER ACCURACY
Calibration error at
25
°C can be removed with a single
temperature trim. Figure 9 shows how to adjust the TMP17’s
scale factor in the basic voltage output circuit.
+V
TMP17
R
100
950
VOUT = 1mV/K
Figure 9. Basic Voltage Output (Single Temperature Trim)
To trim the circuit the temperature must be measured by a
reference sensor and the value of R should be adjusted so the
output (V
OUT) corresponds to 1 mV/K. Note that the trim
procedure should be implemented as close as possible to the
temperature highest accuracy is desired for. In most applications
if a single temperature trim is desired it can be implemented
where the TMP17 current-to-output voltage conversion takes
place (e.g., output resistor, offset to an op amp). Figure 10
illustrates the effect on total error when using this technique.
AFTER SINGLE
TEMPERATURE
CALIBRATION
ACCURACY
WITHOUT TRIM
1.0
0.5
25
105
25
0
0.5
1.0
TEMPERATURE – C
40
Figure 10. Effect of Scale Factor Trim on Accuracy


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