TMP17FS Analog Devices Inc, TMP17FS Datasheet - Page 4

• Manufacturer Part Number: TMP17FS
• Description: IC TRANSDUC TEMP CUR OUT 8SOIC
• Manufacturer: Analog Devices Inc

Specifications of TMP17FS

• Rohs Status: RoHS non-compliant
• Sensing Temperature: -40°C ~ 105°C
• Output Type: Current
• Voltage - Supply: 4 V ~ 30 V
• Accuracy: ±2.5°C
• Package / Case: 8-SOIC (0.154", 3.90mm Width)

TMP17
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 pro-
portional 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 TPC 4.
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 trim-
ming, 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 in the Specifications table make the TMP17
easy to apply in a variety of diverse applications. To calculate a
total error budget in a given system, it is important to correctly
interpret the accuracy specifications, nonlinearity errors, the
response of the circuit to supply voltage variations, and the effect
of the surrounding thermal environment. 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 calibration
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 read directly from
the Specifications table. The error limits are a combination of
initial error, scale factor variation, and nonlinearity deviation
from the ideal 1 µA/K output. TPC 1 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 thermocouples),
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 2 is a plot of typical
TMP17 nonlinearity over the full rated temperature range.
–4–
TRIMMING FOR HIGHER ACCURACY
Calibration error at 25 C can be removed with a single tem-
perature trim. Figure 3 shows how to adjust the TMP17’s scale
factor in the basic voltage output circuit.
To trim the circuit, the temperature must be measured by a refer-
ence sensor and the value of R should be adjusted so the output
(V
should be implemented as close as possible to the temperature
for which highest accuracy is desired. 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 4 illustrates
the effect on total error when using this technique.
Figure 3. Basic Voltage Output (Single Temperature Trim)
OUT
Figure 4. Effect of Scale Factor Trim on Accuracy
) corresponds to 1 mV/K. Note that the trim procedure
–0.1
–0.2
–0.3
–0.5
–1.0
0.2
0.1
1.0
0.5
0
0
–40
–40
–25
–25
Figure 2. Nonlinearity Error
100
950
R
0
0
–
+V
+
TMP17
TEMPERATURE – C
TEMPERATURE – C
TYPICAL NONLINEARITY
25
25
AFTER SINGLE
TEMPERATURE
CALIBRATION
V
WITHOUT TRIM
OUT
+
–
ACCURACY
= 1mV/K
70
REV. A
105
105