AD22103
THERMAL ENVIRONMENT EFFECTS
neglected in the analysis; however, they will sink or conduct
heat directly through the AD22103’s solder plated copper leads.
When faster response is required, a thermally conductive grease
or glue between the AD22103 and the surface temperature
being measured should be used.
The thermal environment in which the AD22103 is used deter-
mines two performance traits: the effect of self-heating on accu-
racy and the response time of the sensor to rapid changes in
temperature. In the first case, a rise in the IC junction tempera-
ture above the ambient temperature is a function of two variables;
the power consumption of the AD22103 and the thermal resis-
tance between the chip and the ambient environment θJA. Self-
heating error in degrees Celsius can be derived by multiplying
the power dissipation by θJA. Because errors of this type can vary
widely for surroundings with different heat sinking capacities, it
is necessary to specify θJA under several conditions. Table I
shows how the magnitude of self-heating error varies relative to
the environment. A typical part will dissipate about 1.5 mW at
room temperature with a 3.3 V supply and negligible output
loading. In still air, without a “heat sink,” the table below indi-
cates a θJA of 190°C/W, yielding a temperature rise of 0.285°C.
Thermal rise will be considerably less in either moving air or
with direct physical connection to a solid (or liquid) body.
MICROPROCESSOR A/D INTERFACE ISSUES
The AD22103 is especially well suited to providing a low cost
temperature measurement capability for microprocessor/
microcontroller based systems. Many inexpensive 8-bit micro-
processors now offer an onboard 8-bit ADC capability at a mod-
est cost premium. Total “cost of ownership” then becomes a
function of the voltage reference and analog signal conditioning
necessary to mate the analog sensor with the microprocessor
ADC. The AD22103 can provide an ideal low cost system by
eliminating the need for a precision voltage reference and any
additional active components. The ratiometric nature of the
AD22103 allows the microprocessor to use the same power sup-
ply as its ADC reference. Variations of hundreds of millivolts in
the supply voltage have little effect as both the AD22103 and
the ADC use the supply as their reference. The nominal
AD22103 signal range of 0.25 V to 3.05 V (0°C to +100°C)
makes good use of the input range of a 0 V to 3.3 V ADC. A
single resistor and capacitor are recommended to provide im-
munity to the high speed charge dump glitches seen at many
microprocessor ADC inputs (see Figure 1).
Table I. Thermal Resistance (TO-92)
Medium
θJA (°C/Watt)
τ (sec)*
Aluminum Block
Moving Air**
60
2
Without Heat Sink
Still Air
75
3.5
An 8-bit ADC with a reference of 3.3 V will have a least signifi-
cant bit (LSB) size of 3.3 V/256 = 12.9 mV. This corresponds
to a nominal resolution of about 0.46°C/bit.
Without Heat Sink
190
15
*The time constant τ is defined as the time to reach 63.2% of the final
temperature change.
**1200 CFM.
USE WITH A PRECISION REFERENCE AS THE SUPPLY
VOLTAGE
Response of the AD22103 output to abrupt changes in ambient
temperature can be modeled by a single time constant τ expo-
nential function. Figure 7 shows typical response time plots for
a few media of interest.
While the ratiometric nature of the AD22103 allows for system
operation without a precision voltage reference, it can still be
used in such systems. Overall system requirements involving
other sensors or signal inputs may dictate the need for a fixed
precision ADC reference. The AD22103 can be converted to
absolute voltage operation by using a precision reference as the
supply voltage. For example, a 3.3 V reference can be used to
power the AD22103 directly. Supply current will typically be
500 µA which is usually within the output capability of the refer-
ence. A large number of AD22103s may require an additional
op amp buffer, as would scaling down a 10.00 V reference that
might be found in “instrumentation” ADCs typically operating
from ±15 V supplies.
100
ALUMINUM
BLOCK
90
MOVING
80
AIR
70
STILL AIR
60
50
40
30
20
10
0
USING THE AD22103 WITH ALTERNATIVE SUPPLY
VOLTAGES
Because of its ratiometric nature the AD22103 can be used at
other supply voltages. Its nominal transfer function can be recal-
culated based on the new supply voltage. For instance, if using the
AD22103 at VS = 5 V the transfer function would be given by:
0
10
20
30
40
50
60
70
80
90
100
TIME – sec
VS
5V
28 m
°C
5V
3.3 V
V
Figure 7. Response Time
is dependent on θJA and the specific heat
capacities of the chip and the package. Table I lists the effec-
tive (time to reach 63.2% of the final value) for a few different
VO
VO
=
0.25 V +
×TA
The time constant
τ
VS
5V
42.42 mV
°C
=
0.378 V +
×TA
τ
media. Copper printed circuit board connections were
REV. 0
–5–
5秒后页面跳转
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