AD22100K AD [Analog Devices], AD22100K Datasheet - Page 5

• Manufacturer Part Number: AD22100K
• Description: Voltage Output Temperature Sensor with Signal Conditioning
• Manufacturer: AD [Analog Devices]

REV. B
For example, with V
output of the AD22100 will be 1.9375 V. At V
nominal output will be 2.1313 V, an increase of 193.75 mV.
A proportionality error of 1% is applied to the 193.75 mV,
yielding an error term of 1.9375 mV. This error term translates
to a variation in output voltage of 2.1293 V to 2.3332 V. A
1.94 mV error at the output is equivalent to about 0.08 C error
in accuracy.
If we substitute 150 C for 25 C in the above example, then the
error term translates to a variation in output voltage of 5.2203 V
to 5.2298 V. A 4.75 mV error at the output is equivalent to
about 0.19 C error in accuracy.
MOUNTING CONSIDERATIONS
If the AD22100 is thermally attached and properly protected, it
can be used in any measuring situation where the maximum
range of temperatures encountered is between –50 C and
+150 C. Because plastic IC packaging technology is employed,
excessive mechanical stress must be avoided when fastening the
device with a clamp or screw-on heat tab. Thermally conduc-
tive epoxy or glue is recommended for typical mounting condi-
tions. In wet or corrosive environments, an electrically isolated
metal or ceramic well should be used to shield the AD22100.
Because the part has a voltage output (as opposed to current), it
offers modest immunity to leakage errors, such as those caused
by condensation at low temperatures.
THERMAL ENVIRONMENT EFFECTS
The thermal environment in which the AD22100 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 vari-
ables; the power consumption of the AD22100 and the thermal
resistance between the chip and the ambient environment
Self-heating error in C can be derived by multiplying the power
dissipation by
for surroundings with different heat sinking capacities, it is nec-
essary to specify
how the magnitude of self-heating error varies relative to the en-
vironment. A typical part will dissipate about 2.2 mW at room
temperature with a 5 V supply and negligible output loading. In
still air, without a “heat sink,” the table below indicates a
190 C/W, yielding a temperature rise of 0.4 C. Thermal rise
will be considerably less in either moving air or with direct
physical connection to a solid (or liquid) body.
Medium
Aluminum Block
Moving Air**
Still Air
**1200 CFM.
*The time constant is defined as the time to reach 63.2% of the final
Without Heat Sink
Without Heat Sink
temperature change.
Table I. Thermal Resistance (TO-92)
JA.
JA
Because errors of this type can vary widely
under several conditions. Table I shows
S
= 5.0 V, and T
JA
( C/Watt)
60
75
190
A
= +25 C, the nominal
S
= 5.50 V, the
(sec) *
2
3.5
15
JA
JA
.
of
–5–
Response of the AD22100 output to abrupt changes in ambient
temperature can be modeled by a single time constant expo-
nential function. Figure 7 shows typical response time plots for
a few media of interest.
The time constant is dependent on
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
media. Copper printed circuit board connections were
neglected in the analysis; however, they will sink or conduct
heat directly through the AD22100’s solder plated copper leads.
When faster response is required, a thermally conductive grease
or glue between the AD22100 and the surface temperature
being measured should be used.
MICROPROCESSOR A/D INTERFACE ISSUES
The AD22100 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 AD22100 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
AD22100 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 AD22100 and
the ADC use the supply as their reference. The nominal
AD22100 signal range of 0.25 V to 4.75 V (–50 C to +150 C)
makes good use of the input range of a 0 V to 5 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).
An 8-bit ADC with a reference of 5 V will have a least signifi-
cant bit (LSB) size of 5 V/256 = 19.5 mV. This corresponds to
a nominal resolution of about 0.87 C.
100
90
80
70
60
50
40
30
20
10
0
0
10
MOVING
AIR
Figure 7. Response Time
ALUMINUM
BLOCK
20
STILL AIR
30
40
TIME – sec
50
60
JA
and the thermal
70
AD22100
80
90
100