adt7401rt-reel7 Analog Devices, Inc., adt7401rt-reel7 Datasheet - Page 10

• Manufacturer Part Number: adt7401rt-reel7
• Description: Smbus/i2c Compatible, ?0.5?c Accurate 12-bit Digital Temperature Sensor With Daisy Chain
• Manufacturer: Analog Devices, Inc.

ADT7401
APPLICATION HINTS
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a
specified accuracy is a function of the thermal mass of, and the
thermal conductivity between, the sensor and the object being
sensed. Thermal mass is often considered equivalent to
capacitance. Thermal conductivity is commonly specified using
the symbol θ, and can be thought of as thermal resistance. It is
commonly specified in units of degrees per watt of power
transferred across the thermal joint. Thus, the time required for
the ADT7401 to settle to the desired accuracy is dependent on
the package, the thermal contact established in that particular
application, and the equivalent power of the heat source. In
most applications, the settling time is probably best determined
empirically.
SELF-HEATING EFFECTS
The temperature measurement accuracy of the ADT7401 may
be degraded in some applications due to self-heating. Errors
introduced are from the quiescent dissipation and power
dissipated when converting. The magnitude of these
temperature errors is dependent on the thermal conductivity of
the ADT7401 package, the mounting technique, and effects of
airflow. Static dissipation in the ADT7401 is typically 10 µW
operating at 3.3 V with no load. In the 6 lead SOT-23 package
mounted in free air, this accounts for a temperature increase
due to self-heating of:
It is recommended that current dissipated through the device is
kept to a minimum as it has a proportional affect on the
temperature error.
SUPPLY DECOUPLING
The ADT7401 should be decoupled with a 0.1 µF ceramic
capacitor between V
if the ADT7401 is mounted remote from the power supply.
Precision analog products, such as the ADT7401, require a well
filtered power source. Since the ADT7401 operates from a
single supply, it seems convenient to simply tap into the digital
logic power supply. Unfortunately, the logic supply is often a
switch-mode design, which generates noise in the 20 kHz to 1
MHz range. In addition, fast logic gates can generate glitches
hundred of millivolts in amplitude due to wiring resistance and
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and GND. This is particularly important
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Rev. PrE | Page 10 of 12
inductance.
If possible, the ADT7401 should be powered directly from the
system power supply. This arrangement, shown in Figure 15,
will isolate the analog section from the logic switching
transients. Even if a separate power supply trace is not available,
however, generous supply bypassing will reduce supply-line
induced errors. Local supply bypassing consisting of a 0.1 µF
ceramic capacitor is recommended.
TEMPERATURE MONITORING
The ADT7401 is ideal for monitoring the thermal environment
within electronic equipment. For example, the surface mounted
package will accurately reflect the exact thermal conditions
which affect nearby integrated circuits.
The ADT7401 measures and converts the temperature at the
surface of their own semiconductor chip. When the ADT7401 is
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT7401
must be considered. Often, a thermocouple or other
temperature sensor is used to measure the temperature of the
source while the ADT7401’s temperature is monitored. Once
the thermal impedance is determined, the temperature of the
heat source can be inferred from the ADT7401 output.
One example of using the ADT7401’s unique properties is in
monitoring a high power dissipation microprocessor. The
adt7401, in a surface mount package, is mounted directly
beneath the microprocessor’s pin grid array (PGA) package.
Figure 15. Use Separate Traces to Reduce Power Supply Noise
Preliminary Technical Data