ADT7420 Analog Devices, ADT7420 Datasheet - Page 23

• Manufacturer Part Number: ADT7420
• Description: 16-Bit Digital I2C Temperature Sensor
• Manufacturer: Analog Devices

Preliminary Technical Data
APPLICATIONS INFORMATION
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a specified
accuracy is a function of the thermal mass of the sensor and the
thermal conductivity between the sensor and the object being
sensed. Thermal mass is often considered equivalent to capaci-
tance. Thermal conductivity is commonly specified using the
symbol, Q, 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. The time required for
the part to settle to the desired accuracy is dependent on the
thermal contact established in a particular application and the
equivalent power of the heat source. In most applications, it is
best to determine the settling time empirically.
SUPPLY DECOUPLING
Decouple the ADT7420 with a 0.1 µF ceramic capacitor
between V
the ADT7420 is mounted remotely from the power supply.
Precision analog products, such as the ADT7420, require a
well-filtered power source. Because the ADT7420 operates
from a single supply, it may seem convenient to 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 hundreds of millivolts in amplitude due
to wiring resistance and inductance.
If possible, the ADT7420 should be powered directly from the
system power supply. This arrangement, shown in Figure 20,
isolates the analog section from the logic switching transients.
Even if a separate power supply trace is not available, generous
supply bypassing reduces supply-line induced errors. Local
supply bypassing consisting of a 0.1 µF ceramic capacitor
is critical for the temperature accuracy specifications to be
achieved. This decoupling capacitor must be placed as close
as possible to the V
TTL/CMOS
CIRCUITS
LOGIC
Figure 20. Use of Separate Traces to Reduce Power Supply Noise
DD
and GND. This is particularly important when
SUPPLY
POWER
DD
pin of the ADT7420.
0.1µF
ADT7420
Rev. PrA | Page 23 of 2
TEMPERATURE MONITORING
The ADT7420 is ideal for monitoring the thermal environment
within electronic equipment. For example, the surface-mounted
package accurately reflects the exact thermal conditions that
affect nearby integrated circuits.
The ADT7420 measures and converts the temperature at the
surface of its own semiconductor chip. When the ADT7420 is
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT7420
must be considered.
When the thermal impedance is determined, the temperature
of the heat source can be inferred from the ADT7420 output.
As much as 60% of the heat transferred from the heat source to
the thermal sensor on the ADT7420 die is discharged via the
copper tracks, the package pins, and the bond pads. Of the
pins on the ADT7420, the GND pin transfers most of the heat.
Therefore, to measure the temperature of a heat source, it is
recommended that the thermal resistance between the GND pin
of the ADT7420 and the GND of the heat source be reduced as
much as possible.
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