ADT7408CCPZ-R2 AD [Analog Devices], ADT7408CCPZ-R2 Datasheet - Page 21

• Manufacturer Part Number: ADT7408CCPZ-R2
• Description: +-2 C Accurate, 12-Bit Digital Temperature Sensor
• Manufacturer: AD [Analog Devices]

APPLICATION 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. Thus, the time required for
the ADT7408 to settle to the desired accuracy is dependent on
the package selected, the thermal contact established in that
particular application, and the equivalent power of the heat source.
In most applications, the settling time is best determined
empirically.
SELF-HEATING EFFECTS
The temperature measurement accuracy of the ADT7408 might
be degraded in some applications due to self-heating. Errors can
be introduced from the quiescent dissipation and power dissipated
when converting. The magnitude of these temperature errors is
dependent on the thermal conductivity of the ADT7408 package,
the mounting technique, and the effects of airflow. At 25°C,
static dissipation in the ADT7408 is typically 778 μW operating
at 3.3 V. In the 8-lead LFCSP_VD package mounted in free air,
this accounts for a temperature increase due to self-heating of
Current dissipated through the device should be kept to a
minimum by applying shutdown when the device can be put in
the idle state, because it has a proportional effect on the
temperature error.
SUPPLY DECOUPLING
The ADT7408 should be decoupled with a 0.1 μF ceramic
capacitor between V
when the ADT7408 is mounted remotely from the power supply.
Precision analog products, such as the ADT7408, require a well-
filtered power source. Because the ADT7408 operates from a
single supply, it might 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 mV in
amplitude due to wiring resistance and inductance.
If possible, the ADT7408 should be powered directly from the
system power supply. This arrangement, shown in Figure 18,
isolates the analog section from the logic switching transients.
Even if a separate power supply trace is not available, however,
generous supply bypassing reduces supply-line-induced errors.
ΔT = P
DISS
× θ
JA
= 778 μW × 85°C/W = 0.066°C
DD
and GND. This is particularly important
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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 ADT7408 V
TEMPERATURE MONITORING
The ADT7408 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 ADT7408 measures and converts the temperature at the
surface of its own semiconductor chip. When the ADT7408 is
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT7408
must be considered. Often, a thermocouple or other temperature
sensor is used to measure the temperature of the source, while
the temperature is monitored by reading back from the ADT7408
temperature value register.
Once the thermal impedance is determined, the heat source
temperature can be inferred from the ADT7408 output. As
much as 60% of the heat transferred from the heat source to the
thermal sensor on the ADT7408 die is discharged via the copper
tracks, the package pins, and the bond pads. Of the pins on the
ADT7408, the GND pin (V
Therefore, when the temperature of a heat source is being
measured, thermal resistance between the ADT7408 V
and the heat source should be reduced as much as possible.
An example of the ADT7408’s unique properties is shown in
monitoring a high power dissipation DIMM module. Ideally,
the ADT7408 device should be mounted in the middle between
the two memory chips’ major heat sources (see Figure 19). The
ADT7408 produces a linear temperature output, while needing
only two I/O pins and requiring no external characterization.
LEFT
Figure 18. Using Separate Traces to Reduce Power Supply Noise
TTL/CMOS
CIRCUITS
LOGIC
BOTTOM
Figure 19. Locations of ADT7408 on DIMM Module
TOP
SO-DIMM THERMAL SENSOR LOCATIONS
SUPPLY
POWER
DD
SS
MIDDLE
pin) transfers most of the heat.
pin.
0.1µF
ADT7408
ADT7408
RIGHT
SS
pin