Datasheet

ADT75
Rev. 0 | Page 21 of 24
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 capacitance. 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 ADT75 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, it is best to
determine empirically the settling time.
SELF-HEATING EFFECTS
The temperature measurement accuracy of the ADT75 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 ADT75 package, the mounting technique, and the effects of
airflow. At 25°C, static dissipation in the ADT75 is typically
798.6 µW operating at 3.3 V. In the 8-lead MSOP package
mounted in free air, this accounts for a temperature increase
due to self-heating of
T = P
DISS
× θ
JA
= 798.6 µW × 205.9°C/W = 0.16°C
It is recommended that current dissipated through the device be
kept to a minimum, because it has a proportional effect on the
temperature error.
Using the power-down mode can reduce the current dissipated
through the ADT75 subsequently reducing the self-heating
affect. When the ADT75 is in power-down mode and operating
at 25°C, static dissipation in the ADT75 is typically 78.6 µW
with V
DD
= 3.3 V and the power-up/conversion rate is 1 SPS
(sample per second). In the 8-lead MSOP package mounted in
free air, this accounts for a temperature increase due to self-
heating of
T = P
DISS
× θ
JA
= 78.6 µW × 205.9°C/W = 0.016°C
SUPPLY DECOUPLING
The ADT75 should be decoupled with a 0.1 µF ceramic
capacitor between V
DD
and GND. This is particularly important
when the ADT75 is mounted remotely from the power supply.
Precision analog products, such as the ADT75, require a well-
filtered power source. Because the ADT75 operates from a
single supply, it might seem convenient to tap into the digital
logic power supply. However, 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 ADT75 should be powered directly from the
system power supply. This arrangement, shown in
Figure 22,
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 ADT75 V
DD
pin.
05326-021
0.1μF
ADT75
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPPLY
Figure 22. Use Separate Traces to Reduce Power Supply Noise