Data Sheet ADT75
Rev. B | Page 21 of 24
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 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 may 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
ΔT = 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 effect.
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
ΔT = P
DISS
× θ
JA
= 78.6 µW × 205.9°C/W = 0.016°C
SUPPLY DECOUPLING
Decouple the ADT75 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
may 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, power the ADT75 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. Place this
decoupling capacitor 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
