ADT6401/ADT6402

Rev. 0 | Page 11 of 12

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 ADT6401/ADT6402 to settle to the desired accuracy is

dependent on the characteristics of the SOT-23 package, 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 ADT6401/

ADT6402 can 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 depends on the thermal conductivity of the

ADT6401/ADT6402 package, the mounting technique, and the

effects of airflow. At 25°C, static dissipation in the ADT6401/

ADT6402 is typically 99 μW operating at 3.3 V. In the 6-lead

SOT-23 package mounted in free air, this accounts for a tempera-

ture increase due to self-heating of

It is recommended that current dissipated through the device be

kept to a minimum because it has a proportional effect on the

temperature error.

SUPPLY DECOUPLING

The ADT6401/ADT6402 should be decoupled with a 0.1 μF

important when the ADT6401/ADT6402 are mounted remotely

from the power supply. Precision analog products such as the

ADT6401/ADT6402 require well-filtered power sources.

Because the ADT6401/ADT6402 operate 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 that are hundreds of millivolts

in amplitude due to wiring resistance and inductance.

If possible, the ADT6401/ADT6402 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 advisable to achieve the temperature accuracy specifications.

This decoupling capacitor must be placed as close as possible to

TTL/CMOS

LOGIC

CIRCUITS

POWER

SUPPLY

ADT6401/

ADT6402

Figure 22. Separate Traces Used to Reduce Power Supply Noise

TEMPERATURE MONITORING

The ADT6401/ADT6402 are ideal for monitoring the thermal

environment within electronic equipment. For example, the

surface-mount package accurately reflects the exact thermal

conditions that affect nearby integrated circuits.

The ADT6401/ADT6402 measure and convert the temperature

at the surface of its own semiconductor chip. When the ADT6401/

ADT6402 are used to measure the temperature of a nearby heat

source, the thermal impedance between the heat source and the

ADT6401/ADT6402 must be as low as possible.

As much as 60% of the heat transferred from the heat source to

the thermal sensor on the ADT6401/ADT6402 die is discharged

via the copper tracks, package pins, and bond pads. Of the pins

on the ADT6401/ADT6402, the GND pin transfers most of the

heat. Therefore, to monitor the temperature of a heat source, it is

recommended that the thermal resistance between the ADT6401/

ADT6402 GND pin and the GND of the heat source be reduced

as much as possible.

For example, the unique properties of the ADT6401/ADT6402

can be used to monitor a high power dissipation microproces-

sor. The ADT6401/ADT6402 device in its SOT-23 package is

mounted directly beneath the pin grid array (PGA) package of

the microprocessor. The ADT6401/ADT6402 require no external

characterization.