Preliminary Technical Data

ADT7420

Rev. PrA | Page 23 of 2

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

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

0.1µF

ADT7420

TTL/CMOS

LOGIC

CIRCUITS

POWER

SUPPLY

Figure 20. Use of Separate Traces to Reduce Power Supply Noise

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.