Preliminary Technical Data

ADT7320

Rev. PrA | Page 23 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. The time required for the part to settle to the

desired accuracy is dependent on 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.

SUPPLY DECOUPLING

The ADT7320 should be decoupled with a 0.1 μF ceramic capacitor

ADT7320 is mounted remotely from the power supply. Precision

analog products, such as the ADT7320, require a well-filtered

power source.

Because the ADT7320 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 ADT7320 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

0.1µF

ADT7320

TTL/CMOS

LOGIC

CIRCUITS

POWER

SUPPLY

Figure 22. Use of Separate Traces to Reduce Power Supply Noise

TEMPERATURE MONITORING

The ADT7320 is ideal for monitoring the thermal environment

within hazardous automotive applications. The die accurately reflects

the exact thermal conditions that affect nearby integrated circuits.

The ADT7320 measures and converts the temperature at the

surface of its own semiconductor chip. When the ADT7320 is

used to measure the temperature of a nearby heat source, the

thermal impedance between the heat source and the ADT7320

must be considered.

When the thermal impedance is determined, the temperature of

the heat source can be inferred from the ADT7320 output. As

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

thermal sensor on the ADT7320 die is discharged via the copper

tracks and the bond pads. Of the pads on the ADT7320, the

GND pad transfers most of the heat. Therefore, to measure the

temperature of a heat source, it is recommended that the thermal

resistance between the ADT7320 GND pad and the GND of the

heat source be reduced as much as possible.