ADT7420UCPZ-R2 datasheet(23/24 Pages) AD

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Data Sheet

ADT7420

Rev. 0 | Page 23 of 24

APPLICATIONS INFORMATION

THERMAL RESPONSE TIME

Thermal response is a function of the thermal mass of the

temperature sensor, but it is also heavily influenced by the mass

of the object the IC is mounted to. For example, a large PCB

containing large amounts of copper tracking can act as a large

heat sink and slow the thermal response. For a faster thermal

response, it is recommended to mount the sensor on as small a

PCB as possible.

Figure 10 shows the typical response time of less than two

seconds to reach 63.2% of its temperature span. The tempera-

ture value is read back as a 16-bit value through the digital

interface. The response time includes all delays incurred on

chip during signal processing.

SUPPLY DECOUPLING

The ADT7420 must have a decoupling capacitor connected

between VDD and GND; otherwise, incorrect temperature

readings will be obtained. A 0.1 µF decoupling capacitor such as

a high frequency ceramic type must be used and mounted as

close as possible to the VDD pin of the ADT7420.

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.

0.1µF

ADT7420

TTL/CMOS

LOGIC

CIRCUITS

POWER

SUPPLY

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

POWERING FROM A SWITCHING REGULATOR

Precision analog devices, such as the ADT7420 require a well-

filtered power source. If the ADT7420 is powered from a

switching regulator, noise may be generated above 50 kHz that

may affect the temperature accuracy specifications. To prevent

this, an RC filter should be used between the power supply and

ADT7420 VDD. The value of the components used should be

carefully considered to ensure that the peak value of the supply

noise is less than 1 mV. The RC filter should be mounted as far

away as possible from the ADT7420 to ensure that the thermal

mass is kept as low as possible.

TEMPERATURE MEASUREMENT

The ADT7420 accurately measures and converts the tempera-

ture at the surface of its own semiconductor chip. Thermal

paths run through the leads, the exposed pad, as well as the

plastic package. 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

because this impacts the accuracy and thermal response of the

measurement.

For air or surface temperature measurements, take care to

isolate the package, leads, and exposed pad from ambient air

temperature. Use of a thermally conductive adhesive can help

to achieve a more accurate surface temperature measurement.

QUICK GUIDE TO MEASURING TEMPERATURE

The following is a quick guide for measuring temperature in

continuous conversion mode (default power-up mode). Execute

each step sequentially.

1. After powering up the ADT7420, verify the setup by

reading the device ID (Register Address 0x0B). It should

read 0xCB.

2. After consistent consecutive readings are obtained from

Step 1, proceed to read the configuration register (0x03),

TCRIT (0x08, 0x09), THIGH (0x04, 0x05), and TLOW (0x06,

0x07) registers. Compare to the specified defaults in Table 6.

If all the readings match, the interface is operational.

3. Write to the configuration register to set the ADT7420 to

the desired configuration.

4. Read the temperature value MSB register, followed by

the temperature value LSB register. Both registers should

produce a valid temperature measurement.

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