ADT7482ARMZ datasheet(23/24 Pages) AD

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ADT7482

Rev. 0 | Page 23 of 24

above ambient. However, the current forced through the remote

sensor is so small that self-heating is negligible. In the case of

the ADT7482, the worst-case condition occurs when the device

is converting at 64 conversions per second while sinking the

maximum current of 1 mA at the ALERT and THERM output.

In this case, the total power dissipation in the device is about

4.5 mW. The thermal resistance, θJA, of the MSOP-10 package is

about 142°C/W.

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and the

ADT7482 is measuring very small voltages from the remote

sensor, so care must be taken to minimize noise induced at the

sensor inputs. Take the following precautions:

•

Place the ADT7482 as close as possible to the remote

sensing diode. Provided that the worst noise sources, that

is, clock generators, data/address buses, and CRTs, are

avoided, this distance can be 4 inches to 8 inches.

•

Route the D+ and D– tracks close together, in parallel, with

grounded guard tracks on each side. To minimize

inductance and reduce noise pick-up, a 5 mil track width

and spacing is recommended. Provide a ground plane

under the tracks,if possible.

5MIL

GND

D+

D–

GND

Figure 24. Typical Arrangement of Signal Tracks

•

Try to minimize the number of copper/solder joints that

can cause thermocouple effects. Where copper/solder

joints are used, make sure that they are in both the D+ and

D− path and at the same temperature.

•

Thermocouple effects should not be a major problem as

1°C corresponds to about 200 mV, and thermocouple

voltages are about 3 mV/°C of temperature difference.

Unless there are two thermocouples with a big temperature

differential between them, thermocouple voltages should

be much less than 200 mV.

•

Place a 0.1 μF bypass capacitor close to the VDD pin. In

extremely noisy environments, an input filter capacitor can

be placed across D+ and D−, close to the ADT7482. This

capacitance can effect the temperature measurement, so

care must be taken to ensure that any capacitance seen at

D+ and D− is a maximum of 1000 pF. This maximum

value includes the filter capacitance, plus any cable or stray

capacitance between the pins and the sensor diode.

•

If the distance to the remote sensor is more than 8 inches,

the use of twisted pair cable is recommended. A total of 6

feet to 12 feet is needed.

•

For long distances (up to 100 feet), use shielded twisted

pair, such as Belden No. 8451 microphone cable. Connect

the twisted pair to D+ and D− and the shield to GND close

to the ADT7482. Leave the remote end of the shield

unconnected to avoid ground loops.

Because the measurement technique uses switched current

sources, excessive cable or filter capacitance can affect the

measurement. When using long cables, the filter capacitance

can be reduced or removed.

APPLICATION CIRCUIT

Figure 25 shows a typical application circuit for the ADT7482,

using discrete sensor transistors. The pull-ups on SCLK,

SDATA, and ALERT are required only if they are not already

provided elsewhere in the system.

The SCLK pin and the SDATA pin of the ADT7482 can be

interfaced directly to the SMBus of an I/O controller, such as

the Intel® 820 chipset.

5V OR 12V

SMBUS

CONTROLLER

FAN CONTROL

CIRCUIT

2N3904/06

CPU THERMAL

DIODE

D1+

D1–

D2+

D2–

VDD

SCLK

SDATA

ALERT

THERM

GND

ADT7482

0.1µF

VDD

TYP 10kΩ

FAN ENABLE

3V TO 3.6V

TYP 10kΩ

Figure 25. Typical Application Circuit

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