ADT7481ARMZ datasheet(23/24 Pages) AD

23 / 24 page

ADT7481

Rev. 0 | Page 23 of 24

generators, data/address buses, and CRTs are avoided, this

distance can range from 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.

GND

D+

D–

GND

5MIL

Figure 22. 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 large temperature

differential between them, thermocouple voltages should be

much less than 200 mV.

• Place a 0.1 μF bypass capacitor close to the V

pin. In

extremely noisy environments, an input filter capacitor may

be placed across D+ and D− close to the ADT7481. This

capacitance can affect the temperature measurement, so care

must be taken to ensure that any capacitance seen at D+ and

D− is a maximum of 1,000 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 of cable is needed.

• For really 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

ADT7481. 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 23 shows a typical application circuit for the ADT7481,

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 and SDATA pins of the ADT7481 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

ADT7481

0.1

μF

VDD

TYP 10k

FAN ENABLE

3V TO 3.6V

TYP 10k

Figure 23. Typical Application Circuit

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