ADT7483AARQZ-R7 datasheet(23/24 Pages) ONSEMI

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ADT7483A

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LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and the

ADT7483A measures very small voltages from the remote

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

sensor inputs. Follow these precautions:

•

Place the ADT7483A as close as possible to the remote

sensing diode. Provided that the worst noise sources such

as 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 pickup, 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

•

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 approximately 200 mV, and thermo-

couple 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 VDD pin. In

extremely noisy environments, place an input filter

capacitor across D+ and D− close to the ADT7483A. 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 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 is needed.

•

For very 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 ADT7483A. 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 ADT7483A,

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 ADT7483A 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

ADT7483A 0.1μF

VDD

TYP 10kΩ

FAN ENABLE

3V TO 3.6V

TYP 10kΩ

ADD0

ADD1

Figure 23. Typical Application Circuit

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