ADM1032ARMZ-2R datasheet(15/18 Pages) ONSEMI | Local System Temperature Monitor

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ADM1032

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Layout Considerations

Digital boards can be electrically noisy environments, and

the ADM1032 is measuring very small voltages from the

remote sensor, so care must be taken to minimize noise

induced at the sensor inputs. The following precautions

should be taken.

1. Place the ADM1032 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 four to eight inches.

2. Route the D+ and D− tracks close together, in

parallel, with grounded guard tracks on each side.

Provide a ground plane under the tracks if

possible.

3. Use wide tracks to minimize inductance and

reduce noise pickup. 10 mil track minimum width

and spacing is recommended.

Figure 18. Typical Arrangement of Signal Tracks

10 MIL

GND

D−

D+

GND

4. Try to minimize the number of copper/solder

joints, which 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 since 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.

5. Place a 0.1

mF bypass capacitor close to the VDD

pin. In very noisy environments, place a 1000 pF

input filter capacitor across D+ and D− close to the

ADM1032.

6. If the distance to the remote sensor is more than

eight inches, the use of twisted pair cable is

recommended. This works up to about 6 feet to

12 feet.

7. For really long distances (up to 100 feet), use

shielded twisted pair, such as Belden #8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

ADM1032. Leave the remote end of the shield

unconnected to avoid ground loops.

Because the measurement technique uses switched

current sources, excessive cable and/or filter capacitance

can affect the measurement. When using long cables, the

filter capacitor can be reduced or removed.

Cable resistance can also introduce errors. 1

W series

resistance introduces about 1

°C error.

Power Sequencing Considerations

Power Supply Slew Rate

When powering up the ADM1032 you must ensure that

the slew rate of VDD is less than 18 mV/ms. A slew rate larger

than this may cause power-on-reset issues and yield

unpredictable results.

THERM Pin Pullup

As mentioned above, the THERM signal is open drain and

requires a pullup to VDD. The THERM signal must always

be pulled up to the same power supply as the ADM1032,

unlike the SMBus signals (SDA, SCL and ALERT) that can

be pulled to a different power rail. The only time the

THERM pin can be pulled to a different supply rail (other

than VDD) is if the other supply is powered up simultaneous

with, or after the ADM1032 main VDD. This is to protect the

internal circuitry of the ADM1032. If the THERM pullup

supply rail were to rise before VDD, the POR circuitry may

not operate correctly.

Application Circuit

Figure 19 shows a typical application circuit for the

ADM1032, using a discrete sensor transistor connected via

a shielded, twisted pair cable. The pullups 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 ADM1032 can be

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

as the Intel 820 chipset.

ADM1032ARMZ-2R Datasheet(PDF) 15 Page - ON Semiconductor

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