ADM1027 datasheet(28/56 Pages) AD

28 / 56 page

REV. A

–28–

ADM1027

Step 1

Determine the Hardware Configuration

Essentially this means choosing whether to use Pin 10 as a PWM2

output or as an SMBALERT output and deciding which SMBus

address is to be used. To set Pin 10 as SMBALERT, set Bit 0

of Configuration Register 3 (Addr = 0x78) equal to 1. The

default state is PWM2, where this bit equals 0.

It also refers to the layout recommendations of the ADM1027

on a motherboard, for example.

ADM1027 Placement Considerations

Motherboards are electrically noisy environments, and care must

be taken to protect the analog inputs from noise, particularly the

D+/D– lines of a remote diode sensor. The following precautions

should be taken:

1. Place the ADM1027 as close as possible to the remote sensing

diode. Provided that the worst noise sources such as clocks

and data/address buses are avoided, this distance can be

4 inches to 8 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. Do NOT run the D+/D– lines in

different directions.

3. Use wide tracks to minimize inductance and reduce noise

pickup. A 10 mil track minimum width and spacing is

recommended.

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. Avoid routing D+/D–

on multiple layers or through vias if possible. These increase

series resistance that will cause temperature error.

5. Place a 0.1

mF supply bypass capacitor close to the ADM1027.

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

the use of shielded twisted pair cable is recommended. This

will work up to 100 feet. Connect the twisted pair to D+/D–

and the shield to GND close to the ADM1027. Leave the

remote end of the shield unconnected to avoid ground loops.

Because the measurement technique uses switched current

sources, excessive cable (adds resistance) and/or filter capacitance

can affect the measurement. A 1

W series resistance introduces

about 0.8

oC error.

ADM1027 Datasheet(PDF) 28 Page - Analog Devices

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